Abstract
The island apple snail, Pomacea maculata, family Ampullariidae, is a large freshwater gastropod native to South America. P. maculata is known as a heavy consumer of aquatic plants. P. maculata was introduced to Florida in the 1990s and has rapidly spread throughout natural and man-made wetlands and waterways in the southeastern United States. Negative ecosystem impacts associated with P. maculata invasion include destruction of macrophyte communities via overgrazing, competitive exclusion of the native Florida apple snail Pomacea paludosa, and the potential transmission of toxins and parasites to predators. Populations of P. maculata have been documented in freshwater tributaries of estuaries such as Mobile Bay, Alabama and the Caloosahatchee Estuary, Florida, and the snails may be moving into the estuaries themselves. The objectives of this study were: 1) to evaluate P. maculata’s potential to harm macrophytes like tapegrass, Vallisneria americana, in low-salinity estuarine environments, 2) to determine how the grazer’s destructiveness might by moderated by environmental context (salinity, temperature, and the presence of other macrophyte species), and 3) to identify management and restoration strategies for V. americana to minimize the harm done by P. maculata. We accomplished these objectives with feeding trials and mesocosm experiments conducted under varying conditions of salinity, temperature, and macrophyte community composition. We found that increasing salinity lowered P. maculata grazing pressure on V. americana but increasing temperature increased grazing pressure. Herbivory on V. americana was not reduced and was sometimes intensified when other aquatic plant species were present. The results of two mesocosm experiments suggested that salinity and snail presence have a nonadditive, antagonistic, effect on V. americana. I.e., in the absence of snails the plant performed best at 0 psu, whereas when snails were present the plant did best at 5-10 psu due to reduced snail grazing. Due to the significant sub-lethal impacts of salinity on P. maculata’s grazing and health it is unlikely that the snail’s invasion will proceed beyond the lowest salinity portions of estuaries. These estuarine regions can therefore serve as a valuable refuge for V. americana populations, providing that effective water management keeps salinity below the approximately 10 psu threshold where significant direct harm occurs to the plants.