Grazing Effects of the New Zealand Mud Snail Across a Productivity Gradient in the Greater Yellowstone Ecosystem

Accurately predicting the effects of introduced species on native commumtles and ecosystems is a challenge. Utilizing methods of food web ecology, we measured grazing effects of the invasive freshwater New Zealand mud snail, Potamopyrgus antipodarum, in streams within the Greater Yellowstone Ecosystem. Previous results indicate that P. antipodarum can significantly reduce algal standing stocks in less than one week, but it is not yet known if grazing effects vary across streams differing in benthic algae production. In this study, we measured the strength of P. antipodarum grazing on algal resources across six streams varying widely in ambient primary production. In field enclosure experiments within each stream, we estimated direct grazing effects of snails on algae by measuring chlorophyll a, gross primary production and chlorophyll a-specific primary production. In most streams, P. antipodarum decreased overall algal standing stocks, as measured by chlorophyll a, even though gross primary production was not affected. As a result, chlorophyll-a specific primary production increased in productive streams. Finally, standardized comparisons of P. antipodarum-algae interactions indicated that grazing effects were largest in the most productive streams. The overall impact of P. antipodarum on native stream communities will be greatest in the most productive streams if these assemblages are also capable of supporting dense P. antipodarum populations.