HISTORICAL ECOLOGY OF FLORIDA SPRING-FED FLUVIAL SYSTEMS USING FOSSIL AND MODERN MOLLUSKS

In Florida’s spring-fed fluvial systems, molluscan fossil and death (Pleistocene to Holocene) assemblages are preserved alongside the current, living community. These three assemblage types provide a historical perspective on different timescales, tracking changes in molluscan biodiversity, community composition, and spatial structuring within changing systems. To study these changes, three types of samples were collected from Florida’s Silver, Ocklawaha, and Wakulla rivers: live mollusks (life assemblages), surficial shell accumulations (death assemblages), and in situ bank sediments/marls (fossil assemblages). The 259 bulk samples collected yielded 46,978 specimens, representing 24 molluscan taxa. Radiocarbon dating and taphonomic analysis show that these death assemblages are formed from multiple sources and may be characterized by multimillennial time-averaging. Consequently, comparison of the living and fossil assemblages, without the inclusion of death assemblages, is more appropriate for assessing recent ecosystem changes.

Fossil assemblages are higher in diversity than life assemblages despite the added presence of recently introduced exotic species that are absent from fossil deposits. Additionally, modern Wakulla River communities in downstream sites are often dominated by the brackish-tolerant neritid Vitta usnea, a species that is completely absent in the local fossil assemblages. Nonmetric multidimensional scaling indicates that, for all three rivers, life assemblages are distinct in faunal composition from their corresponding fossil assemblages. Additionally, life assemblages in the Silver and Ocklawaha rivers are similar to one another, and distinct from those of the Wakulla River. While there is some spread within fossil assemblages, there is still overlap between all three rivers, showing similar faunal composition in the fossil record. These results suggest that Florida river systems once hosted a broadly similar mollusk association that has become more spatially heterogeneous. Recent shifts in faunal composition and an increase in spatial heterogeneity likely reflect localized incursions of brackish taxa, differential success of introduced species across river systems, and regionally variable loss of native taxa. These trends are hypothesized to represent the combined effects of global climate changes and regional human-induced ecosystem stresses.