Tradeoffs and Species Coexistence

A 'super-species' is the best at colonizing new patches of habitat, at outcompeting all neighbors and at avoiding predators. Such a species does not exist. The benefits obtained from performing one ecological function well come at the cost of performing a different function. Such differences among species are referred to as tradeoffs. The population dynamics of different species are influenced by interspecific tradeoffs, which subsequently influence the possibilities for coexistence among species. It is important to note that in this context, tradeoffs represent niche differentiation among species (see above).

One way to consider a tradeoff is as a negative functional interaction between traits. In plants, for example, shrubs and trees must invest much of their resources into support structures to obtain height. This comes at the expense of investment in photosynthetic tissue. In contrast, vines must use host plants for structural support in order to reach any considerable height. The use of host plants for support results in a greater allocation of energy (which would otherwise be invested in stem tissue) toward photosynthetic tissue.

In spatially structured landscapes, the competition-colonization tradeoff is an important mechanism that can generate coexistence. Simply, while some species are good competitors, others are better colonizers. No one species can be good at both: there is a tradeoff, such that species with high fecundity and dispersal are poorer competitors, while species that are good competitors necessarily have lower fecundity and poorer dispersal. Under a fairly strict set of conditions, relating to the dynamics of competing populations, the competition-colonization tradeoff predicts the coexistence of a potentially infinite number ofspecies. These conditions include the idea that there is a strict competitive hierarchy among species (i.e., competitive asymmetry), such that the superior competitor will always win competitive interactions with competitively inferior species. Being a good competitor comes at the cost of being a poorer colonizer, such that inferior competitors that are better colonizers can occupy sites that have not been colonized by the superior competitor.

For plants, the interspecific competition-colonization tra-deoffhas been linked to the life-history tradeoffrelating seed size to seed number. From a finite pool of resources, a plant can make either many small seeds or few large seeds. There is fairly clear empirical evidence that seedlings from large seeds out-compete seedlings from small seeds during early seedling growth. The larger production of (smaller) seeds in other species leads to superior colonization. These differences in seed size and seed output neatly match the requirements of the competition-colonization tradeoffmodel for coexistence. Theoretical work, matched with neat empirical research, by Mark Rees and his research group reveals the complex nature of the competition-colonization tradeoff in annual plant communities. Importantly, the tradeoff appears to work as the sole mechanism of coexistence in the real world only when competitive asymmetries are extreme.

One form of coexistence (called density-dependent coexistence) requires that life-history strategies differ such that an advantage at one stage of the life cycle implies a disadvantage at another stage in the life cycle. With two species, each gains a relative advantage at some level of total density, and where densities fluctuate, both species may persist indefinitely. Such life-history tradeoffs appear in small-bodied fish assemblages in freshwater lakes, for instance, although density-dependent coexistence has not been demonstrated unequivocally in nature as yet.

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