Does ecosystem and evolutionary stability include rare species?

Abstract

Abstract 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 higher rates of local extinction and recolonization for rare species in the community. There is also evidence that many rare species, because they are trophically or otherwise specialized, are less tightly integrated into ecosystems than abundant species. Higher evolutionary turnover in rare species is seen in their higher rates of extinction and speciation compared to abundant species. A simulation with foraminifera communities shows that high numbers of individuals must be preserved and sampled to properly characterize original community species diversity. Despite the benefit of time-averaging which can often enhance fossil completeness, taphonomic studies indicate that a substantial fraction of the original species in a community are ultimately unsampled in subfossil and young fossils. This problem is probably worse in older (e.g., Paleozoic) fossils. We show how spatial and temporal patterns of abundance are mixed in a community. Because spatio-temporal abundance patterns of species may be linked biologically, with rare species having a patchier (less uniform) distribution in both time and space, the intermixing of spatial and temporal patterns in the record may still provide important data. Rare species thus seem to have greater temporal and spatial abundance variation on both ecological and geological time scales. The biased perception of community stasis may depend on the degree of resolution. Observation of Recent through young fossil communities provides higher resolution that permits perception of individualistic behavior of rare species via higher rates of extinction, speciation and migration. Coarse scales of observation, such as the biofacies, largely filter out the “noise” of rare species community flux and one sees suites of coexisting abundant species, with minor and covariant abundance changes. These co-occurring abundant species are extinguished only by large-scale disturbances. But the perceived coordinated stasis 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.