Are snails important competitors in stream ecosystems of the Pacific Northwest (USA)

Juga silicula (Gould) is an omnivorous pleurocerid snail that inhabits many streams in NW USA. In some streams it comprises over 90% of invertebrate standing crop bio-mass. Analysis of benthic samples from 20 stream reaches showed that densities of many invertebrate taxa were inversely correlated with snail density. Experimental reduction of snail density resulted in higher algal standing crops and higher densities of many although not all invertebrate taxa. The taxa most strongly affected by Juga were those that 1) overlapped most extensively with Juga in habitat use and 2) were sedentary. Juga also affected the between habitat distribution of individuals. These effects were generally species-specific and related to habitat preferences of each species. Juga appears to be a competitive dominant in some streams and may profoundly influence both abundances of individual species and community structure. Furthermore, because individuals of Juga are not generally eaten by vertebrate predators, this snail may also affect trophic dynamics by reducing energy transfer to higher trophic levels