Ecospace Utilization in High-Diversity Shallow Shelf Marine Communities of the U.S. Gulf Coastal Plain

Diversity may change in two ways - changing the number of species within an ecological group, or by changing the number of ecological groups. This second method has been invoked to explain changes in global diversity, but is it responsible for changes in diversity at smaller spatio-temporal scales? The middle to late Eocene has previously been identified as a time of molluscan diversity decline in the U.S. Gulf Coastal Plain. Here we assess the magnitude of this decline and examine associated changes in ecospace utilization.

We compiled a database of bivalve, gastropod and coral abundance data derived from published datasets, unpublished theses and our own collections. Data come from the upper Lisbon, Gosport, Moodys Branch, and Yazoo formations. The resulting dataset contains over 70,000 individuals of 546 different species from 188 collections. To assess changes in diversity we calculate alpha and gamma diversity. We rarefy diversity to 300 individuals to account for differences in sampling intensity.

To assess changes in ecospace utilization, we assign each species to a mode of life based on a modified version of the three-dimensional ecospace framework proposed by Bush et al. (2007). The three axes that make up the framework are tiering (e.g. pelagic), motility (e.g. creeping), and feeding (e.g. predator). We base our assignments on extant genera or families.

Results indicate that gamma diversity declined by 25% from the upper Lisbon formation to the Moodys Branch formation. However, the Moodys Branch formation encompasses more modes of life. Diversity loss coincides with a shift in dominance from the highly diverse “creeping predator” mode of life in the upper Lisbon formation to the less diverse but highly abundant “infaunal suspension feeder” mode of life in the Gosport Sand and Moodys Branch formations. Changes in diversity are primarily accommodated by large shifts within the existing ecological structure.