Tom Olszewski (Department of Geology and Geophysics, Texas A&M University)
The composition and diversity of ecological communities reflects a dynamic balance between processes that add species (e.g., immigration and speciation) and processes that exclude species. Competition is generally regarded as one of the processes thought to reduce diversity because species that are too similar in their resource requirements and modes of life cannot coexist indefinitely. However, model results presented here suggest that ecological communities with a high diversity of ecologically similar species can develop a dynamic of competitive accommodation rather than exclusion. Although such states are transient, they can allow a large number of species to coexist for millions of generations, a duration comparable to the geologic lifespan of fossil species. Although communities showing accommodation dynamics display quite stable population sizes and are resistant to invasion, they are very sensitive to externally imposed perturbations. When perturbed, they become open to invasion by new species and reordering of common and rare species before rapidly returning to a pseudo-steady accommodated state. Disruption of accommodated communities may provide an explanation for patterns of long-term ecological stability punctuated by sudden reorganization observed in the fossil record, such as that seen in high-diversity Middle Permian brachiopod communities from west Texas (270-260 million years ago). New collections from the Bell Canyon Formation provide an opportunity to observe fossilized communities through a series of geologic intervals before, during, and after a major environmental disruption associated with a drop in sea level. Analysis of abundance distributions show statistically significant differences among all of the sampled intervals, suggesting that communities were always in flux and never reached a steady state. The environmental perturbation did, however, result in substantial changes in the identity of dominant taxa, including the appearance of invaders from outside of North America. It also changed the gross shape of the pre-perturbation abundance distribution from equitable to very uneven among dominant taxa. The time interval after the event shows a gradual return to an abundance distribution like that prior to the perturbation, but with a reordering of rare and common taxa relative to the pre-event communities. The observed pattern is consistent with the predicted effects of a disruption to ecological communities experiencing accommodation dynamics. The implication is that species in such ecological communities may spend most of their evolutionary history in a state where competition acts to maintain diversity rather than reduce it. The selective regime in such circumstances could be quite different than that normally expected in resource-limited systems.
SFI Host: Doug Erwin