Evolution of Species Assemblages: Effects of Energetic Constraints and Species Dynamics on the Diversification of the North American Avifauna

Characteristics of the terrestrial avifauna of North America can be viewed as adaptations by a taxonomically, geographically, and ecologically defined assemblage of many species to the constraints imposed by its own biology and by the environment. We have identified distinctive patterns in the variation among species in population density, body size, area of geographical range, and trophic status. The patterns observed in bivariate plots of log-transformed variables can be characterized provisionally in terms of polygons that enclose combinations of the variables exhibited by species. The sides of these polygons may be either abrupt or indistinct. We suggest that sharp, clear-cut boundaries separating combinations of characteristics that species possess from those combinations that are not observed in any species are the result of absolute constraints. As a trivial example, the maximum size of the geographical range is determined by the size of the continent. A more interesting example of an apparently absolute constraint is an energetic trade-off between maximum population density and body size. Boundaries separating combinations of characteristics that species possess from those not possessed can also be diffuse and relatively poorly defined. We suggest that such boundaries result from the probabilistic processes of origination and extinction, such that the number of species declines gradually across the boundary. An example is the increase in minimum area of geographical range with increasing body size, which is hypothesized to reflect the probability of extinction. We summarize our hypotheses to account for the observed patterns in a model for the adaptive evolution of the North American terrestrial avifauna. With appropriate modifications, a similar model could be developed for any biota. Our analyses provide neontological evidence for the kinds of patterns observed in the fossil record and used by paleontologists to argue that a process analogous to natural selection at the level of individuals within populations operates at the level of species within biotas. It is useful to view certain attributes of contemporary species as the result of some combination of absolute constraints and the dynamics of speciation, colonization, and extinction processes.