SEARCHING FOR AN EQUILIBRIUM POINT IN GLOBAL BIODIVERSITY: AN EXAMPLE USING A STROPHOMENATE BRACHIOPOD GUILD THROUGH THE PALEOZOIC

The question of whether interspecific competition influences diversity at broad geographic scales has been tied to whether a taxic equilibrium point exists, above which no additional taxa could coexist in the given space. Some studies reject the idea of an equilibrium point on the grounds that global diversity appears to have increased primarily through expansion into new niches (marine infauna, invasion of land, flight etc.). However, (1) equilibrium points may fluctuate with the intensity of competition relative to non-equilibrium processes (predation, disturbance, spatial complexity); and (2) expansion into new niches does not preclude the possibility of equilibrium points. Equilibrium, if it exists, occurs within guilds; taxa expanding into new niches no longer compete with the original guild. Thus, equilibrium should be sought within guilds through time. As an illustrative example, I examined diversity of the guild comprising free-lying (non-cemented), small (< 6 cm, max. dim.) strophomenate brachiopods living in subtidal level-bottom habitats from the Ordovician-Permian. Diversity at the stage level was summed for guild members at three different geographic scales: global; continental (North America); and biogeographic realm (Southern Laurentia). Raw diversity was then normalized for stage duration and for sea-level. The free-lying strophomenate guild experienced a minor peak in the Early Devonian, a major peak during the Lower Permian, and extinctions during the Upper Ordovician and Devonian. Excepting the Permian peak, diversity remained relatively constant (t-test, correlations), despite major turnovers in taxic composition. These patterns occur at all three geographic scales. In addition, the diversity pattern indicates that clade replacement occurred several times. In each case of replacement, final extinction of the incumbent occurred after a brief recovery from the prior mass extinction; a pattern predicted for cases of true clade replacement (Sepkoski, 1996). These results suggest that an equilibrium point may have existed for this guild through most of the Paleozoic. Peaks above equilibrium may be a response to conditions dominated by non-equilibrium processes; e.g. increased predation and spatial complexity in the Devonian.