Competition is an important structuring force in communities and especially among herbivores. Gastropods are important freshwater herbivores and non-native species have been known to limit native species with similar niches. Anecdotal observations in Florida suggest the non-native apple snail, Pomacea maculata, is excluding the native Florida apple snail, P. paludosa, upon invasion of new wetlands. Using survey data we report on such a pattern in four wetlands in southern Florida and use a field experiment to test for effects of juvenile density and neighbor identity on growth of native juvenile P. paludosa. In our experiment, similar sized juvenile Pomacea were placed in 1-m2 mesh cages embedded in the slough of a shallow wetland. Cages had either low or high snail densities and high-density cages had monospecific and mixed species treatments.
Results/Conclusions
Surveys of snails and egg masses (laid above the water surface) from 2009-2019 indicate P. paludosa went locally extinct in one of the four wetlands following invasion by P. maculata around 2013. In the experiment snail density reduced P. paludosa growth by 22%, and when controlling for density native growth was inhibited by 31% in the presence of P. maculata. Survival was high across all treatments and algal quality and abundances did not vary between treatments. While the mechanism of growth inhibition is unclear antagonistic interactions with P. maculata may explain recent declines in native P. paludosa populations.