PATCHINESS AND SPATIAL RESOLUTION IN MOLLUSK DEATH ASSEMBLAGES IN SAN SALVADOR, BAHAMAS

Mollusk death assemblages are widely used in modern and paleontological studies as markers of environmental settings, ecosystem health, and biodiversity. Many studies use comparative analyses among fossil and/or modern sites, and ascribe differences in faunal compositions to changes in overall ecosystem diversity or to factors influencing this diversity such as climate change or environmental shifts. In order to fully utilize faunal assemblage information obtained from fossilized marine ecosystems, paleontologists must sample these ecosystems with knowledge of their spatial heterogeneity in mind. To do this, we must first understand the nature of spatial heterogeneity in present-day depositional settings. We assessed the degree to which the modern mollusk fauna of subtropical marine carbonate systems, an environment which is common in the fossil record, display spatial patchiness. We also examined environmental factors that potentially influence the composition of mollusk assemblages.

We addressed these questions using mollusk death assemblages from 14 localities on the island of San Salvador, Bahamas. At each locality, we collected bulk samples at 10 meter intervals along onshore-offshore transects ranging in length from 50 to 130 meters. We found that the mollusk assemblages varied in composition among transects, despite all localities being from a relatively small island. Mollusk assemblages from within transects were more similar than assemblages from adjacent transects. In addition to within-habitat patchiness, several environmental factors also seem to influence mollusk faunal assemblages, including seagrass proximity and the level of anthropogenic disturbance.

Our results suggest that mollusk death assemblages exhibit and retain a high level of patchiness, and that spatial information resulting from environmental variables and habitat types may be well preserved within shell death assemblages at relatively fine scales (e.g. tens of meters). Paleontologists who are interested in measuring diversity in fossil habitats may be able to find small scale environmental changes preserved in the fossil record, and need to sample in ways that account for patchiness, keeping in mind the degree of spatial variation in faunal assemblages that may be present among and within habitat types.