TRACKING CHANGES IN THE HISTORICAL ECOLOGY OF FLORIDA’S RIVERS USING RECENT AND FOSSIL FRESHWATER MOLLUSKS

Aquatic communities of Florida’s freshwater ecosystems have been studied by ecologists for over 50 years. In contrast, death and fossil assemblages, which preserve molluscan-components of past communities and can provide a unique historical perspective, have received limited attention. These historical approaches can be used to quantify recent ecosystem changes, including faunal composition, biodiversity, and spatial structuring of regional faunal associations. To address these issues, three types of samples were collected from Florida’s Silver, Ocklawaha, and Wakulla rivers: living mollusks (life assemblages), surficial shell accumulations (death assemblages), and in situ river bank sediments (fossil assemblages). 82 bulk samples collected so far yield 24,492 specimens representing 20 species of mollusks.

Quantitative analyses indicate that diversity is highest in fossil assemblages, intermediate in death assemblages, and lowest in life assemblages. Many live and dead samples are dominated by recent invasive taxa, such as Corbicula fluminea and Melanoides tuberculata, while fossil samples include species that are rare or absent in life and death assemblages, suggesting changes in mollusk communities that predate modern ecological research. Nonmetric multidimensional scaling indicates that, for all three rivers, life assemblages are distinct in faunal composition from both the corresponding fossil assemblages as well as the life assemblages of other rivers. Across all river systems, fossil assemblages are more similar to each other in faunal composition than their counterpart life assemblages. These results tentatively suggest that river systems that once hosted similar mollusk associations are now more disparate faunally, likely reflecting differential success of invasive species and variable loss of native taxa. The local and regional shifts documented here point to the emergence of novel ecosystems with a diminished local diversity and increased regional variability.