FOOD WEB COMPRESSION DUE TO HABITAT REDUCTION: TOWARDS AN UNDERSTANDING OF THE ECOLOGICAL EFFECTS OF SUBTROPICAL MASS EXTINCTIONS THROUGH TROPIC ANALYSIS OF A MODERN MOLLUSCAN FOOD WEB

Marine molluscan communities of the western Atlantic experienced a regional Plio-Pleistocene mass extinction event, in part due to sea-level regression leading to habitat loss. Habitat loss provides a high potential for community changes and trophic cascades. However, tropical and subtropical systems with higher diversity and more complex food webs have shown weaker trophic cascades than those observed in temperate, low diversity environments. Therefore, understanding habitat-loss mitigated food web alterations in (sub)tropical environments may provide important context for the Plio-Pleistocene extinction. The prevalence of recent historical data for the molluscan food web at Baymouth Bar in Alligator Harbor, Florida, (BMB) provides an opportunity to examine the effects of habitat reduction on a modern seagrass community. Between 1963 and 2015, the size of the intertidal seagrass habitat in BMB was reduced by 80%. Community census and field experiments suggest this has resulted in a flattened food web structure, with trophic position (TP) reductions primarily at the top trophic levels. We used stable isotopes of nitrogen and carbon, a powerful tool for reconstructing spatially and temporally integrated estimates of diet and TP, to quantitatively examine these conclusions. Bulk soft tissue analyses of d¹³C and d¹⁵N were measured for live-collected bivalves and gastropods. While precise TP calculations are currently limited by the absence of a robust proxy for the littoral baseline, the preliminary data confirm that habitat reduction of BMB has flattened the food web, primarily at the highest gastropod trophic level. The nitrogen signature of the Florida horse conch, Triplofusus giganteus, confirms that this top predator has a lower-than-expected nitrogen signature, below the lettered olive, Oliva sayana, like that of the pear whelk, Fulguropsis spirata, and only slightly above the banded tulip, Cinctura hunteria. It can also be noted that the C and N signatures of the naticids Neverita delessertiana and Sinum perspectivum, suggest little evidence for TP reduction at the omnivorous, middle tiers of the food web. These results could help to refine our understanding of how food web structure of Plio-Pleistocene seagrass communities in Florida responded to disturbance associated with sea level regression.