A REAPPRAISAL OF THE RELATIONSHIP BETWEEN SEA LEVEL AND SPECIES RICHNESS

MacArthur and Wilson’s (1963) equilibrium theory of island biogeography gave both neontologists and paleontologists substantial empirical evidence to explain the nature and controls on diversity. Their hypothesis that species-level diversity was dependent upon area (at least based on empirical data for oceanic islands) raised the hope of paleontologists that this relationship could be readily applied to the fossil record of marine organisms and hence to the history of life. The paleontologic application was founded on the assumption that the species-area relationship should hold for benthic marine organisms responding to changes in shelf area driven by sea-level fluctuations. Therefore, diversity declines and increases chronicled in the fossil record, at least as far as benthic organisms are concerned, would largely reflect regression and transgressions, respectively. Although a range of papers touting the efficacy of this mechanism were published during the 1960’s and 70’s, more recent work has suggested that this linkage is quite weak in the paleontologic record. However, data from Late Cretaceous inoceramids from the Western Interior Seaway as well as a re-evaluation of the Devonian brachiopod data compiled by McGhee suggest that the correlation is much better than has previously been believed. In this study the species richness data is compared to individual third-order sea-level sequences rather than combining the species richness data over longer intervals of geologic time. In the latter case, there is an extremely weak correlation in both cases, but when the former approach is applied, the correlation increases significantly. Interestingly, in both data sets when a mass extinction occurs – the Cenomanian-Turonian and the Frasnian-Fammenian for the inoceramids and brachiopods, respectively – the correlation decreases significantly. These results suggest that there is an "evolutionary memory" within ecosystems, and, therefore, the level of species richness will fluctuate through time even if sea level values are equivalent. Furthermore, given the rate of speciation in most invertebrate groups in comparison to the rate and magnitude of sea-level changes during the Quaternary, it is difficult to evaluate whether the current level of biodiversity reflects high or low sea-level conditions.