AN EVENNESS STUDY OF THE PLEISTOCENE BERMONT FAUNA OF FLORIDA

Previous studies of tropical western Atlantic molluscan species richness conflict regarding the number and severity of possible extinction events from the Plio-Pleistocene to the Recent. One issue that has been consistently overlooked in past studies, however, is whether evenness, a diversity metric of the relative abundance of species within a fauna, has changed. A less even, though equally diverse fauna with few abundant species and many rare species, for example, could appear to have low diversity at low sampling intensity. Evenness is also an important descriptor of community structure. In the present study, we apply evenness indices for the first time to the paleoecological analysis of the lower Pleistocene Bermont “Formation” of southern Florida.

The evenness indices EHurlbert, E(1/D), E(1-D) and E-ln(D), the Shannon-Weaver diversity index, and the Simpson dominance index were calculated for a standardized number of individual gastropods from two Bermont beds: upper 3C and lower 6B. The evenness indices chosen vary on a scale of 0 to 1, with 1 being maximum evenness (where each species constitutes an equal percentage of the total individuals). In addition, diversity and evenness were compared along a transect of bed 3C to determine the extent of lateral variation.

Random samples of 300 individuals (>0.35 cm) from the upper Bermont unit 3C, taken every 50 m along a 200 m transect, varied in species-level evenness from 0.42 to 0.49 (EHurlbert) or 0.62 to 0.70 (E(1-D)). Macrogastropods (>1.5 cm), 0.42 (EHurlbert), 0.63 (E(1-D)), displayed similar evenness to microgastropods (0.35-1.5 cm), 0.44 (EHurlbert), 0.71 (E(1-D)), for a sample of 750 random individuals. Differences reflect the abundance of the genus Olivella in the microgastropod samples. The upper and lower Bermont beds sampled showed much more pronounced differences in evenness. For a random sample of ~3100 (>0.35 cm) individuals from the upper bed 3C evenness was 0.42 (EHurlbert), or 0.69 (E(1-D)) versus 0.64 (EHurlbert) or 0.89 (E(1-D)) from the lower bed 6B. The gastropods Cerithium muscarum (53% of total individuals in bed 3C vs. 4% in bed 6B) and Prunum apicinum (4% of bed 3C vs. 22% of bed 6B), are the major determinants of this evenness disparity. These data suggest a previously unreported ecological variability during deposition of the Bermont “Formation”.