Alien molluscs are widely distributed in U.S. streams. While some raise economic concerns on the order of billions of dollars, documentation of widespread ecological effects has, in some instances, been more elusive. As one way to evaluate this issue we used data from a probability survey of wadeable streams of the coterminous U.S.. Samples were collected from 2000 through 2005 based on a sample of 1,382 probability sites. Macroinvertebrates and a wide range of other data were collected with consistent sampling protocols at all sites.

We evaluate the ecological association between the presence of the most frequently observed alien mollusc and biotic integrity in streams of the western United States. We represent macroinvertebrate assemblage biotic integrity in terms of the ratio of the observed number of taxa present and the expected number of taxa, O/E. We represent the vertebrate assemblages in terms of a multimetric index of biotic integrity. We account for the effects of multiple factors which may have an effect on these biotic indices – e.g., water chemistry, disturbance, excess sediment, natural fish cover, stream size, climate and dams in the upstream watershed. We then evaluate the unexplained variation in biotic indices associated with the presence of the most prevalent alien mollusc.

Results/Conclusions

Analysis of the survey data identified one alien mollusc, Corbicula fluminea, Asian clam, as the most prevalent. This species was estimated to be present on 10.5% of the perennial stream length – 109,000 km. It was present in over 15% of the stream length in the coastal plains of the southeast, the southern Appalachians, the southern Plains, the southwestern mountains and in the xeric California ecoregions.

In portions of the West, the presence of C. fluminea was associated with a substantial reduction in both macroinvertebrate and vertebrate assemblage indices of biotic condition.