A Reappraisal of the Relationship between Sea Level and Species Richness

The relationship between area and species richness was documented as early as the mid-17^th century (see discussion in Rosenzweig, 1995), but it was not until the publication of MacArthur and Wilson’s (1967) The Theory of Island Biogeography that the hypothesis became ingrained in ecological theory. Their work forcefully presented substantial empirical evidence that explained the nature of, and possibly the controls of, diversity, at least on oceanic islands. Their hypothesis that species-level diversity is dependent upon area raised the hopes of paleontologists that this relationship could readily be applied to the fossil record of marine organisms and hence to the history of life. The paleontologic application of this concept was founded on the belief that the species-area relationship should hold for benthic marine organisms responding to changes in shelf areas primarily affected by sealevel fluctuations. Therefore, diversity increases and declines chronicled in the fossil record would largely represent transgressions and regressions, respectively, as far as benthic organisms are concerned. These patterns are overprinted by plate tectonic, evolutionary, and mass-extinction events, but nevertheless sea-level changes should be a dominant control.

A number of early studies pointed to the potential applicability of the species-area effect for various intervals of geologic time (e.g., Johnson, 1974; Schopf, 1974; Simberloff, 1974). In addition, building on earlier work by Newell (1967), there were attempts to relate Phanerozoic compilations of species-level diversity, such as that by Raup (1976a), to sea-level fluctuations (e.g., (Sepkoski, 1976); but see reinterpretation by (Flessa and Sepkoski, 1978). The species diversity reflected in these compilations were largely controlled by sampling vagaries, especially controlled by outcrop area and rock volume available for study (Raup, 1976b), and certain groups, intervals, and regions were and continue to be better studied than others. Furthermore, the fauna was treated in toto, rather than focusing on individual groups has been the case in neontologic work. More recent work focused on specific taxonomic groups and geologic intervals, however, has suggested otherwise. Valentine and Jablonski’s (1991) study of Pleistocene and Holocene sea-level fluctuations suggests that the rapid and substantial sealevel changes over the past 1 Myr had no effect upon diversity – the existing data show virtually no faunal differences between these sea-level highstands. McGhee (1991, 1992), based on species richness as well as evolutionary rates in Devonian brachiopod species as a response to sea-level change, concluded that sea level, as well as the rate of sea-level change, showed virtually no correlation with either variable. This pointed to a minimal control by sea level, hence changes in shelf area, in regulating benthic organisms and suggested that patterns documented in modern oceans may be a very recent phenomenon or simply fortuitous.