Species Coexistence

Ecologists have long sought to determine the factors that allow competing species to coexist in nature. Competitive exclusion (extinction) is one possible outcome of competitive interactions, and ecologists have determined several potential outcomes and mechanisms for species coexistence, typically associated with differences among species as implied by CEP. For the past several decades, Lotka-Volterra equations have been further developed and elaborated to identify conditions in which coexistence is possible. One conclusion from several theoretical studies in the 1960s pointed out that n species can coexist only when the number of resources or limiting factors is greater than the number of species, k > n. Other work has pointed to nonlinear functional response relationships with resources, varying densities, and temporal variation in resources as other conditions facilitating coexistence.

Early studies by Tansley and later by Park showed that the nature of competitive interactions may differ, in fact may be reversed, under different environmental conditions. In Tansley, conspecifics of the plant genus Galium differentially dominated in different soil types, and Park found Tribolium beetle species to differentially dominate under different temperature and humidity conditions. Recent examples of this common pattern highlight that the strength of competitive interactions can vary with habitat conditions, and coexistence may be occurring at larger spatial scales by habitat heterogeneity. Therefore, species coexistence and exclusion are contingent on habitat conditions and spatial scale.

Resource competition can also lead to character displacement, whereby competitors evolutionarily diverge in their resource use, reduce niche overlap, and thereby coexist. This competition-driven process assumes that individuals of species who overlap in their niche (and this being directly related to the degree of competition)

will have lowered fitness, causing directional selection. Species will then evolve decreased niche overlap and competition, which may also be reflected in morphological differences. The evidence required to support the presence of character displacement should include (1) nonrandom size differences, (2) genetic basis for trait differences, (3) differences resulting from evolutionary shifts, (4) morphological differences resulting in differential resource use, and (5) competitive interactions must be established.

The ecologist G. E. Hutchinson contributed immeasurably to the development of the niche concept. Hutchinson's 'Homage to Santa Rosalia' article addressed why so many species were able to coexist and was a pivotal landmark in bridging the relationship between the niche and competition. He pointed out that species that were too similar morphologically (e.g., body size) and ecologically, will result in competitive exclusion of one species. However, coexistence between closely related species will occur when the species are sufficiently different in body size and therefore their resource use. This minimum difference in body size was estimated to be 130% or a factor of 1.3, which came to be known as the Hutchinsonian ratio; the measuring of size ratios as an estimate of prior character displacement became the focus of much research for a generation of ecologists trying to understand community structure and species coexistence. Ultimately, this approach faced an array of criticisms based on statistical and inferential grounds. For example, in many communities, the size differences were not different than randomly selected sets of species, the patterns were not different than random distributions of size differences. Further, species that differ in body size may not always compete less than similarly sized species.

For several decades, there has been a decline in the use of the term 'niche', but the tide has turned. In recent years, there has been a resurgence and renewed interest in the niche as a central concept and explanation for species coexistence, which has resulted because of recent reformulations and definitions of the term. Further, the 'niche' has been repackaged (e.g., species traits) and promoted as a central theme for the future of ecology, reflecting previous work on fundamental and realized niche. Another update on the view of the niche has been the concept of niche tradeoffs as a mechanism for species coexistence (Figure 2).

One criticism of the niche explanations for coexistence is that they have difficulty explaining high levels of species diversity. In recent years, the metacommunity concept has emerged to address community dynamics at different spatial scales, local and regional. Processes at the local community scale are the traditional, dominant factors affecting communities, including species interactions with each other (e.g., competition, predation) and their environment. At the regional community scale, o c o o ra ~o

Figure 2 An example of tradeoffs among species A-D along two niche axes, competitive ability and predator tolerance, that will hypothetically facilitate coexistence.

processes among local communities dominate, including dispersal and habitat heterogeneity. The processes at local and regional scales contribute to community dynamics and patterns of abundance and diversity. A number of perspectives have emerged within this framework for explaining diversity and abundance patterns. One explanation for diversity patterns that has emerged in recent years has been the neutral theory. In these models, species are assumed to be identical ecologically and demographically, and diversity and relative abundance is driven by metacommunity size, and random variation in demographic (births and deaths), dispersal, and speciation rates. Empirical support has been variable for neutral theory, but it has proven to be an influential alternative to the niche perspective. It appears that both niche and neutral processes act in community dynamics. While most approaches to the niche viewed differentiation on a single dimension (except for Hutchinson's n-dimensional hypervolume), niche tradeoffs view two or more niche axes where species traits have a negative functional relationship (Figure 2). For example, a species' ability to compete may come at the cost of defending against predators (Figure 2), which has been documented in numerous aquatic and terrestrial communities. Further, ecologists are increasingly becoming aware of the importance of spatial dynamics that can contribute to species coexistence. The competitive-colonization ability tradeoff has been hypothesized as a mechanism for species coexistence in space with some empirical support. Consequently, species niche differences can be exhibited along numerous niche axes along on spatial and temporal planes, potentially explaining high levels of diversity.

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