Distribution of the apple snail Pomacea canaliculata in Pampean streams (Argentina) at different spatial scales

Abstract

Studies dealing with the distribution of freshwater mollusks in the Pampean plains are very scarce and have all focused on the determinants of their presence among different waterbodies. The aims of this study were to investigate the distribution of the apple snail Pomacea canaliculata among and within Pampean streams. The main sampling scheme included heterogeneous sections within different sites belonging to all the streams of the Encadenadas del Oeste basin (Buenos Aires Province), in which apple snails? presence and abundance as well as several environmental variables were recorded. P. canaliculata was present in long streams originating in the Piedmont area, with the exception of one that suffered extensive dredging works but it was absent in short streams originating in the plains. Lower altitudes and higher organic matter contents characterized the inhabited sites among the long streams. At a smaller scale, higher concentrations of Na+ and lower levels of organic suspended matter characterized the inhabited sections. Within the short streams P. canaliculata was only found at sites located downstream from a connection to an inhabited long stream and hence constituted sink populations; short streams presented alkaline waters with high conductivity, very slow currents and no trees. The microhabitat of P. canaliculata was investigated at two sites from two different streams in which environmental variables were registered at individual snails? locations and at systematically distributed points. Apple snails were more frequently found in places located close to the shore, where current velocity was usually low, sediments fine and rich in organic matter, and macrophytes abundant. These variables were all correlated at this small scale thus making their individual interpretation difficult. All the streams in this basin appear to be habitable for P. canaliculata, but the populations would only persist autonomously in the medium and low reaches on the long streams. The distribution within long streams is probably governed by stochastic patterns of extinction-colonization as a consequence of the variability of the climatic and hydrological conditions in this region.