Coexistence theory
Coexistence theory is a framework to understand how competitor traits can maintain species diversity and stave-off competitive exclusion even among similar species living in ecologically similar environments.
In such communities, any species that becomes rare will experience positive growth, pushing its population to recover and making local extinction unlikely.
As its name implies, these processes act in a way that push the competitive abilities of multiple species closer together.
Any factor that reduces R*s between species (like increased harvest of the dominant competitor) is classified as an equalizing mechanism.
Environmental variation (which is the focus of the Intermediate Disturbance Hypothesis) can be considered an equalizing mechanism.
some species may lose components of their competitive advantage which were useful in the previous version of the environment.
Similarly, if species are differently affected by environmental variation (e.g., soil type, rainfall timing, etc.
Stabilizing mechanisms increase the low-density growth rate of all species.
[citation needed] In 1994, Chesson proposed that all stabilizing mechanisms could be categorized into four categories.
A general way of measuring the effect of stabilizing mechanisms is by calculating the growth rate of species i in a community as[7]
where: In 2008 Chesson and Kuang showed how to calculate fitness differences and stabilizing mechanisms when species compete for shared resources and competitors.
Each species j captures resource type l at a species-specific rate, cjl.
Each unit of resource captured contributes to species growth by value vl.
Each consumer requires resources for the metabolic maintenance at rate μi.
Given predation and resource capture, the density of species i, Ni, grows at rate
where the summation terms is contributions to growth from consumption over all j focal species.
The average fitness of a species takes into account growth based on resource capture and predation as well as how much resource and predator densities change from interactions with the focal species.
which represents the amount to which resource consumption and predator attack are linearly related between two competing species, i and j.
The direct expressions for intraspecific and interspecific competition coefficients from the interaction between shared predators and resources are
A 2012 study[9] reviewed different approaches which tested coexistence theory, and identified three main ways to separate the contributions of stabilizing and equalizing mechanisms within a community.
In an invasion analysis, one species (termed the "invader") is removed from the community, and then reintroduced at a very low density.
An invasion analysis could be performed using experimental manipulation, or by parameterizing a mathematical model.
The authors reviewed 323 papers (from 1972 to May 2009), and claimed that only 10 of them met the above criteria (7 performing an invasion analysis, and 3 showing some negative-density dependence).
However, an important caveat is that invasion analysis may not always be sufficient for identifying stable coexistence.
Conversely, high order interactions in communities with many species can lead to complex dynamics following an initially successful invasion, potentially preventing the invader from persisting stably in the long term.
[12] For example, an invader that can only persist when a particular resident species is present at high density could alter community structure following invasion such that that resident species' density declines or that it goes locally extinct, thereby preventing the invader from successfully establishing in the long term.
The 2001 Neutral theory by Stephen P. Hubbell[13] attempts to model biodiversity through a migration-speciation-extinction balance, rather through selection.
[14] It assumes that all members within a guild are inherently the same, and that changes in population density are a result of random births and deaths.
Few studies have attempted to measure fitness differences and stabilizing mechanisms in plant communities, for example in 2009[16] or in 2015 [17] These communities appear to be far from neutral, and in some cases, stabilizing effects greatly outweigh fitness differences.
In addition to the core ecological concepts described above, which CCT summarizes as limited similarity, limited competition, and resilience, CCT argues the following features are essential for cultural coexistence: Cultural Coexistence Theory fits in under the broader area of sustainability science, common pool resources theory, and conflict theory.
