VARIATIONS OF COMMUNITY DYNAMICS BETWEEN SIZE CATEGORIES IN THE TAMIAMI FORMATION, FLORIDA

Marine paleoecological studies explore the ecosystems of Earth’s past, but they are often limited to the larger specimens that are easiest to collect and identify. Analysis of taxonomic variation between different size categories of organisms can reveal distinctive characteristics in community structure that may be missed when examining only macro taxa. Here, we compare taxonomic composition and abundance of two different size categories within a single sample.

One 4 L bulk sample was analyzed from the Plio-Pleistocene Tamiami Formation at the SMR Aggregates Quarry in Florida. The samples were cleaned and divided into two size categories, > 3 mm (large) and 1 - 3 mm (small). One random subsample of the small category was taken to match the sample size of the large category. All specimens were identified to genus and counted. Richness and evenness were calculated, as well as relative abundance of select taxa.

Bivalves and gastropods dominated the sample, with some barnacles, corals, crabs, and urchins. The small category had a total of 409 bivalves and gastropods; the large sample had 404 bivalves and gastropods. The small category had a richness of 48 and the large category had a richness of 65. Both categories had a similar evenness, the small being 0.88 and the large being 0.83. The relative abundances of gastropods and bivalves in the small category were 50% each. For the large category, the relative abundance of gastropods was 25% and 75% bivalves. Perhaps the gastropods did not grow large enough to occupy the large category, or bivalves were less successful than the gastropods in the small category. Numerous taxa existed in moderate to high abundance in one size category, but are absent in the other. Examples include the bivalve Astarte, with 16% relative abundance in the small category and 0% in the large. The bivalve Lucinisca, absent in the small category, was 13% in the large category. The bivalve Plicatula was also absent in the small category but had 4.5% in the large category. These distinctive values of richness, evenness, and abundance indicate ecological differences between the small community and the large community. Future efforts will focus on gathering predation data from both size categories as well as adding to the sample size of the small category.