Community Assembly

Graphical models of small numbers of interacting species at steady state might seem to offer limited insights for ecological communities composed of many more species. However, the contributions of the theory presented above become clearer in the contexts of community assembly, succession, and metacommunities. These related concepts recognize that many communities begin their history in sparsely populated habitats created by a major disturbance, and are colonized over time by species arriving from established communities in similar habitats. As this succession proceeds, species are sorted into those that persist locally, and those that do not.

Ecologists have long recognized that inhibitory and facilitative interactions such as competition, commensal-ism, and mutualism play a part in this sorting. Stoichiometric theory and related approaches can identify properties of the species that ultimately persist. Basically, species that coexist stably must have tradeoffs in their abilities to use different resources, between resources that they require and the substances they produce as resources or toxins for other species, or between abilities to use resources and withstand predation or allelopathy. Stable coexistence based on the right tradeoffs and the right stoichiometry imparts a measure of determinism to local community assembly and species composition. As a result, nonrandom patterns are predicted in the species properties summarized by their ZNGI graphs, for example, and in their distributions in relation to environmental gradients of resources.

Stable coexistence is not the only possible consequence of the interactions presented above. Especially for predation and allelopathy, scenarios consistent with priority effects are plausible. The stoichiometric relationships that impart stability are not biologically necessary, permitting destabilization leading to priority effects. When priority effects characterize interspecific interactions, community assembly becomes stochastic. There are multiple endpoints, that is, alternative stable states, and long-term species composition depends on the order in which colonists of different species arrive. Even so, on the larger scale of the metacommunity - the collection of similar communities linked by dispersal and colonization - it is possible that patterns with stoichiometric signatures emerge. When examining a productivity gradient with increasing supplies of nutrient resources, priority effects in the interactions discussed above would make species composition in more productive local communities more stochastic, and reduce compositional similarity among localities. Higher species diversity would also be associated with higher productivity at the metacommunity scale, while local communities would display a peak in diversity at intermediate productivity.

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