Florida applesnail

In this thesis, I explored the abiotic and biotic factors that explain the variation in reproduction, survival, and individual growth of the Florida Apple Snail (Pomacea paludosa), and I combined reproduction, survival, and individual growth to determine the factors limiting Florida Apple Snail populations. First, I examined how the variation in reproduction of the Florida Apple Snail and another non-native congener (P. maculata) can be explained by depth, temperature, photoperiod, and adult densities. I also examined how metaphyton total phosphorus (TP) influenced individual growth rates of both species. I found that P. paludosa optimally reproduce in shallower water than P. maculata, that both species respond positively to increased metaphyton TP, and that P. paludosa respond weaker to variation in metaphtyon TP than P. maculata. Then, I examined individual growth across seasons, and examined how size-dependent survival varied across seasons in and out of the presence predators. I also investigated predators that strongly influence survival rates. I found that individual growth was slower in the dry season than the wet season, that survival was not size-dependent nor seasonal when predators were excluded from cages, and that survival was size-dependent in the dry season but not the wet season when in the presence of predators. Greater Sirens and Giant Water Bugs were found to be strong predators of P. paludosa < 10 mm Shell Length (SL), Greater Siren and Giant Water Bugs seasonal abundances were responsible for the differences in size-dependent survival across seasons. Finally, I incorporated differences in ideal hydrological and temperature conditions for reproduction, seasonal differences in individual growth, and changes in survival in and out of the presence of predators onto a zero-population growth isocline. I found that optimal hydrological and temperature conditions, and increased growth in the dry season, could not make up for losses of snails < 10 mm SL by predators which suggested that predators are limiting P. paludosa populations in our study area.

In this thesis I examined factors limiting population success of apple snails (Pomacea spp.) in Florida wetlands. First, I examined effects of summertime hydropattern in replicated wetlands on reproduction and juvenile growth. Annual reproductive effort of Florida apple snails (Pomacea paludosa) and the invasive apple snail P. maculata under was not affected by deeper water in the summer. When juvenile P. paludosa were forced to grow in reportedly favorable and relatively deeper summertime depths survival was high between treatments and growth was unaffected. In the other chapter I examined interactions between the two snails with a series of observations and experiments. I examined historical data on the assemblages and found that P. paludosa were locally extinguished in one of the wetlands within six years of invasion by P. maculata. Two field experiments gave conflicting results about the importance of interspecific inhibition of P. paludosa by the invasive snail, but in either case resource reduction could not be demonstrated. Lab exposure to P. maculata chemical cues strongly inhibited P. paludosa growth, although P. paludosa did not behaviorally avoid mucus of P. maculata. In mesocosms Pomacea paludosa growth was reduced with increasing exposure to adult P. maculata waterborne cues, and the results suggest that contact with mucus at a low exposure may even have a greater inhibiting effect. This interspecific direct chemical growth inhibition was novel for gastropods, but could be more generally important, and its potential impacts to P. paludosa populations should be explored further.