DOES ECOSYSTEM AND EVOLUTIONARY STABILITY INCLUDE RARE SPECIES

The appearance of faunal stability in the fossil record occurs because mainly abundant species are preserved and sampled. The paleontologist does not therefore observe many rare species with high ecological and evolutionary turnover rates. High ecological turnover refers to highe r rates of local extinction and recolonization for rare species in the community. There is also evidence that many rare species, because the y are trophically or otherwise specialized, are less lightly integrate d into ecosystems than abundant species. Higher evolutionary turnover in rare species is seen in their higher rates of extinction and specia tion compared to abundant species. A simulation with foraminifera comm unities shows that high numbers of individuals must be preserved and s ampled to properly characterize original community species diversity. Despite the benefit of time-averaging which can often enhance fossil c ompleteness, taphonomic studies indicate that a substantial fraction o f the original species in a community are ultimately unsampled in subf ossil and young fossils. This problem is probably worse in older (e.g. , Paleozoic) fossils. We show how spatial and temporal patterns of abu ndance are mixed in a community. Because spatio-temporal abundance pat terns of species may be linked biologically, with rare species having a patchier (less uniform) distribution in both time and space, the int ermixing of spatial and temporal patterns in the record may still prov ide important data. Rare species thus seem to have greater temporal an d spatial abundance variation on both ecological and geological time s cales. The biased perception of community stasis may depend on the deg ree of resolution. Observation of Recent through young fossil communit ies provides higher resolution that permits perception of individualis tic behavior of rare species via higher rates of extinction, speciatio n and migration. Coarse scales of observation, such as the biofacies, largely filter out the ''noise'' of rare species community flux and on e sees suites of coexisting abundant species, with minor and covariant abundance changes. These co-occurring abundant species are extinguish ed only by large-scale disturbances. But the perceived coordinated sta sis of abundant species may be real. Abundant species tend to be more eurytopic and biotically interconnected, to form the ''framework'' of the community, with rare species in spatial and temporal flux through this framework.