American Society of Naturalists

A membership society whose goal is to advance and to diffuse knowledge of organic evolution and other broad biological principles so as to enhance the conceptual unification of the biological sciences.

“On the consequences of the interdependence of stabilizing and equalizing mechanisms”

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Chuliang Song, György Barabás, and Serguei Saavedra (Nov 2019)

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The essence of the Darwinian view of life is that species better adapted to their environments outcompete those less well adapted. This, however, raises the question of why there are more than one species on Earth, instead of just a single “best” one. The basic answer is that it is impossible to be the best at everything simultaneously. Ground finches on the Galápagos Islands, from the genus Geospiza, all eat seeds, for instance, but each species has a different characteristic beak size. Large beaks are excellent for breaking and eating large seeds, but are useless in eating small ones. And conversely, small beaks are much better at capturing small seeds than large ones. Thus the finches avoid competition and are able to coexist, because they compete for different things. Ecologists have a way of classifying how such differences between otherwise similar species contribute to their coexistence. Differences are classified into two categories: “stabilizing” differences are those helping coexistence (such as eating different seed types), while “equalizing” differences are those that, all else equal, cause one species to be better adapted than the other (two finch species which eat the exact same seed type may differ in how well they are able to make use of those seeds – the difference in their abilities is the “fitness inequality” between them). While this classification scheme is intuitive and theoretically sound, the authors show that these two types of differences, widely considered independent and opposing forces, are actually interdependent, and are so in intricate ways. This means that one cannot use these concepts in an overly simplistic way to understand why species coexist in nature. However, by perceiving the stabilizing and equalizing contributions as net effects, rather than general ecological causes, we may get a better understanding of coexistence.


Abstract

We present an overlooked but important property of modern coexistence theory (MCT), along with two key new results and their consequences. The overlooked property is that stabilizing mechanisms (increasing species’ niche differences) and equalizing mechanisms (reducing species’ fitness differences) have two distinct sets of meanings within MCT: one in a 2-species, and another in a general multispecies context. We demonstrate that the 2-species framework is not a special case of the multispecies one, and therefore these two parallel frameworks must be studied independently. Our first result is that, using the 2-species framework and mechanistic consumer-resource models, stabilizing and equalizing mechanisms exhibit complex interdependence, such that changing one will simultaneously change the other. Furthermore, the nature and direction of this simultaneous change depend sensitively on model parameters. The second result states that while MCT is often seen as bridging niche and neutral modes of coexistence by building a niche-neutrality continuum, the interdependence between stabilizing and equalizing mechanisms acts to break this continuum under almost any biologically relevant circumstance. We conclude that the complex entanglement of stabilizing and equalizing terms makes their impact on coexistence difficult to understand, but by seeing them as aggregated effects (rather than underlying causes) of coexistence, we may increase our understanding of ecological dynamics.