Corbicula at its southernmost invasion front in Patagonia: Unusual low density and asymmetric trait responses to varying environmental conditions

Abstract

Freshwater clams of the genus Corbicula are successful global invaders, but the processes that determine their geographic range limits remain poorly understood. Here we explored how this invasive clam responds to varying environmental pressures at the southernmost invasion front of its global distribution. We assessed population density, heavy metal content, body weight and shell growth of individuals collected from areas subjected to different farm drainage and runoff conditions in the Río Negro estuary, Patagonia, Argentina. We found that clams from a habitat directly exposed to this source of pollution exhibited higher accumulation of copper, lower body weight, but higher individual growth than conspecifics from a downstream habitat less exposed to pollution, which followed the opposite trend. This asymmetric pattern supports the idea of resource-allocation strategies to varying environmental pressures occurring even over a small geographical range. Density showed no spatial differences, yet it was unusually low in spite of Corbicula having been established in the area for more than 15 years, which is consistent with a negligible population growth rate at the invasion front. This is the first study to report such an extended lag period for this successful worldwide invader. We discuss the implications of low densities for life-history traits that are selected for optimizing population growth at the invasion front.