Effects of the invasive species Potamopyrgus antipodarum (Hydrobiidae, Mollusca) on community structure in a small Mediterranean stream

Abstract

Resistance to biological invasions can depend on the properties of the host ecosystem and on the richness of native species. Mediterranean streams are characterized by natural seasonal disturbances, a high capability of resistance and resilience, and high diversity and endemism, which may reduce the establishment of invaders. One example of freshwater invasive species in Mediterranean streams is Potamopyrgus antipodarum, a grazing mudsnail which arrived in Europe in the late 19th century from New Zealand. We studied the effects during one year of the invasion of P. antipodarum on macroinvertebrate community structure at two different scales along a pollution gradient in a small Mediterranean catchment at the Iberian Peninsula. At reach scale, we analyzed environmental tolerance of P. antipodarum, changes of macroinvertebrate community structure along the stream, and chlorophyll-a standing stocks in field experiments. At microhabitat scale, we assessed how the abundance of P. antipodarum was mediated by biotic and abiotic factors. The abundance of P. antipodarum along the pollution gradient showed a unimodal distribution, being higher at intermediate impaired conditions. Sites with a dominance of P. antipodarum did not necessarily have a different taxonomical structure than sites without it. The effect of P. antipodarum on chlorophyll-a standing stocks was not significant. At microhabitat scale, both the biotic (competition with other taxa) and abiotic (flow velocity and substrate) factors are supposed to interfere with the abundance of P. antipodarum generating structural differences among microhabitats. Our results show that P. antipodarum had relatively high abundances in our stream but with relatively low impact, what strongly contrasts to some western US studies. Its weak effect on community structure suggested low abilities of P. antipodarum to get higher densities in the harsh hydrologic condition of Mediterranean streams characterized by seasonal droughts and floods.