IS NICHE CONSERVATISM OR NICHE EVOLUTION MORE COMMON IN GEOLOGIC TIME?

Most neontological analyses of native species’ response to exotic species introductions or climate change assume that species conserve the parameters of their ecological niche when faced with changing environmental conditions. This assumption, however, is rarely tested. The lack of testing is due partly to the limited temporal duration available when studying exclusively extant taxa. Because the fossil record preserves members of the same species at multiple temporal horizons, it is possible to assess how local populations or entire species responded to shifting environmental conditions (biotic or abiotic) using GIS-based tools such as ecological niche modeling (ENM) to reconstruct the parameters of a species’ ecological niche before, during, and after the environmental shift.

Theoretically, populations may exhibit two end-member responses: niche conservatism or niche evolution. Under niche conservatism, species would maintain the parameters of their ecological niche in the face of environmental change, and maintain population stability by tracking their preferred set of environmental conditions laterally as conditions change. Alternately, populations may exhibit niche evolution in which the parameters of their original ecological niche are altered through natural selection and the species becomes adapted to the new set of environmental conditions.

Since most species are morphologically stable for up to one million years or longer, niche conservatism at the species level is arguably to be expected. Niche evolution, however, is likely to be common at the population level. Congruent niche evolution across all populations in a species could result in gradual morphological change, while individualistic niche evolution by one (or several) population(s) may result in cladogenesis.

New analyses on Late Ordovician brachiopods from the Cincinnati, Ohio region specifically examine the question of niche stability through geologic time. Initial results from these analyses suggest that species niche parameters are more stable during intervals of biotic overturn than intervals of abiotic change. Results from these analyses are compared with published results from Neogene continental and Devonian marine taxa to examine the generality of the Late Ordovician results for taxa in general.