EFFECTS OF SAMPLING ON THE OBSERVED RICHNESS AND EVENNESS OF MOLLUSCAN ASSEMBLAGES IN THE PLIOCENE PINECREST BEDS OF SOUTHWEST FLORIDA

Do Florida's Plio-Pleistocene shell beds record a late Pliocene extinction event? No studies have quantied the effects of sampling intensity and evenness on observed richness patterns. Evenness is a metric of diversity that measures the relative abundance of species within a given community. In a less even community, lower sampling intensity will produce an artificially low richness level. Our goal here is a preliminary study of diversity, including richness and evenness, of a pre-extinction phase of Florida molluscan communities, within a sample-standardized framework.

We collected 14 one-gallon bulk samples from six beds within the lower Pinecrest Beds (middle Pliocene) at SMR Aggregates Quarry in Sarasota, Florida. We analyzed each bed separately to examine differences in diversity within the lower Pinecrest that might reflect ecological or taphonomic processes. The samples were sieved at a mesh size of 5mm. All specimens were sorted, and gastropods as well as bivalves were identified to species level whenever possible, using literature sources. Abundance data were collected by counting gastropod apices and bivalve umbos. The total number of species identified was 160 gastropods (>750 individuals), and 84 bivalves (>1800 individuals). We used rarefaction curves to ensure sample-standardization of the richness data obtained and to compare richness among samples. We then applied the following evenness indices: E(Hurlbert), E(1/D), E(1-D), and E-ln(D), in addition to the Shannon-Weiner and the Simpson dominance indices.

Preliminary bivalve richness per layer shows no strong trend within the lower Pinecrest Beds, although it is punctuated by a non-significant decrease in richness in the middle of the section. Evenness (calculated using E(1/D)) ranges from 0.33 – 0.67 across the entire section, with no overall trend. In the middle of the section, however, evenness does fluctuate rapidly from sample to sample within and between beds. Further data analyses will help us to better understand these differences. Our preliminary data suggest that richness changes independently of evenness in these beds, and is thus not merely a product of changes in sampling or evenness.