Biotic and abiotic drivers of snail population dynamics in a seasonally flooded wetland

Background/Question/Methods: The origins of population dynamics depend on interplay between abiotic and biotic factors; the relative importance of each changing across space and time.  The predator permanence hypothesis proposes that fish predators replace invertebrate predators along a spatial gradient of water permanence.  Spatial variation could be traded for seasonal variation in ecosystems that are connected to permanent water bodies with consequences for population dynamics of resident species.  We test for seasonal variation in the relative importance of water permanence, resource quality, and seasonally shifting predator types on population dynamics of the snail, Planorbella duryi in a sub-tropical seasonally flooded wetland at four sites that varied in resource quality with a 12 year time series, field mortality estimates, and predator performance trials.

Results/Conclusions: Intraspecific density was negatively correlated with the rate of change in snail population size at all four sites suggesting they are self limiting and potentially compete for limited resources.  Crayfish density negatively affected the rate of snail population change at an enriched site where snail densities were among the highest of all sites throughout the time series revealing the potential negative density dependence in affecting snail populations.  Adult molluscivorous fishes were sparsely distributed but were most common in the wet season.  Fish consumed more and larger snails than crayfish.  Field-estimated mortality was higher in the wet season primarily because crushing predators, like molluscivorous fishes, added to mortality by entry-based based predators like crayfish.  Therefore, resource quality affected snail density most; crayfish predators were a chronic source of mortality, and molluscivorous fishes represent an additional source of mortality during the wet season.  We found evidence supporting the concept that the predator permanence hypothesis could be traded for seasonal variation in predator density at one site.  This site resembled a permanent pond with fish predators in the wet season, a fish-less permanent pond in the dry season, and a temporary pond in the middle of the dry season.