© Douglas W. Morris

Lakehead University


Communication requires that both sender and receiver understand the message. In language, clear definitions are essential to clear communication. The definitions that follow represent an attempt to introduce students of ecology and evolution to the lexicon of both disciplines. While I encourage the use of simple language, jargon and short-hand are, nevertheless, crucial to efficient communication. An inherent risk is that the jargon itself can create artificial sub-divisions of inter-dependent disciplines. I hope that the following list will help foster increased and informed communication between ecologists and evolutionary biologists. Neither discipline can advance productively without the other.


Definitions create huge problems for the lexicographer because they arise from a variety of sources, and change with use. One seldom knows the ultimate origin(s) of the definitions they use. The problem, paradoxically, is most acute for those terms that are so important that they are in common, but perhaps improper, use. Most of us, quite appropriately, borrow definitions freely and without shame. Consistent use should produce redundant definitions, even from independent sources. One should find, therefore, dramatic similarities among written definitions that might appear to violate normal rules regarding precedence of use and proper citation. I have attempted to minimize these effects by citing (when known) the author(s) who introduced the original productive use of terms. The definitions we use in such cases have often been ascribed by others. We are all indebted to those anonymous authors.

I have been influenced by the superb glossaries that exist in most texts in ecology and evolution, and by countless articles, lectures, and discussions. There has been no consistent attempt to duplicate verbatim any definition from any of those sources. I hope that authors who suspect that their definitions appear here without credit will recognize the principle of consistent use, that their definitions will often have arisen through a similar process, and that mimicry, in language as in evolution, is the highest form of respect.

Absolute Fitness:

The measure of the number of gene copies produced by an individual genotype without reference to the number produced by other genotypes within the population. - contrast with "relative fitness"


In the context of development, a form of peramorphosis in which the exaggerated trait in the descendant is achieved through an increased rate of development relative to the ancestor.


The superb fit of species to their environment.

A heritable trait that increases the fitness of individuals possessing the trait relative to those individuals that lack it.

The process producing adaptedness.

Adaptationist Programme: (Stephen Gould and Richard Lewontin)

The assumption that natural selection operating with sufficient time can, and has, produced organisms superbly adapted to their environments such that most structure, form, function, and behaviour has been produced by natural selection. Also called the "Panglossian paradigm", and occasionally used in a disparaging or deprecating manner. - author's note - whereas it is true that there are many constraints to adaptation and natural selection, this does not mean that one should abandon natural selection as a prime organizer of biological pattern and diversity - compare with "Darwinian"


The ability of individual organisms to reproduce and survive in their environment.


The description of a trait that increases the fitness of individuals possessing the trait relative to individuals that lack it. - contrast with "maladaptive"

Adaptive Function: (Richard Levins)

The line or plane that identifies the weighted measure of fitness in two or more different environments.

Adaptive Landscape: (Sewall Wright)

A metaphor describing variation in fitness in changing environments. The adaptive landscape varies in space and time, and also with population density and the frequency of different alleles (or strategies) in a population.

Adaptive Radiation:

The process of the diversification of a lineage (clade) into many different taxa.

Additive Genetic Variance:

The variation in a population of individuals attributed to the additive effects of genes.


A measure of the "reflectivity" of a surface. High reflectance (as off of snowcover or unvegetated lands) yields a high albedo.

Allee Effect: (Warder Allee)

The concept that population growth may, at low densities, be positively related to density. - see "inverse density dependence"


One of many possibly different forms of a single gene.

Allometric equation: (y = bxa)

A mathematical description of the relative rates of growth of two morphological traits (y and x). The slope (b) is a measure of the speed of overall growth whereas the allometric coefficient (a) describes relative growth rates of y and x (a = 1, rates are equal; a > 1, y increases faster than x; a < 1, x increases more rapidly than y [e.g., metabolic rates {E}typically increases less rapidly than increases in body mass {M} {E = bM.75}]). Many ecological variables also vary allometrically with body size. - compare with "allometry"


The differential growth rate of morphological traits during ontogeny. Similar processes also apply to many ecological characters. - compare with "allometric equation"

Allopatric Speciation:

- see "geographic speciation"


The existence of separate species, or populations within a species, in separate geographical locations. - contrast with "sympatry"


An enzyme-encoding gene whose various alleles may be detectable by the enzyme's characteristics (e.g., its electrical charge).

Altruistic Behaviour:

Any behaviour that increases the fitness of non-related recipients while reducing the fitness of the actor. - contrast with "kin selection"

Amplification of Altruism: (Elliot Sober and David Sloan Wilson)

The concept that altruism need not evolve by overcoming its initially high cost to the individual performing the altruistic act, but that it might often arise through "second-order public good" where the collaborative acts of many individuals that reinforce altruistic behaviours allows the cost of altruism to be shared among those individuals. The subtle distinction is that second-order public goods provider receives no individual benefit from reinforcing group behaviour (e.g., social control through penalties against selfish behaviour).


Occurs when there is gamete dimorphism. In sexual species, female gametes are larger and more costly to produce than male gametes. The larger investment in zygotes by females (anisogamy) results in a two-fold advantage for asexual over sexual reproduction, and sets the stage for sexual selection. - contrast with "isogamy"

Antagonistic epistasis:

An epistatic effect where the expression of one gene or mutation causes an opposite effect (either increased or reduced fitness) on others. Antagonistic deleterious mutations increase phenotypic fitness; antagonistic beneficial mutations reduce phenotypic fitness. - see "synergistic epistasis"

Apparent Competition: (Robert Holt)

A reciprocal negative interaction between two or more species caused not by direct competition, but mediated instead, through a shared predator.

Artificial Selection:

The progressive change in traits toward some pre-defined goal and caused by selective breeding by humans.

Ashmole's Hypothesis:

An explanation for the latitudinal increase in clutch size caused by high competition of resident birds with variable numbers of migrant species. Competition with migrants reduces the survival of residents over winter. During the next reproductive season, competition is reduced, and surviving individuals can afford large clutches. Clutch size should be directly related to the proportion of migrant species in an avifauna.

Assembly Rules: (Jared Diamond)

The rules governing how species are assembled into ecological communities.

Assortative Mating:

The process where there is a clear pattern in certain genotypes selecting others as mates. Also called non-random mating. - contrast with "random mating"

Attraction Selection: (Douglas Morris)

A form of competition selection leading to the evolution of secondary traits which function to attract others for the purpose of transferring or exchanging genes. - see "dominance selection" and "intersexual selection"


A point in the appropriate state-space (often population densities) toward which the values of the state variables converge.

A specific value of the joint densities of two or more species toward which their respective densities converge.

A point of attraction for population trajectories.

Averaging Fallacy: (Elliot Sober and David Sloan Wilson)

The mistake of ascribing undue credit to individual selection by subsuming groups into its definition. The fallacy occurs when average fitness is calculated for a unit larger than the groups within which the (group) trait can confer group benefit and improve the fitness of members of one group over that of others.

Background Extinction:

The rate of species extinction under all causes other than those attributable to humans.

Basin of Attraction:

The state space (usually of population densities) that converges on a single attractor.


The general body plan or "blueprint" of a taxon (usually at the scale of family, order, class, or phylum). Implicit in use of the term is the assumption that the bauplan represents an integrated set of characters that cannot easily undergo independent evolution.

The set of points lying within a fitness set.


Plural of bauplan.

Bayesian Forager:

An organism that uses the information gained by foraging to modify its foraging behavior. - contrast with "omniscient forager"

Bean-bag Genetics:

A slang (sometimes disparaging) term used to describe the branch of genetics that assumes a direct correspondence between alleles and the phenotype. Also used to describe the independent assortment and inheritance of alleles. - contrast with "quantitative genetics"

Bergmann's Rule: (Bergmann)

A biogeographical rule describing the pattern where populations of a single species tend to be of larger body size at more northern latitudes than they are farther south. Often assumed to have a physiological basis related to the conservation of energy in more demanding environments.


The form of evolution that emerges from selection when environments vary through time. Traits that may confer high fitness during good times may yield very low fitness during bad times. Such traits can possess a high mean fitness (based on addition, mean fitness can be high because fitness can be exceptionally high in a single generation) but will be replaced by traits that maximize geometric mean fitness throuh time (the geometric mean is based on multiplication). Bet-hedging strategies evolve because the conservative traits cancel out very bad times with acceptable success during good times (they minimize temporal variation in fitness). An alternative bet-hedging strategy is to diversify such as through the production of offspring with high phenotypic variability or phenotypic plasticity. Andrew Simons has suggested that bet-hedging may represent the key that links micro and macroevolution.


The variety of life as well as the processes and patterns it creates.


The biological study of spatial patterns and relationships.

The scientific study of processes that create spatial patterns in ecology.

Black-hole Sink: (Robert Holt)

An area or population within which population growth is negative at all densities.


A rapid reduction in population size of short duration. Often associated with coincident founder effects, increased inbreeding and genetic drift. - also see "genetic bottleneck"

Bottom-up Control:

The argument that the biomass of each trophic level is determined by the availability of nutrients and energy in the trophic level immediately below it. - contrast with "top-down control"

Broad-sense Heritability (H):

The total genetic similarity between offspring and their parents.

The phenotypic variation in a population caused by genetic differences.

Canalization: (C. H. Waddington)

The process that limits phenotypic expresions through develpmental constraints. The analogy is based on canals that channel water along specified paths rather than let it seek its own pathway in the watershed. - contrast with "reaction norms"

Carrying Capacity (K):

A deceptively complex term describing the population size at which the population growth rate would be zero in the absence of any interspecific effects.


A rare and cataclysmic event causing a rapid increase in the death rate of individuals. Often associated with habitat destruction, changes in demography and population dynamics, and in the structure of ecological communities.

A cataclysmic event that has negative effects on survival and persistence, regardless of evolutionary adaptation.

Cellular Automaton:

A method of simulating spatial population dynamics by allowing propagules within a matrix (or lattice) of sites to colonize adjacent cells (sites). More generally, cellular automata refer to models of lattice systems that allow the state of each cell to change during the simulation.

A lattice wherein the state of each cell is determined by the states of surrounding cells

Centrifugal Organization: (Michael Rosenzweig and Zvika Abramsky)

A form of community organization based on two or more species sharing a preference for one habitat, while exhibiting distinct preferences on two or more additional habitats.

Centrifugal Speciation: (W. L. Brown Jr.)

A form of geographical speciation where large isolated populations are the primary source of adaptive radiation rather than smaller ones. Favourable mutations arise more frequently in the large population that evolves rapidly but occasionally produces new conservative species following the creation of barriers to gene flow.


see "chaotic population dynamics"

Chaotic Population Dynamics:

Population dynamics where a small change in initial densities can lead to widely different population trajectories caused by non-linear density dependence. Forecasts of abundance become increasingly unreliable over time. Usually reserved for systems that are otherwise completely deterministic. Despite our inability to predict the abundance of any particular population at any point in time, we may be able to define the statistical distribution of abundances quite precisely. - see "ergotic chaotic population dynamics"

Character Displacement:

Occurs when a character (usually related to competitive ability) or trait is more different where populations or species overlap (sympatry) than where they occur separately (allopatry).


In systematics, the group of related species descended from a common ancestor.

A monophyletic group of species.


A classification of species and higher taxa based on shared derived characters.

A classification of species that attempts to reflect phylogeny.


A visual display of the assumed phylogeny of species.


A gradual (usually spatial) gradient in phenotypes.

Coefficient of Inbreeding (F):

The probability that the two alleles in a diploid individual arose from the same ancestor allele (the probability that they are identical by descent). When F is greater than zero, the population is inbred, and the frequency of heterozygotes is less than would occur if mating was random (e.g., in Hardy-Weinberg equilibrium).

Coefficient of Relatedness (r):

A measure of the genetic similarity among individuals caused by common ancestry (also called shared descent).


The adaptations of two or more species in direct response to the adaptations of other sympatric and interacting species.


The ability of two or more species to live together for prolonged periods of time.

Colonization-competition Hypothesis:

The hypothesis that ecological succession proceeds because late-succession species are better competitors, but poorer colonizers, than are early succession species. Late-succession species will, therefore, always replace early colonizers, regardless of how rich or poor the environment happens to be. - compare with "successional niche hypothesis"


The group of interacting species that share an area.

Community Matrix:

A species species matrix, A, whose elements (Aij) correspond to the per capita effect of species j on the growth rate of species i for all pairs of species. It is important to note that the elements of the matrix may not be easily converted to pairwise interaction coefficients (Aij/Aii) unless the interactions among all species are linear (additive). The elements represent partial derivatives evaluated at equilibrium densities and include, thereby, the effects of all other species in the community.

Community Structure:

The description of the relative abundances and distribution of species within a community.

Comparative Method:

A research program that tests hypotheses in ecology and evolution based on comparisons among spatially segregated or phylogenetically distant taxa.

Competition Coefficient (aij):

A measure of the average effect of a single individual of species j on the population growth rate of species i.

Competition Selection: (Douglas Morris)

A general term for the kind of multilevel selection that accounts for the evolution of secondary characters that enhance the exchange of genes between individuals and groups. Sexual selection is a subset of competition selection. - see "dominance selection" and "attraction selection"

Competitive Exclusion Principle:

see "principle of competitive exclusion"

Competitive Release:

The reduction in interspecific competition that occurs when a species exists in a community (often on an island) with fewer competing species than under normal conditions. Often associated with niche expansion or character divergence.

Competitive Speciation: (Michael Rosenzweig)

A process of ecological speciation where density-dependent habitat selection causes populations to become segregated in different habitats. Habitat segregation results in disruptive selection and eventual speciation. - see "ecological speciation" and "sympatric speciation" - contrast with "geographic speciation"


A somewhat vague term used by landscape ecologists to describes the number and identity of different habitats in a landscape, but occasionally, the characteristics of single habitat patches. - see "context" and "connectivity"

Compression Hypothesis: (Robert MacArthur and Edward Wilson)

The hypothesis that an increase in the number of competing species is more likely to result in a contraction in habitat use, than in diet.


A member of the same genus (congeneric).


A term used by landscape ecologists to describe how landscape "structure" affects the movement of organisms through space. Organisms living in landscapes with high connectivity are generally thought to have less restrictions on their movement and occupation of the landscape than they would have in a less connected landscape.- see "structural connectivity and "functional connectivity" as well as "composition" and "context"

Conservation Biology:

The multi-disciplinary study, application, and advocacy associated with conservation and conservation strategies. - contrast with "conservation ecology"

Conservation Ecology:

The scientific study of the ecological and evolutionary processes and patterns associated with the conservation of biodiversity. - contrast with "conservation biology"


A member of the same species.


Any factor or process that limits adaptation.

Contest Competition: (Alexander J. Nicholson)

That form of competition whereby individuals actively interfere with other individuals of the same or different species (often referred to as interference competition). - contrast with "scramble competition"


A term used by landscape ecologists to describe the intermingling of habitat patches within an intervening matrix. - see "composition" and "connectivity"

Convergent Evolution:

Occurs when similar traits or adaptations have evolved among groups of unrelated species.

Co-operative Breeding:

A breeding system in which adult individuals who are not the parents aid in the rearing of young. Helpers (also called auxiliary breeders) are usually related to the parents and are maximizing their inclusive fitness.


see "semivariogram"

Core-taxa Hypothesis: (Evan Siemann & James H. Brown)

An explanation for the observation that gaps in body-size distributions can be explained by the widespread distribution of related taxa among biomes within a continent, and by differences in the kinds of taxa that inhabit different continents. The hypothesis predicts that gaps should occur at different body sizes in similar biomes on different continents, and at similar body sizes among different biomes on the same continent. - contrast with "textural-discontinuity hypothesis"


A strip of habitat in a matrix of unusable space along which propagules can disperse from one area to another.

Cost of Delay Hypothesis: (R. H. Drent and S. Daan)

A version of state-dependent life history theory (specifically related, in this instance, to the evolution of clutch size) predicting that when breeding seasons are restricted, parents in poor condition may be able to maximize their reproductive contribution to future generations by delaying breeding until their improved body condition allows them to produce a clutch large enough to exceed the associated cost of reduced offspring value. The hypothesis has been formalized by Rowe et al. (1994; "the condition-dependent individual optimization hypothesis").


see "population cycle"

Darwin Machine: (H. Plotkin)

The concept that numerous "managed" processes function similarly to our understanding of evolution by natural selection. The concept is especially helpful in understanding cognition, multi-level selection and cultural evolution. Cultural evolution, for example, allows multiple ideas to be derived and tested against one another (the "Darwin" component) as well as a process for selecting those ideas that are adaptive (the "machine"). Most importantly, Darwin machines can themselves be products of evolution.


A loose and unfortunate term used to describe scientists (or scientific programs) whose world view is centred on natural selection and adapation as the main processes of evolution and evolutionary change - compare with "adaptationist programme"

Defense Diversity Hypothesis (DDH): (Laurie and Tarja Oksanen)

The argument that herbivores and plants possess a variety of defensive strategies that protect them from higher trophic levels and thereby minimize trophic cascades. - see "world-is-green and world-is-prickly-and-tastes-bad hypotheses"


A local group of randomly mating individuals.

A local panmictic population.

Demographic Stochasticity:

Fluctuations in population density caused by the probabilistic survival and reproduction of individuals possessing different demographic traits (e.g., sex or age classes). Most severe in small populations - contrast with "environmental stochasticity"

Density Dependence:

Occurs when a process in ecology or evolution is either a curvilinear or non-linear function of population density. - contrast with "frequency dependence"

Density-dependent Habitat Selection:

Any process of active habitat choice where the density of individuals in different habitats varies with population size.

Density Independent:

Description of a process in ecology or evolution that has no relationship with population density.

Derived Character (Trait):

A novel trait that occurs among a subset of species sharing a common ancestor.

Despotic Distribution:

see "ideal-despotic distribution"

Differential Allocation Hypothesis: (Nancy Burley)

A sexual-selection hypothesis whereby females increase their Darwinian fitness by linking their investment in offspring to the attractiveness of their father. The hypothesis assumes that there is a trade-off between current and future investment in offspring, and that females allocate more resources to offspring when they have an attractive mate, than they do on average.

Diffuse Competition: (Robert MacArthur)

The cumulative, not necessarily linear, negative effect of a complex of competing species.

Direct Benefits Hypothesis:

A hypothesis of intersexual selection whereby the secondary traits preferred by one mate (usually the female) act directly to increase its fitness (e.g., increased territory quality, increased parental care etc.), rather than indirectly through improved "genetic quality" of offspring.

Directional Selection:

The form of natural selection when phenotypes toward one extreme have higher fitness than others. Results in a shift in the modal phenotype and a skewed distribution of phenotypes in the population.


The movement of individuals between habitats, areas, or populations.

The movement of individuals from their natal site to another.

Disruptive Selection:

A process of selection where peaks of high fitness are separated by valleys of low fitness. More than one modal phenotype has high fitness. A necessary mechanism in most models of sympatric and ecological speciation.

Distinct-Preference Organization: (Stuart Pimm and Michael Rosenzweig)

A form of community organization based on the premise that interacting species possess distinct preferences for qualitatively different habitats. Frequently leads to the "ghost of competition".

Dominance Selection: (Douglas Morris)

A form of competition selection where individuals and groups compete amongst others for the opportunity to transfer or exchange genes. - see "attraction selection" and "intrasexual selection"


- see "genetic drift"

Early Bird Hypothesis: (Peter and John Smallwood)

The hypothesis that sex ratios will be linked to the timing of reproduction when early reproduction creates asymmetries in future breeding success. Sex ratios in early broods will be male biassed, for example, when males from those broods have a higher reproductive success than females, or males produced from later broods.

Ecological Speciation:

Any speciation event where the process of speciation depends principally on ecological context. - see "competitive speciation" and "sympatric speciation" - contrast with "geographic speciation"

Ecological Succession:

A more-or-less predictable sequence of species as a community is assembled through time.

Ecological Trap Hypothesis: (Dwernychuk and Boag, Gates and Gysel)

Occurs when environmental change decouples the cues used to assess habitat quality from the habitat's true quality.

Frequently associated with the attraction of animals (often birds) to a habitat (e.g., edge) that may appear as (or previously functioned as) a source, but which now functions as a sink (e.g., as could be caused by nest predation).


The branch of science that seeks to understand interactions between organisms and their environment.

The complex of interactions describing a species' fit to nature.

Ecology of Fear: (Joel Brown)

The concept that many of the effects of predators on prey, and on ecological populations and communities are transmitted not by effects on population size through mortality, but instead by their effects on the behaviour of prey. Often results in incredibly fascinating evolutionary games between one or more prey species and their predators.


The complex of communities and the physical/chemical environment in an area whose patterns are based on shared biological interactions.


The blended and unique habitat that occurs along the boundary between habitats.

Edge Effect:

Occurs when the pattern of population density, and the process of population dynamics, is different near a boundary between two habitats than can be explained by the independent effects of the two habitats.

Edge Species:

A species that specializes on ecotones between habitats.

Effective Population Size (Ne):

The size of a hypothetical population that would lose its heterozygosity at the same rate as the population under investigation. Effective population size is almost always smaller than the actual population being studied.

If one knows the number of males (Nm) and the number of females (Nf) in a population, Ne can be estimated as:

Ne = 4(NmNf)/(Nm + Nf)

Endogenous Factors:

A term used in population regulation to specify dynamic feedback processes that influence population numbers (these may be instaneous or temporally lagged). - contrast with "exogenous factors"

Enemy-free Space: (Jeffries and Lawton)

Usually refers to indirect competition caused when a common predator (or group of predator species) causes two or more prey species to "compete" for "enemy-free space" where the predator's negative impact is reduced. - see "apparent competition"


A context-dependent term used to describe the external relationships of genes, individuals, populations, and species.

Environmental Stochasticity:

Fluctuations in population density associated with a probabilistic environmentally induced change in population growth rate or carrying capacity - contrast with "demographic stochasticity"


The process whereby one gene (or even the allelic frequencies of that gene) modify the expression (and fitness) of another. - see "antagonistic" and "synergistic epistasis

Equilibrium Theory of Island Biogeography: (Robert MacArthur and Edward Wilson)

The theory that attempts to explain the number of species existing on an island as a dynamic balance between colonization and extinction.

Ergotic Chaotic Population Dynamics:

Chaotic population dynamics that converge on the same statistical distribution of abundances. Ergotic chaotic populations with different parameters of population growth will converge on different probability distributions of abundance. - see "chaotic population dynamics"

Escape-and-Radiate Hypothesis: (Paul Ehrlich and Peter Raven)

A highly debated coevolutionary hypothesis whereby angiosperms develop new secondary compounds by mutation and recombination, some of which are toxic or repulsive to herbivores. Free of their enemies, the plants diversify in an evolutionary radiation. Insects follow suit, first evolving resistance to the new compounds, then diversifying themselves.

Escape-from-Enemy Hypothesis:

The hypothesis that invasive species exhibit high fitness because natural enemies in the species' native range (e.g., competitors, predators, parasites and other pathogens) are absent in the introduced range. Free of its enemies, the invading species has high success. Note that the hypothesis depends explicitly on interspecific interactions ( also called the enemy-release hypothesis).


The dynamics of genes in space and time.

The change in gene or allelic frequencies through space and time.

A change in heritable traits through common descent.

Evolutionarily Stable Strategy (ESS): (John Maynard Smith)

A frequency-dependent fitness strategy (e.g., the sex ratio of a population) that cannot be invaded by any other alternative strategy (e.g., a different sex ratio).

An evolutionary strategy that cannot be replaced by any other currently existing strategy. - see "evolutionary game theory"

Evolutionary Arms Race:

An evolutionary process whereby a trait continues to evolve toward extreme values because its selective advantage is eroded by counter-adaptations by other individuals or species.

Evolutionary Branching:

The process of cladogenesis caused by disruptive selection at a fitness minimum

Evolutionary Ecology:

The branch of ecology that evaluates how evolution by natural selection creates and modifies ecological processes and patterns.

Evolutionary Game Theory: (John Maynard Smith)

That branch of game theory evaluating the stability of existing strategies against all currently available strategies. - see "evolutionarily stable strategy"

Evolutionary Strategy:

The relationships among traits that combine to increase a phenotype's fitness.

Exogenous Factors:

Used in population regulation to specify processes that modify changes in populations without being influenced by population size or density. - contrast with endogenous factors

Expected Heterozygosity (He):

A measure of genetic diversity based on an estimate of the average heterozygosity. The estimate is obtained by sampling several loci, then calculating the mean heterozygosity across all sampled loci.

Exploitation Competition:

see "scramble competition"

Exploitation Ecosystems Hypothesis (EEH): Laurie Oksanen et al.

The argument that entire guilds of endotherm predators limit endotherm herbivore guilds, and plants consequently compete heavily in productive ecosystems, but not in unproductive ones where predators are absent and natural herbivory is high - see "world-is-green hypothesis"


Any group of organisms that are currently alive.

Extinction Filter: (Andrew Balmford)

The concept that initial extinctions associated with major climatic or other changes act as a filter removing highly susceptible species and taxa, while those that remain will often be resilient in the face of continuing change. Having cleared the initial hurdle of species extinction, remaining taxa have a high probability of clearing others (Coope, 1995). Long periods of human occupation and disturbance may be one such filter where the extinction of many species in areas with high human density and disturbance has left a disproportionate number of taxa that can withstand, or even flourish, in the presence of human activities.

Fitness (W):

A measure of the number of gene copies produced by individual genotypes. - see "absolute fitness", "inclusive fitness", and "relative fitness"

Fitness Minimum:

A point on the adaptive landscape where the fitness gradient is 0 and where fitness is at a minimum (the antithesis of an ESS). Frequency-dependent selection can cause such minima to be evolutionary attractors that later diverge via disruptive selection.

Fitness Set: (Richard Levins)

The plot, for all possible phenotypes, of fitness in one environment versus that in another. Used in combination with the adaptive function to identify the best possible phenotype (ESS).

Fluctuating Asymmetry:

The pattern of bilateral variation in organisms where small random and independent asymmetries occur on both the "left" and "right" sides. Fluctuating asymmetry tends to be greater (greater bilateral variation) when individuals experience stress during development. The link to stress has led many researchers to use flucuating asymmetry as a measure of an individual's quality or fitness.

Food Chain:

A colloquial term describing the trophic position of species within ecological communities.

A simplistic linear representation of trophic relations and links within ecological communities.

Food Web:

A graphical or statistical description of the set of trophic links within an ecological community.

Fractal Geometry: (Benoit Mandelbrot)

The branch of mathematics devoted to describing "self-similar" structures across spatial scales. Snowflakes are an example, because they have similar geometry no matter how large. In ecology, examples include natural habitat edges and coastlines because they are irregular at any measurement scale.


The process of creating fragments of habitat from a larger pre-existing habitat.

Frequency Dependence:

Occurs when a process in ecology or evolution varies with the frequency of individuals, genotypes, phenotypes or strategies. - contrast with "density dependence"

Functional Connectivity: (Tischendorf and Fahrig)

A term used in landscape ecology to describe the behavioural response of individual species to landscape "structure".- compare with "structural connectivity"

The relationship between the number of prey consumed by a predator and the number of prey present (per capita consumption rate). Though there is an invinite number of possible functional responses, three have received the most attention from ecologists. Predators with type I responses consume prey in direct proportion to prey abundance. Predators with type II responses forage with diminishing returns through time. Type III, or sigmoidal, responses typify predators that switch to alternative species when prey abundance is low. - contrast with "numerical response"

Fundamental Niche: (George-Evelyn Hutchinson)

The set of environmental conditions that could be exploited by a population or species.

The set of environmental conditions where a population or species could live and replace itself. - compare with "realized niche"

Fundamental Theorem of Natural Selection: (Ronald Fisher)

The rate of change of fitness by natural selection is proportional to a population's additive genetic variation.

An important corollary is that natural selection cannot decrease fitness because the additive genetic variance can never be less than zero.

Game Theory:

The branch of mathematics concerned explicitly with processes that are frequency dependent.

Gap Analysis:

The creation, overlay, spatial analysis and interpretation of maps of variables such as cover, soil type, land use, species and ecosystem distribution to identify spatial gaps requiring protection. In practice, gap analysis is usually conducted at the landscape scale using remote sensing data and geographic information systems.


Any genetic code that is heritable as a single unit.

Gene Flow:

The migration and integration of genes into a different gene pool from which they originated.

Gene Pool:

The complex of genes contained within a single inter-breeding population.


A phenotype or species that has equally high fitness across a broad range of environments.

A phenotype or species that consumes a variety of resources with equal efficiency.

A phenotype or species that occupies a wide variety of habitats.

Genetic Bottleneck:

A dramatic reduction in population size and genetic variation followed by an increase in population size without an immediate increase in genetic variability.

Genetic Drift:

A change in allele frequency caused by chance alone. Results in non-adaptive evolution and is most common in small populations where random events in survival and reproduction can most easily alter allele frequencies.

Genetic Load:

The reduction in a population's mean fitness caused by the accumulation of deleterious alleles.


The complex of genes contained within a single individual.

Geographic Mosaic Coevolution: (John Thompson)

The hypothesis that coevolution among species varies spatially within a selection mosaic that differs from place to place. The model has three components. 1. A selection mosaic that favours different coevolutionary trajetories in different populations. 2. The existence of "coevolutionary hot spots" where reciprocal selection occurs among community members, and by inference, the simultaneous existence of "cold spots". 3. The "remixing" of traits geographically by processes such as gene flow, genetic drift, and local extinctions.

Coevolutionary dynamics that vary from place to place.

Geographic Range:

The space or distance, measured in one or more of many different ways, corresponding to the geographical limits of a species' distribution, or to the number of sites over which it exists. There is no standard method for measuring geographic ranges, and all methods possess a different bias in their estimate of area occupied and the spatial limits (extent) of a species' occurrence.

Geographic Speciation:

The process of speciation that depends on geographical barriers to gene flow. - see "allopatric speciation"

Ghost of Competition Past: (Michael Rosenzweig)

Occurs when competition between or among species causes them to occupy different habitats when coexisting at a stable competitive equilibrium. Also used as a metaphor by Joseph Connell in an inclusive sense to refer to any case where the effects of past competitors still exist on the distribution, abundance, adaptations or ecological interactions of species.

Giving-up-density (GUD): (Joel Brown)

The amount of resource remaining after foraging. GUD is highly correlated with quitting-harvest rates (QHR) when animals forage in patches with diminishing returns. When individuals forage in identical patches, GUD can be used to estimate variables such as relative habitat qualities, differences in predatory risk, and differences in missed opportunities among habitats or among foragers.

Group Selection:

A historically controversial form of selection invoking group benefit at the expense of individual fitness. More properly viewed as any process where differential reproduction and survival among groups effects group differences in heritable traits. A collection of groups will have, thereby, a different temporal/spatial or statistical distribution of traits than will the same number of individuals that are not structured into groups. Thus, one should be able to test for group selection by comparing the "rate" of evolution among populations that are, and are not, structured as groups. It is crucial to appreciate that goup selection ceases when competing groups are eliminated. Selection will then occur only at lower levels of group organization where those sub-groups compete amongst one another. The point would appear to be brought home by numerous human cultural examples of "global domination" followed by disintegration and anarchy. - see "multilevel selection"

Guild: (Barry Root)

A group of interacting species that consume shared resources in similar ways.


The physical (but not necessarily abiotic) environment in which an organism lives.

The classification of the physical (but not necessarily abiotic) environment into units corresponding to different preferences and perceptions by the organisms that live in them.

Habitat-dependent Population Regulation: (Douglas Morris)

The concept that an understanding of population regulation must include the effect of density-dependent habitat selection. The form of regulation will depend on the relative relationships between fitness and density in each habitat. Examples include "congruent", "convergent", "cross-over" "divergent" and "parallel" regulation.

Habitat Matching: (H. Ronald Pulliam and Thomas Caraco)

The concept that food-limited consumers should distribute themselves such that the number of foragers using any two patches matches the productivity of those patches. Often called the "habitat-matching rule" - see "undermatching"

ni / nj = Ki / Kj

where n is the number of individuals in a patch and K is carrying capacity.

Habitat-matching Rule:

see "habitat matching"

Habitat Selection:

The process whereby individual organisms choose to occupy one or more habitats preferentially over others. - see "density-dependent habitat selection"

Habitron: (Douglas Morris)

An experimental facility that allows ecologists to control the density and movement of individuals between pairs of habitats.

Hamilton's Rule: (William Hamilton)

Natural selection will favour any allele that reduces individual fitness as long as the fitness benefit (weighted by the degree of genetic relatedness) obtained by relatives exceeds that cost.

Natural selection will favour any allele that reduces individual fitness as long as inclusive fitness is increased by at least that same amount. - see "kin selection"

rb - c > 0

where r is the coefficient of relatedness, b is the benefit to the recipient, and c is the cost to the "actor".

Hamilton-Zuk Hypothesis: (William Hamilton and Marelen Zuk)

A model for the "handicap principle" whereby sexual displays are considered to be reliable indicators of health. Though the display causes reduced viability, it can nevertheless be selected for if it is expressed differentially in the healthiest individuals. Diseased individuals either do not express the display, or express a display of reduced quality.

Handicap Principle: (Amotz Zahavi)

The originally controversial but now widely-accepted concept that extreme secondary sexual characters are selected because they signal high mate quality despite being a handicap that would otherwise reduce the fitness of the individual. Mates that select individuals with the handicap obtain genes of high viability for their offspring. They also receive the "handicap". The principle has potentially wide application and includes any interaction where "signallers" advertise their quality to "receivers". - see "Hamilton-Zuk" hypothesis


A mating system (especially common in the Hymenoptera - social insects including the ants, bees and wasps) in which males are haploid (develop from unfertilized eggs) and females are diploid (develop from fertilized eggs). Full sisters are more closely related to one another (on average, sharing 75% of their genome) than to either parent (50% of the genome).

Hard Selection:

Occurs when the survivors of a selective event are determined independent of population density or of their fitness relative to other individuals within the population. - contrast with "soft selection"


see "broad-sense heritability" and "narrow-sense heritability"


Differences in the rate, or timing, of development that often lead to evolutionary differences in the structure and shape of morphological traits. One of several mechanisms where altered development produces detectable evolutionary changes in traits. - compare with "heterotopy"


A term used to signify that heterozygotes have higher fitness than homozygotes.

A term used to signify that outbred or crossbred individuals have higher fitness than inbred ones.


An evolutionary change in the location of a phenotypic trait within an organism caused by changes in development. One of several mechanisms where altered development produces detectable evolutionary changes in traits. - compare with "heterochrony"


A population measure estimating the mean proportion of heterozygous loci possessed by a randomly-selected individual.

A measure of the frequency of heterozygotes in a population.

Higher-order Effect:

Occurs when the effect of one species on another depends on which other species are present in the community.


A derived trait possessed by two or more taxa (e.g., species).

A common ancestral (but not necessarily unmodified) trait possessed by two or more taxa.

Hot Spot:

An area with an exceptionally large number of species (often endemics).

Humpty-Dumpty Effect: (Stuart Pimm)

The situation in which a community cannot be restored even though all species are still available to colonize the site. The founding conditions on which the community was based no longer exist.


A form of peramorphosis in which the exaggerated trait in the descendant is achieved through prolonged development relative to the ancestor.


A clear and concise statement of falsifiable predictions designed to test a scientific theory.

Icarus Effect: (Robert Colwell and David Winkler)

Results in a conservative neutral model of community structure that can obscure the role of competition because the "correlation between vagility and morphology can obscure the effects of competition in morphological comparisons of mainland and island biotas". - see Narcissus Effect & J. P. Morgan Effect

Ideal-free Distribution (IFD): (Stephen Fretwell and Henry Lucas)

The distribution of habitat-selecting individuals between two or more habitats when the mean fitness is identical in each. Assumes that individuals occupy habitats in a way that maximizes individual fitness and that each is free to occupy any habitat that it chooses. - see "inclusive fitness distribution"

Ideal-despotic Distribution: (Stephen Fretwell and Henry Lucas)

The distribution of habitat-selecting individuals between two or more habitats when the mean fitness differs because dominant individuals actively interfere with the habitat choices of subordinates. Sometimes referred to as the "ideal-dominance distribution".

Ideal-pre-emptive Distribution: (H. Ronald Pulliam and Brent J. Danielson)

The distribution of habitat-selecting individuals between two or more habitats when the mean fitness differs because early colonizing individuals pre-empt the best available breeding sites from individuals that arrive later.

The distribution of habitat-selecting individuals that results when each individual occupies the best-available breeding site.

Inbreeding Depression:

A reduction in the fitness of offspring caused by mating between close relatives. Mechanisms include reduced heterosis and the creation of individuals homozygous for deleterious alleles. - contrast with "outbreeding depression" and see "coefficient of inbreeding"

Incidence Function (j): (Jared Diamond)

The pattern of occupation of a species across a variety of sites (often islands).

Incidental Predation:

The form of predation that occurs when predators, nourished by common prey, also consume other 'non-target' prey types. When predators are abundant, incidental predation can have large negative effects on rare prey species.- see "apparent competition"

Inclusive Fitness: (William Hamilton)

General meaning: The measure of the number of gene copies produced by a genotype plus the number produced by that genotype's relatives.

Proper restricted meaning: The measure of the number of gene copies produced directly by an individual plus the number produced in relatives that are caused by that individual's social behaviour.

Inclusive Fitness Distribution (InFD): (Morris, Lundberg & Ripa)

The expected distribution of individuals among habitats assuming that individual's are maximizing their inclusive fitness. Leads to increased "undermatching" of density in the better habitat with increasing degree of relatedness. - see "ideal-free distribution"

Incumbent Replacement: (Michael Rosenzweig and Robert McCord)

The evolutionary process whereby a currently existing species (the incumbent) is replaced by competitors that have a "constraint-breaking" adaptation. Incumbent replacement is intricately dependent on habitat selection. Replacement is associated with low densities of incumbent species that allows competitors to invade the incumbent's habitat(s).

Independent Contrasts: (Joe Felenstein)

Also called "standarized independent contrasts". A method of correcting comparative data for phylogenetic bias. A key component of the "comparative method". The technique operates by creating contrasts among (usually pairs) taxa that can be assumed to have independent evolutionary histories (taxa sharing a single common ancestor). The phylogeny is then "pruned" of these independent contrasts, and the technique repeated. Each contrast on the pruned tree is transformed to account for the bias in estimating ancestoral taxa, and every contrast is standardized by the mean value of each trait, and by its standard deviation that is assumed to be proportional to the degree (or time) of evolutionary divergence. The technique assumes that evolution can be modelled as a continuous process of small changes.

Indirect Effects:

Occur among groups of species when the influence of a "donor" species on a "receiver" species is made through either direct or indirect influences on intermediate "transmitter" species.

Individual-based Model (IBM):

A method of simulating population dynamics and community structure by successively evaluating the survival, reproduction, and movement of each individual within the population or community of organisms.

Individual Optimization Hypothesis: (Pettifor, Perrins & McCleery)

The concept that parents produce an optimal clutch size tailored to the parents' abiolity to produce and rear young (quality). - see "optimal investment hypothesis"

Insular Distribution Function (IDF): (Mark Lomolino)

The plot of the presence/absence of species as a function of island area (ordinate) and isolation (abscissa). The boundary between presence and absence represents the combinations of area and isolation where extinction and immigration rates are equal.

Instantaneous Value Assessment Rule:

see "marginal-value theorem"; contrast with "potential value assessment rule"

Interaction Chain:

Occurs when the indirect effects of one species on another is caused by changes in the abundance of intermediate or "transmitter" species (e.g., apparent competition and intra-guild predation.

Interaction Coefficient:

A coefficient representing the sign and magnitude of the average effect of individuals of species j on the population growth rate of species i. - contrast with "competition coefficient"

Interaction Modification:

Occurs when one species indirectly influences another by modifying the per capita effect effect of an intermediate species on the receiver species (e.g., the ecology of fear.

Interference Competition:

see "contest competition"

Interior Species:

A species that can maintain its population only in the interior of large patches of habitat (often forest).

Intersexual Selection:

Sexual selection associated with mate choice. A special case of attraction selection.

Inter-specific Competition:

Any form of reciprocal negative interaction on population dynamics or fitness between two or more species. - see "contest competition" and "scramble competition" - contrast with "intra-specific competition"

Intolerant Species:

A species whose average fitness declines rapidly outside of a narrow range of habitat(s). - contrast with "tolerant species"

Intra-guild Predation: (Robert Holt)

The concept that generalized predators often consume prey within their own trophic level.

Intrasexual Selection:

Sexual selection associated with competition among members of the same sex for access to mates. A special case of dominance selection.

Intra-specific Competition:

Any form of reciprocal negative interaction on fitness among individuals within a single species. - contrast with "inter-specific competition"

Intrinsic Rate of Natural Increase (r):

The (usually) maximum rate of instantaneous population growth. Sometimes called the "innate rate of increase" or the "instantaneous rate of increase". Identical to the "Malthusian parameter", a measure of fitness commonly used in population genetics.

Inverse Density Dependence:

A term used to imply an increase in fitness (or population growth rate) with increased density. This clumsy jargon should be replaced with clear descriptions of the relationship between density and fitness (or population growth rate). The problem is exacerbated by the use of "negative" density dependence to imply a positive effect. - see "Allee effect"


A line along which the rate of change (the first derivative) of one state variable is constant. In ecology, isoclines describe the sets of densities of two species (or resouces) along which their respective rates of population growth are constant. - see "zero-growth isocline"

Isodar: (Douglas Morris)

A line or surface along which different evolutionary strategies have equal fitness. Usually used to describe the ESS "solution" to density-dependent habitat selection. Named after "Darwin".


Occurs when gametes are of similar size and cost. - contrast with "anisogamy"

Isoleg: (Michael Rosenzweig)

A line describing the sets of densities of two species where each exhibits equal choice between a pair of habitats. From the Greek "iso" - the same, and "lego" - to choose.


The form of reproduction where individuals reproduce many times during their lifetime.

J. P. Morgan Effect: (Robert Colwell and David Winkler)

Results in a conservative neutral model of community structure that can obscure the role of competition because the "weaker the taxonomic constraints on sampling the harder it becomes to detect competition". - see Icarus Effect & Narcissus Effect


A symmetrical (usually square) "window" (e.g., of N by N cells) commonly used in spatial modeling and analysis. Rates of change between the "centres" of adjacent kernels can be used to map landscape heterogeneity. The pattern generated will depend on local heterogeneity as well as kernel "size".

Kin Selection:

The form of natural selection based on an individual's inclusive fitness. - see "Hamilton's rule" and contrast with "altruistic behaviour"


A German word (roughly translated as culture followers) that identifies species that persist and increase in human-modified habitats and landscapes (see Rosenzweig 2003 for a discussion of their importance in conservation).


A German word (roughly translated as culture avoiders) that identifies species that cannot tolerate human-modified habitats and landscapes (see Rosenzweig 2003 for a discussion of their importance in conservation).

Laissez-faire Competition:

see "scramble competition"

Landscape Ecology:

The study of ecological processes, and the description of ecological patterns, at the spatial scale of landscapes.

That branch of ecology where processes and patterns at the landscape scale depend on interactions among its components.

Large Population Size Theory: (Ronald A. Fisher)

A gradual process of adaptation based on mutation and selection where the average fitness of very large panmictic populations increases through time. Assumes that populations integrate across all local environmental variation. Can lead to geographic speciation through barriers to gene flow. - contrast with "shifting balance theory"

Lazarus Taxa:

Species (or higher taxonomic levels) that "disappear" for long periods of time following mass extinctions (presumably because they occupy refugia in small numbers), then "reappear" to re-colonize at least some of their former geographic range.


An aggregation of individuals of the same sex (usually males) that advertise to members of the opposite sex visiting the aggregation for the purpose of mating. Usually assumed to occur in species where males cannot defend either females or the resources they require, often because the females and their resources are widely distributed.

Levins' Rule: (Hanski et al. 1996)

A heuristic rule emerging from models of metapopulation dynamics. The rule should not be used by managers because its limiting assumptions are likely to be violated in many real-life situations.

"A sufficient condition for metapopulation survival is that the remaining number of habitat patches following a reduction in patch number exceeds the number of empty but suitable patches prior to patch destruction."

Life History:

The description of an organism's survival and reproduction through life.

Life-history Strategy:

The relationships among traits related to survival and reproduction that maximizes an organism's fitness.

Limiting Similarity: (Robert MacArthur and Richard Levins)

The concept that the assembly of species into a community is based on limits to how similar they are in morphology, resource and habitat use.

Linkage Disequilibrium:

Occurs when the distribution of genotypes at one locus depends on the distribution of genotypes at another locus (linked traits).

Linkage Equilibrium:

Occurs when the genotypes at one locus are distributed randomly with respect to those at another locus (independent assortment).

Keystone Species: (Robert Paine)

A species whose large impacts on a community are greater than expected by its abundance. Used originally to identify predators that had unusually great impacts on community structure and assembly.

Macroecology: (James Brown, Brian Maurer)

The study of how large-scale (usually corresponding to geographic range sizes) processes influence local populations and communities.

The description of large-scale patterns in ecology.


The change in species and higher taxa through long periods of time.

Evolution at a scale where it is no longer possible to transmit characteristics "horizonatally"


The description of a trait that reduces the fitness of individuals possessing the trait relative to individuals that lack it. - contrast with "adaptive"

Malthusian Parameter (r):

see "intrinsic rate of natural increase"

Malthusian Principle:

Populations will increase as their maximum rate until they become limited by their environment.

Marginal-value Theorem: (Eric Charnov)

The proof that an optimally-foraging individual with perfect information should exploit resources in any patch until the instantaneous harvest rate within the patch equals the average within the individual's foraging range. - also known as the "instantaneous value assessment rule"; contrast with "potential value assessment rule"

Maternal Investment Hypothesis:

see "Trivers and Willard Hypothesis"

Mesopredator Release: (John Terborgh)

Occurs when medium-sized omnivorous predators become abundant because intra-guild predation has been reduced by the reduction or elimination of larger predators. The hypothesis is most frequently invoked with respect to increased abundance of nest-predators (in North America these include raccoons, opossums, and skunks) that may increase extinction risks ground-nesting landbirds.

Mesoscale: (Robert Holt)

The spatial scale where populations are aggregated into sub-units.

The spatial or landscape scale corresponding to a metapopulation.

Metacommunity: (Robert Holt)

The interaction of a metapopulation with other species across a variety of spatial scales.

Metapopulation: (Richard Levins, Ilkka Hanski)

Spatial aggregations of individuals of a single species across the landscape.

The composite of several more-or-less independent local populations of a single species.

Metapopulation Dynamics:

The change in numbers of individuals associated with the differential survival and colonization of several more-or-less independent local populations.


The dynamics of allele frequencies within populations and species.

Evolution at a scale where there can be lateral (or horizontal) exchange of characteristics among populations.

Mid-domain Effect: (Robert Colwell and David Lees)

The tendency for increased spatial overlap of species toward the centre of a geographically constrained area caused by the geometrical limits of the area. Similar arguments can be made about geometric constraints on other properties of species (e.g., density, niches). Mid-domain effects arise because species with large geographic ranges can occupy only large areas, whereas species with small spatial requirements can exist in both small and large areas. Thus, as one approaches a boundary (e.g., polar limits of distribution), the probability of encountering new species with large ranges decays, while the probability of encountering species with small ranges remains more-or-less constant. Species richness will peak near the middle of the geometrically constrained space, whether it be an island, a patch of habitat, or the entire world. The mid-domain effect, and more generally, geometric theories of distribution, thus serve as the appropriate null model against which to assess patterns of species richness and abundance.


The movement of individuals and subsequent integration of their genes from one population to another.

The temporal (usually biannually) movement of individuals from one part of their geographic or population range to another.

Minimum Viable Population (MVP):

The smallest population size that has a given probability of remaining extant for some specified period of time.

Modern Synthesis:

The name given to our improved understanding of evolution through population genetics, often associated with theories advanced by Ronald A. Fisher, John B. S. Haldane, and Sewall Wright.

Molecular Clock: (E. Zuckerkandl and Linus Pauling)

The concept that mutations occurring at a constant rate can be used to age divergence of populations or clades.

Moran Effect: (P. A. P. Moran)

The ability of environmental (density-independent) perturbations to synchronize the dynamics of populations that have the same density-dependent structure (e.g., time-lags, cycling; the structure must be the same, but not necessarily the mechanisms creating that structure). The effect is important because it suggests that causes of synchronized dynamics need not be the causes of density-dependent regulation.

Mosaic Evolution:

The more-or-less independent evolution of characters that yields different rates of evolution for different characters, even within a single lineage. The emphasis is on the evolution of characters or features rather than on the whole organism.

Muller's Ratchet: (H. J. Muller)

An explanation for the evolution of sexual reproduction whereby it is argued that genetic drift in small asexual populations allows the gradual but inevitable increase in the number of mutations each individual carries. The ratchet turns most rapidly when small populations have high mutation rates where mutants have minor negative effects on fitness.

Multilevel Selection:

Recognition that natural selection occurs across numerous levels from the most fundamental elements from modification through common descent (e.g., nucleotides) to the largest groups for which one can identify an evolutionary trait. The evolution of a trait that occurs at several different levels occurs through the balance of selection acting across all applicable levels. see "group selection"


Any process that creates new genes or alleles.


An interaction between two or more species where each gains either a direct or indirect per capita benefit.

Narcissus Effect: (Robert Colwell and David Winkler)

Produces conservative null models of communities that can obscure the role of competition when "sampling from a post-competition pool underestimates the role of competition, since its effect is already reflected in the pool". - see Icarus Effect and J. P. Morgan Effect

Narrow-sense Heritability (h2): (h2 = R/S)

The ratio of additive genetic variation to total phenotypic variation.

The proportion of variation between parents caused by genes.

Natural Selection: (Charles Darwin)

The change in allele frequencies caused by the differential survival and reproduction of individuals living in variable environments. Natural selection requires heritable variation, a "struggle for existence" where there is the potential for production of more offspring than can survive, and that the heritable variation influences the struggle.

Nested Sub-set: (Bruce Patterson)

The pattern of incidence of a species among a set of sites (often islands) where the composition of communities of few species is a sub-set of the composition of communities with more species.


A form of paedomorphosis in which the juvenile features of the descendant's trait are achieved by a decreased rate of development relative to the ancestor.

Net Reproductive Rate (R):

The number of female descendants produced by a female over her lifetime.

Neutral Theory: (Motoo Kimura)

The theory arguing that most mutations have minor, if not zero, effects on fitness. Neutral mutations allows the accumulation of a high degree of heterozygosity within the population.

Niche: (Joseph Grinnell)

The set of environmental conditions exploited by a population or species.

Niche Breadth:

A measure of the variance in resource use by a population or species.

Niche Complementarity:

A pattern among coexisting species when high overlap on one niche dimension is associated with low overlap on another.

Niche Ovlerap Hypothesis: (Eric Pianka)

The hypothesis that species compensate for increased competition by reducing the overlap in their niches.

Norm of Reaction:

see "reaction norm"

Normalizing Selection:

see "stabilizing selection"

Numerical Response

The relationship between the number of predators and the number of prey. - contrast with "functional response"


An area (usually high elevation) that remains unglaciated when all nearby areas are covered by glacial ice. Often functions as an important reservoir of species whose geographic ranges have become disjunct.

Omniscient Forager:

A forager that possesses "perfect" information on such things as the quality and renewal rates of patches and its instantaneous intake rate. -contrast with "Bayesian forager"

Optimal Investment Hypothesis: (Douglas Morris)

The hypothesis that parents allocate limited resources to reproduction in such a way as to optimize the investment in each offspring. The optimum will occur whenever juvenile survival and parental mortality increase with the proportion of resources invested in reproduction. - see "individual optimization hypothesis" and the "principle of proportional investment"

Optimal Virulence Hypothesis: (R. M. Anderson and R. M. May)

A model of parasite virulence based on a supposed trade-off between virulence (or parasite transmission) and parasite-induced host mortality. Transmission rates increase toward an optimum at intermediate levels of host mortality. High reproduction by the parasite increases virulence and host mortality. Parasites should thus evolve an optimal virulence (and reproduction) level that maximizes reproductive success. The hypothesis applies to some pathogens, but not to those where virulence is caused primarily by sources other than parasite reproduction (e.g., as in interactions between hosts, parasites and their shared environments).


An archaic and discredited view of progressive evolution assuming that evolution is directed, linearly, toward a fixed goal. Once a structural "advance" has occurred within a taxon, evolution continues to "enhance" that change forever, or until the taxon goes extinct.

Outbreeding Depression:

A reduction in fitness of offspring caused by outcrossing of individuals from different populations. Mechanisms include reduced local adaptation as well as the breakup of coevolved genes (linkage equilibrium). - contrast with "inbreeding depression"

P* Rule: (Robert Holt)

A "rule of thumb" for species coexistence demonstrating that the prey species that persists with a shared predator, in the absence of competition, will be the one that maintains the highest equilibrium density of the predator species.

P** Rule: (Holt, Grover and Tilman)

A "rule of thumb" for species coexistence that demonstrates that the prey species that persists in a three-link food web is the one that maintains the highest equilibrium density of predators when it itself is at equilibrium with its resources.

Paedomorphic Trait:

A trait whose development has "terminated" at an early stage in descendants such that the adult expression of the trait resembles the juvenile form in the ancestor. Common mechanisms of paedomorphosis include "neoteny", "progenesis" and "post-displacement". - contrast with "peramorphic trait" and compare with "heterochrony"

Panmictic Population:

The term used to describe a population within which mating is random (panmixis).

Parallel Evolution:

The evolution of similar traits, among closely-related species, that are not caused by common descent.


A form of asexual reproduction that occurs by the otherwise normal production of offspring in the absence of fertilization.

Patch Residence Time (PRT):

The amount of time that a forager spends in a single foraging patch. Typically this will include the time spent searching for prey, the time spent handling prey, and the time spent in other activities including grooming, social behaviour, and predator vigilance. Foraging models vary in the number of activities included, and PRT will vary accordingly.

Peramorphic Trait:

A trait whose development is altered such that its form is "exaggerated" relative to its expression in ancestors. Common mechanisms of peramorphosis include "acceleration", "hypermorphosis", and "pre-displacement". - contrast with "paedomorphic trait" and compare with "heterochrony"


A measure of stability based on the length of time that a population, species, or community exists in one state.

Phase Plane:

see "state space"

Phase Plot:

A graph of the trajectory of instantaneous states of two or more species through time. - see "phase space"

Phase Space:

In ecology, the space defined by species' densities within which each point corresponds to a possible instantaneous state of the system. A collection of such points through time yields the trajectory of population dynamics through time. - see "phase plot"


The expression of genes within a single organism.

Phenotypic Gambit: (Alan Grafen)

The approach where one assesses the evolution of a trait assuming that it is controlled by simple genetics.

A research strategy where one assesses evolutionary strategies without regard to underlying genetics.


The process whereby a single gene has more than one effect on the phenotype.


A mating system in which a single female has two or more male mates.


A mating system in which several females share two or more male mates.


A mating system where a single male has two or more female mates. Often arises because some males are able to monopolize limited resources or habitats.


The diet of an organism that consumes a wide variety of foods (polyphagous).


Occurs when a single genotype produces alternative (usually morphological) phenotypes in response to different environmental cues. A classic example is the protective morphology (helmets, spines) that develop in some Daphnia (water flea) species in the presence of predators.


The group of interacting con-specific individuals that share an area.

Population Cycle:

A controversial pattern of population dynamics where the density of one or more species (often northern herbivores and their predators) shows more-or-less consistent multi-annual cycles of low and high density. Competing hypotheses include time lags associated with specialist predators versus poorly understood intrinsic mechanisms of population regulation.

Population Density:

The number of individuals in a population or sub-population divided by the cumulative area or volume that they occupy.

Population Dynamics:

The change in numbers of individuals in a population associated with their differential survival and reproduction.

Population Genetics:

The branch of genetics that studies the patterns of, and change in, allele frequencies within populations.

Population Size:

The total number of individuals living within a population.


A form of paedomorphosis in which the juvenile features of the descendant's trait are acquired by a delay in the onset of development relative to the ancestor.

Post-zygotic Isolation:

A mechanism of reproductive isolation among members of different populations that is caused by failures in zygote development or sterility in hybrid offspring. - contrast with "pre-zygotic isolation"

Potential Value Assessment Rule: (Ola Olsson & Noél Holmgren)

The rule stating that Bayesian foragers should abandon a foraging patch when the estimated gain rate during the remaining time in the patch equals the expected long-term gain rate for the habitat. - contrast with the "instantaneous value assessment rule"

Predator Facilitation

A form of indirect mutualism where the presence of one predator species alters prey behaviour or distribution so that they are more susceptible to consumption by a second coexisting predator species. Similar processes can operate to increase prey capture success within a single species of predator.


A form of peramorphosis in which the exaggerated trait in the descendant is achieved through an earlier onset of development than in the ancestor.

Pre-emptive Distribution:

see "ideal-pre-emptive distribution"

Pre-zygotic Isolation:

A mechanism of reproductive isolation among members of different populations that is caused by differences in the timing of breeding or in mate choice. - contrast with "post-zygotic isolation"

Principle of Allocation: (Richard Levins)

The concept that the allocation of energy or resources by an individual to one activity cannot be simultaneously used for another. Often referred to as the "trade-off principle" or "jack-of-all-trades master-of-none".

Principle of Competitive Exclusion: (G. F. Gause)

The concept that coexisting species must occupy different niches, otherwise all but one will be excluded. Sometime stated as "complete competitors cannot coexist" (Hardin).

Principle of Equal Opportunity: (Robert MacArthur)

The concept that species diversity will be adjusted among habitats until the degree of interspecific interactions (especially competition) is equalized among them. If one habitat has more ecological opportunity than another, species should migrate preferentially toward that habitat and evolve specializations to it.

The principle of equal opportunity can also be applied to the cost-benefit analysis of any "evolutionary" or "behavioural" decision. At the scale of habitat selection, for example, individuals should move from one habitat to another until benefits equal costs, that is, until there is equal opportunity afforded within each one.

Principle of Proportional Investment: (Douglas Morris)

The mathematical rule that if parents allocate limited resources equally to each of n offspring, each will receive 1/n of the total investment.


A form of paedomorphosis in which the juvenile features of the descendant's trait are achieved by earlier termination in the onset of development relative to the ancestor.


Any group of one or more individuals, at any life stage, who can act as founders of a population when colonizing a new (often island) area.


A form of sequential hermaphroditism where females change their sex to become male. (EM>- compare with "protrandry"


A form of sequential hermaphroditism where males change their sex to become female. (EM>- compare with "protogyny"

Pseudo-sink: (Andrew Atkinson and William Sutherland)

A habitat that has net immigration, but nevertheless can maintain a population in the absence of migration. A habitat that is a pseudo-sink in one landscape may be a source in another because the population will have positive growth at low density.

Punctuated Equilibrium: (Niles Eldredge and Stephen Jay Gould)

The concept that the diversification of life occurs during short bursts of macro-evolutionary change, followed by long periods of stasis, rather than as a gradual progression.

Quantitative Trait:

A trait that is influenced by the interactions among several genes.

Quantitative Genetics:

The branch of genetics dealing with the inheritance and evolutionary effects of quantitative traits. - contrast with "bean-bag genetics"

Quitting-harvest Rate (QHR):

The instantaneous harvest rate (consumption of resource) at which a forager leaves a foraging patch.

R* Rule: (David Tilman)

A "rule of thumb" of species coexistence demonstrating that the prey species that persists in the absence of predators is the one that maintains the lowest equilibrium density of resources (the most efficient species).

R** Rule: (Holt, Grover and Tilman)

A "rule of thumb" of species coexistence demonstrating that the prey species that persists in a three-link food web is the one that maintains the lowest equilibrium density of resources when it exists in equilibrium with its predator(s).

Random Mating:

The process where there is no consistent pattern in the selection of mates. - contrast with "assortative mating"

Reaction Norm:

The range of phenotypes produced by a single genotype in different environments. - contrast with "canalization"

Realized Niche: (George-Evelyn Hutchinson)

The set of environmental conditions where a population actually lives.

The set of environmental conditions actually exploited by a population or species.

The set of environmental conditions where a species actually lives and replaces itself. - compare with "fundamental niche"

Reciprocal Altruism:

The concept that altruistic behaviour evolves through the "promise" of future reciprocity from the individuals that benefit from the original voluntary altruistic act.

Reconciliation Ecology: (Michael Rosenzweig)

A set of conservation strategies built around the concept that conservation biologists must learn how to share human habitats with other species to ensure that those species maintain their geographic ranges.

"the science of inventing, establishing, and maintaining new habitats to conserve species diversity in places where people live, work, or play" (M. L. Rosenzweig, 2003, p. 7)

Red-Queen Hypothesis: (Leigh Van Valen)

Van Valen's metaphor from "Through the Looking Glass" whereby evolution continues because a phenotype's selective advantage is eroded by the adaptations of other individuals or species. Encapsulated in the phrase "the faster evolution progresses, the more things stay the same". Often refers to evolution associated with predator-prey dynamics.

Never-ending coevolution.


A small geographical area representing a subset of a species or higher taxon's former geographic range, and where it occurs in reduced numbers. Usually associated with major catastrophic or climatic events.

Relative Fitness:

The proportional measure of the number of gene copies produced by a genotype in comparison to the number produced by other genotypes in the population.- contrast with "absolute fitness"

Reproductive Skew:

The degree to which individuals in a group bias the total reproduction by the group toward themselves. A high skew group has a few dominant breeders (and consequently many "helpers"). A low skew group has a more equitable distribution of breeding individuals.

Reproductive Value (Rv):

A measure of fitness calculated as a female's age-specific expectation of future reproduction.

A female's age-specific production of future descendants.


A measure of ecological stability that quantifies how quickly a population or community returns to a pre-disturbed state.


A measure of ecological stability that quantifies how difficult it is to disturb a population or community from an existing state.


Any consumable item that has the potential to limit the size of a biological population.

Resource Dispersion Hypothesis (RDH):

A model to explain group living when there are no obvious direct benefits. The hypothesis assumes that when resources are unpredictable in space or time, individuals will be able to ensure access to resources only if they defend much larger territories than when resources are more predictable. Pulses of resource abundance will nevertheless exceed the requirements of individuals who benefit from sharing the cost and profit of the enlarged territory with other individuals. Group sizes should tend to be larger in more variable environments.

Resource Selection Function (RSF): (B. F. J. Manly and M. S. Boyce)

A function that represents the proportional use of a resource relative to its availability. Often calculated using presence/absence data, then associating the presence of individuals with underlying variables via logistic regression. Resource selection functions can be converted to resource selection probability functions, integrated with geographic information systems, and used to predict spatial patterns in population size under different management or disturbance scenarios.

Resource Selection Probability Function (RSPF):

Closely related to resource selection functions, and often calculated from used versus unused "units" in logistic regression.

Response to Selection (R): (R = h2S)

The difference between the mean phenotype (e.g., mean body size) of the offspring from selected parents and the mean phenotype of the offspring from all individuals in the parental population.

Runaway Selection: (Ronald A. Fisher)

A model of intersexual selection where genes for mate choice become linked to genes for the secondary sexual character. The process will continue to exaggerate the secondary character until its fitness disadvantage is opposed by natural selection.


A major mutation in one or more phenotypic traits that is often associated with rapid evolutionary change and radiation - see "punctuated equilibrium"

Scramble Competition: (Alexander J. Nicholson)

Competition within or among species whereby individuals independently consume resources in short supply. - contrast with "contest competition"

Secondary Sexual Characters:

Sexually dimorphic characters that would normally be opposed by natural selection but that are necessary for sexual reproduction when individuals compete for mates.

Seed Bank:

The accumulated viable seeds present in soil.

Seed Rain:

The large-scale distribution of dispersing seeds.

Seed Shadow:

An area of reduced seed dispersion often associated with physical features that alter wind dispersal.


see "natural selection"

Selection Coefficient (s):

The difference in fitness between one genotype and another.

Selection Differential (S): (S = R/h2)

The difference between the mean phenotype (e.g., mean body size) of selected individuals (e.g., survivors) and the mean phenotype of the entire population.


The form of reproduction where individuals reproduce only once during their lifetime.


Often called simply a variogram and closely related to correlograms and covariance functions. A graph usually used as a spatial or geostatistic that plots the autocorrelation between estimates of a variable measured at increasing distance (times) from one another. The semivariance will increase as more autocorrelated variance is accounted for with increased distance, but will level off when no further increase is encountered. In the case of population density, the semivariance can be estimated as the sum of the squared differences in samples of density between lagged distances -x and +x divided by twice the density at laggged distance x. The intercept with the Y axis is called the "nugget" and represents the distance at which there is no spatial autocorrelation (e.g., purely random variance in density with distance or white noise). A population with no spatial pattern in density would possess a flat variogram.


A decline in reproduction with age.

A decline in physiological function with age.

Sensory Drive:

One of many hypotheses that relate the evolution of secondary sexual characters to mate preference. The hypothesis assumes that mate choice is adaptive, and that the evolution of mating traits evolve under both sexual and natural selection. The traits are viewed as "communication signals" that coevolve with perception by the sex most clearly involved in mate choice. Signals vary among habitats because some variants will be more conspicuous in one habitat than in another. Signals must also correspond to the receiver's perception, but not be so conspicuous as to dramatically reduce fitness via natural selection (e.g., increased predation, reduced foraging efficiency).

Sex Ratio:

The proportion of a population composed of males. Theory predicts, when offspring of each sex are equally expensive, that negative frequency-dependent selection will favour genotypes biassed toward production of the sex that has the lowest frequency in the population. Otherwise, the ratio of investment in male and female offspring is equalized to produce an optimum sex ratio, at birth, biassed in favour of the least expensive sex. - see "Shaw-Mohler Theorem"

Sexual Conflict:

The outcome of an evolutionary arms race between males and females when they have opposing strategies for optimizing reproduction.

Sexual Selection: (Charles Darwin)

The change in secondary sexual characters caused by the differential success of individuals at securing mates. Often, the characters would be selected against by natural selection. - see "inter- and intrasexual selection"

Shared-preference Organization: (Stuart Pimm and Michael Rosenzweig

Occurs when two or more competing species share a preference for the same quantitatively superior habitat.

Shaw-Mohler Theorem: (coined by Stearns and Hoekstran)

When an organism reproduces sexually, an evolutionary equilibrium will occur when the product of male and female fitness is maximized. If male and female offspring are equally expensive and have equal fitness, then the sex ratio should be 1:1. But if these conditions do not hold, the evolutionary equilibrium will be biassed toward the sex that yields the greatest net fitness. - see "Sex Ratio"


A form of sylviculture designed to generate an even-aged forest stand by sequentially removing trees in an older one that provides shelter while the new forest regenerates. The sequence of cuts is usually as follows: 1. Preparatory cut to remove damaged, diseased, or old trees so that a vigorous group of seed trees remains. 2. Regeneration cut to provide growing area for regeneration while providing shelter of larger trees. 3. Removal cut to harvest the remaining large trees after regeneration is complete. Note that though regeneration is natural, this form of sylviculture is not likely to restore the original heterogeneous forest structure.

Shifting Balance Theory: (Sewall Wright)

Adaptive evolution occurring through the combined processes of random genetic drift in small spatially-structured demes, epistasis, genotype-environment interactions, local selection and migration. Random gene reassortments change allele frequencies and convert nonadditive genetic variance into additive variance that can be acted on by natural selection. Successful gene combinations migrate across the adaptive landscape. Restrictions to gene flow can be an effective means of speciation. Originally viewed as "too complicated", but now recognized as increasingly realistic in variable environments. - contrast with "large population size theory"

Sieve habitat: (Christian Thomas and William Kunin)

A habitat (or population) that has net emigration like a source habiat, but in which net emigration exceeds net recruitment (Births - Deaths). Sieves, even though they contribute propagules to other habitats and poppulations, can persist only through "rescue" by immigrants. Sieves are especially important as colonization stepping-stones in the maintenance of metapopulations.

Signor-Lipps Effect:

An "underestimate" of the date of extinction of a taxon caused by incomplete sampling (an incomplete fossil record) whereby the last known fossil occurrence of a taxon predates its actual extinction. The bias can be important in estimating rates of extinction. - contrast with the "Sppil-Rongis" effect


An area or population that has negative population growth and receives immigrants from other areas or populations with positive population growth (source). - contrast with "black-hole sink"

A habitat whose net migration is positive.

Sister Species:

A restricted example of "sister taxa".

Sister Taxa:

The name given to pairs of taxa diverged from a single and adjacent ancestor.

Pairs of taxa sharing a dichotomous node in a cladogram.

Size-advantage Hypothesis: (M. T. Ghiselin)

A hypothesis of sex-change in sequentially hermaphroditic species. If Darwinian fitness increases more rapidly with size in one sex than in another, an individual that changes from sex with lower reproductive potential will be favoured over one that does not change, or one that changes in the opposite direction. Protogyny (change from female to male), for example, is often associated with sexual selection whereby large males gain preferential access to many females. The predictions of protogyny are compromised by female size distributions and sperm competition (Munoz and Warner 2003).

Soft Selection:

Occurs when survival through a selective event depends on population density and, thereby, on their fitness relative to other individuals within the population. - contrast with "hard selection"


An area or population that has positive population growth at low density and that exports individuals to areas or populations with negative population growth (sink).

A habitat whose net migration is negative.

Source-sink Dynamics: (H. Ronald Pulliam)

The process of connected population dynamics where areas of negative population growth (sinks) are maintained by immigration of individuals from areas with positive population growth (source).


A phenotype or species that has high fitness in a small range of environments.

A phenotype or species that consumes only a few resources with high efficiency.

A phenotype or species that occupies a narrow range of habitats.

A phenotype or species with a narrow range of abilities.


The process that creates new species.

The establishment of reproductive isolation among previously interbreeding populations.


A group of individuals with common ancestry that have the natural potential to successfully exchange genes.

A group of very similar individuals that share a common ancestry.

A group of inter-breeding individuals that are evolutionarily independent from other inter-breeding groups.

Species Abundance Distribution:

A graph of the number of species (frequency) [ordinate] existing in classes representing different abundances (numbers) of individuals [abscissa - frequently on a log2 scale]. Also plotted frequently as a rank/abundance plot where abundance [ordinate - log scale] is graphed against the rank of species (called species sequence) from the most to least abundant.

Species Diversity:

Any of several measures of the number of species in a community or area corrected by their relative abundances. - contrast with "species richness"

Species Flock:

An unusually large number of closely-related species that live in a relatively small area, often with unusual levels of sympatry with little apparent ecological separation, and who have few or no close relatives elsewhere. Examples include the flowering plants in Mediterranean heathlands of southwestern Australia (kwongan) and the Cape Region of South Africa (fynbos), as well as various freshwater taxa in many of the world's great lakes (especially Lake Baikal, and those of Africa's Rift Valley (Tanganyika, Malawi, Victoria). - also called a "species swarm"

Species Richness:

The number of different species living in a community or area.

Species Swarm:

see "species flock"

Sperm Competition: (G. A. Parker)

A form of male-male competition in polyandrous or promiscuous species where resolved by one male's sperm is more successful at siring offspring than those of others (often associated with the amount and quality of sperm produced). The sperm of different males compete to fertilize eggs.

Sppil-Rongis Effect:

An "overestimate" of the date of origination of a taxon caused by incomplete sampling (an incomplete fossil record" whereby the first known fossil occurrence of a taxon post-dates its origin. The bias can be important in estimating the rate of diversification and adaptive radiation in the fossil record. - contrast with the "Signor-Lipps effect"

Stabilizing Selection:

The form of natural selection when the average phenotype in the population has maximum fitness. Typically, the phenotypic variance declines through time, but there is not change in the mean phenotype produced. Also called "normalizing selection".

Stalingrad Effect: (J. S. Brown et al.)

Occurs when the state of foragers is so desperate, and the marginal value of energy is so concomitantly high, that they forage their environment to an extraordinarily low value while enduring extreme predation risk. Named after the otherwise inexplicable behaviour of starving German soldiers prior to their surrender at Stalingrad during the second world war.

State Space:

A graph of the set of variables that describe a system.

The n-dimensional plot of the sets of densities of each of n species censussed in a biological community.

synonym = "phase plane"


A probabilistic event.


The set of possible trait values on which adaptive evolution can operate.

The outer negatively-sloped boundary of a fitness set.

Structural Connectivity:

A term used by landscape ecologists to describe the physical connection among landscape patches independent of the properties of any species living in the landscape.- compare with "functional connectivity"


The group of interacting con-specific individuals associated with a particular area or habitat.


see "ecological succession"

Successional Niche Hypothesis: (S. W. Pacala and M. Rees)

The hypothesis that ecological succession in areas with relatively small disturbances (e.g., tree blowdowns that spare supple saplings), early successional species capitalize on available resources and dominate for a short time before late-succesional species that are suppressed take advantage of higher resource efficiency (or longer life spans) to replace the "early" species. - compare with "competition-colonization hypothesis"

Sympatric Speciation:

A controversial process of speciation where sympatric populations diverge into separate species. - see "competitive speciation" and "ecological speciation" - contrast with "geographic speciation"


The zone of geographical overlap of separate species. Occasionally refers to sympatric populations where each occupies a different habitat in the same area. - contrast with "allopatry"

Synergistic epistasis:

An epistatic effect where the expression of one gene or mutation causes a reinforcing effect (either increased or reduced fitness) on others. Synergistic deleterious mutations reduce phenotypic fitness; synergistic beneficial mutations increase phenotypic fitness. - see "antagonistic epistasis"


The branch of biology devoted to classifying organic diversity.


Study of the processes that preserve fossils, and the contexts in which fossils occur.

Tangled Bank: (Charles Darwin)

Darwin's metaphor (he used the term "entangled bank") to describe the myriad of interactions occurring among species in the "struggle for existence".


Any formal grouping of organisms that share common traits.

Textural-continuity Hypothesis: (C. S. Holling)

An explanation for the observation that gaps in body-size distributions are caused by structural discontinuities in habitat and landscape. The hypothesis predicts that the greatest gaps in body size should, within similar biomes, occur at the same size on different continents but at different sizes among biomes on a single continent. - contrast with "core-taxa hypothesis"


A set of logical testable predictions based on a clearly defined set of defensible assumptions.

Tolerant Species:

A species whose average fitness is similar across a wide array of habitats. - contrast with "intolerant species"

Top-down Control:

The argument that the biomass of one trophic level is controlled by the biomass of the trophic level immediately above it. - contrast with "bottom-up control"

Trade-off Principle:

see "principle of allocation"


Any detectable difference in phenotypes that can be acted on by evolution.

Triver's and Willard Hypothesis: (R. L. Trivers and D. E. Willard)

The hypothesis that parents should invest preferentially in the sex with the greatest variance in fitness. An important corollary in polygynous species is that only females of highest quality will be able to reap the rewards of investing in offspring of the most expensive sex (usually males). - also called the "maternal investment hypothesis"

Trophic Cascade: (Paine; Carpenter, Kitchell & Hodgson)

A model of community organization where it is argued that the effects of removing (or adding) a top carnivore will 'cascade' across lower trophic levels as a series of negative and positive effects (e.g., removal of carnivores will enhance herbivore biomass with a concomitant reduction in the biomass of primary producers). - see "top-down control"

With top-down control, primary producers are limited by nutrients in communities with an odd number of trophic levels, but are limited by consumers in communities with an even number of trophic levels. Most ecologists now agree that trophic cascades are dampened as food chains/webs increase in length and complexity.

Many ecologists follow Polis (1999) and differentiate between "species-level" and "community-level" cascades. These can be differentiated as follows:

Community-Level Trophic Cascade:

A substantial change in the distribution of plant biomass caused by changes in the abundance of carnivores.

Species-Level Trophic Cascade:

A change in the distribution and abundance of a sub-set of plant species caused by changes in the abundance of carnivores.

Note that all trophic cascades represent examples of indirect interactions and typically depend on strong direct interactions between carnivores and their prey.

Trophic Level:

The functional location of a group of organisms within a food-chain. The trophic level of a species (or group of species) is determined by counting of the number of number of levels of energy transfer.


The concept that under habitat selection, a variety of factors can cause the population to be distributed such that fewer individuals occupy the "better" habitat than one would expect based on its resource renewal. - see "habitat matching"

Unified Neutral Theory of Biodiversity and Biogeography (UNTBB): (S. P. Hubbell)

A theory of biodiversity whereby the loss of an individual in a community is replaced by a new randomly-selected individual belonging either to an existing species (with a defined probability), or to a new one. The distribution of species abundances will thus fit a "zero-sum multinomial distribution".


The creation of barriers to gene flow associated with geological processes.

World-is-green Hypothesis: (Hairston, Smith and Slobodkin)

The argument that, in terrestrial ecosystems, predator limitation of herbivores allows plants to flourish and compete for nutrients and light. - see "world is green-yellow-and-white and world-is-prickly-and-tastes-bad hypotheses"

World-is-green-yellow-and-white Hypothesis: (Laurie Oksanen - named by Stuart Pimm)

An "extension" of the world-is-green hypothesis arguing that predator limitation of herbivores can occur only in very productive (green) environments. In less productive areas (yellow), predators are absent and herbivores limit plants. In very unproductive (white) environments, herbivores are excluded by low productivity. - see "world-is-green and world-is-prickly-and-tastes-bad hypotheses"

World-is-prickly-and-tastes-bad Hypothesis: (William Murdoch - named by Stuart Pimm)

The argument that, in terrestrial ecosystems, physical and chemical defences of plants limit the biomass of herbivores. Sometimes called the "green-desert" hypothesis. - see "world-is-green and world-is-green-yellow-and-white hypotheses"

Zero-growth Isocline (ZNGI):

An isocline along which the population growth rate of a species is equal to zero. Used most often to evaluate when a pair of populations can exist in stable equilibrium (the point where the two isoclines cross).

Some Notable Quotes

"Clear writing brings a grave danger: People may begin to understand you!" (Michael L. Rosenzweig 1995, p. xvi)

"For the green prehuman earth is the mystery we were chosen to solve, a guide to the birthplace of our spirit, but it is slipping away." (Edward O. Wilson, 1992, p. 344)

"More effective conservation is impossible in the face of grinding human poverty on the one hand, and blinding human greed on the other." (John H. Lawton, 2002, p. 2)

"A sad commentary is that our ability to respond and to defend natural systems has been eroded within academe by scientific elitism against natural history and systematics." (Paul K. Dayton, 2003, p. 10)

"Science progresses funeral by funeral." (Anonymous)

"A wise man knows what to ignore." (Ancient Chinese Proverb; cited by D. S. Wilson, 2002, p. 29)

"The careful foot can walk anywhere." (Chinese Proverb; cited by M. L. Rosenzweig, 2003, p. 1)

"A theory is required to see the things that are in front of our faces." (David Sloan Wilson, 2002, p. 125)


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