HISTORICAL ECOLOGY OF FLORIDA FRESHWATER SPRING AND RIVER MOLLUSKS

Florida’s spring-fed rivers contain molluscan fossil (late Pleistocene-Holocene) and death assemblages preserved alongside living mollusk communities. These assemblages can provide a perspective on long-term historical ecology, community restructuring, and other changes in response to environmental, anthropogenic, and biological impacts. In this study, we collected fossil (in situ underwater bank sediments), death (loose shell accumulations), and live (living specimens) molluscan assemblages from the Silver, Ocklawaha, and Wakulla Rivers in Florida. Between 2017-2023 these collections yielded 325 samples containing 59953 specimens of 24 taxa of both gastropods and bivalves.

Fossil assemblages included 16 taxa, while live assemblages were reduced to 13, even though live assemblages contained both introduced and brackish-tolerant taxa that were absent in the fully native freshwater fossil assemblage. Death assemblages had the highest species richness at 24 taxa, suggesting that they are likely a mixture of both recently dead and reworked fossil material. Nonmetric multidimensional scaling indicates that, for all three rivers, live communities are always distinct in faunal composition from their corresponding fossil assemblages, and often distinct from their respective death assemblages (and that this difference also persists when invasive species are excluded from the analysis). Live communities from the Ocklawaha and Silver Rivers are similar, as the latter is a tributary of the former, though both are distinct in composition from the Wakulla River. Death and fossil assemblages show broadly similar composition across all three rivers. These results suggest that while mollusk communities were similar across Florida during the late Pleistocene and early Holocene, they have more recently locally diverged. The recent dissimilarities between populations may be due to a range of local and regional factors including variable success of introduced taxa, differential loss of native species, habitat destruction, or disturbances related to anthropogenic or storm impacts.