Interactions of exotic and native Pomacea with wetland vegetation structure in the Greater Everglades, Florida, USA

Invasions of exotic species are a global threat to native species, biological diversity, and ecological restoration projects. The snail Ponzacea maculata is an exotic herbivore with broad environmental tolerances and high fecundity that is spreading across the southeastern USA. This study examined snail-plant interactions and resource utilization by P maculata and a native congener, Pomacea paludosa, in littoral zone wetlands of Lake Okeechobee, Florida. Questions concerning the preferences of the two snail species for wetland plant species, and potential risks to native snail populations and plant communities in an already stressed ecosystem were addressed. Vegetation communities associated with native, exotic, and mixed snail populations were significantly different. The exotic plant Hydrilla verticillata was the greatest contributor to plant community similarities between sites supporting P maculata, whereas the native plant species Eleocharis cellulosa, Nymphaea odorata, and Schoenoplectus americanus influenced similarities among plant communities supporting P paludosa. Within mixed snail populations, native emergent Paspalidium geminatum, and submerged Vallisneria americana and Potamogeton illinoensis wetland plant species were the most important contributors to site similarity. As generalist feeders, native and exotic Ponzacea may spread their resource use among native and exotic wetland plant communities, respectively, thus reducing both competitive and top-down pressures. However, limited evidence in mixed populations suggests that coupled top-down herbivory pressure may impact some important native restoration indicators in the Greater Everglades, such as V americana. Future research should examine the potential risks posed by relatively cryptic facilitative interactions of exotic P maculata with other exotic invaders such as H. verticillata.