EXTINCTION PATTERNS IN MOLLUSCAN COMMUNITIES OF THE FLORIDA NEOGENE: EFFECTS OF SAMPLING INTENSITY AND EVENNESS

When considering the observed taxonomic richness of a community, the effects of sampling intensity and evenness on diversity patterns must be considered. Evenness is a measure of the relative abundance of a species in a collection, where the probability of sampling all species decreases as abundance distributions become more uneven. Sampling intensity also affects richness estimates because many species are only rarely encountered. A low sampling intensity and an uneven community will produce a richness level that is artificially low. The goal of this study is to examine the evenness and richness of molluscan communities in Florida, spanning the Miocene to the Pleistocene, to determine if a late Pliocene extinction event took place or if a drop in diversity is due to low sampling intensity or low evenness. To do this we constructed the first molluscan database of the Early Miocene Chipola Formation using standardized samples and then compared these samples to unpublished data of late Neogene molluscan collections.

Sample collection in the Chipola Formation occurred along a horizontal transect and at different bedding planes in the section along the Apalachicola River. The samples were collected in 1-gallon bags as bulk samples and sieved with a 5 mm and 3 mm stacked mesh. Only gastropods with a complete apex and bivalves with a complete umbo were used for analysis. Each specimen was identified to species level when possible using literature and museum collections. The PIE, E(1/D), E(1-D), and E –ln(D) evenness indices were applied along with the Simpson Dominance Index and the Shannon Diversity Index.

Comparisons of rarefaction curves for the Early Miocene Chipola Fm., the Pliocene Pinecrest Beds and Caloosahatchee Fm., and the Pleistocene Bermont and Fort Thompson Fms. at a standardized cutoff of 95 gastropod specimens recovered richness levels of 35, 30, 37, 21 and 25 species, respectively. Changes in gastropod richness occur independently of evenness until the Bermont, where a slight decrease in evenness appears to exaggerate diversity loss across the Plio-Pleistocene boundary. These results point to a major shift in community diversity and structure at the Plio-Pleistocene boundary, with no evidence for recovery through the Late Pleistocene. Analyses of bivalve diversity patterns will also be presented.