Does density affect interactions between a native and non-native aquatic gastropod?

 

Presence of the invasive New Zealand mud snail (Potamopyrgus antipodarum) in the Snake River raises concerns for the long-term viability of native gastropods in this region.  Previous studies examining interactions between P. antipodarum and native gastropods have revealed evidence of competition with some species and facilitation with others.  Our goals were to explore the demography of native Valvata sp. in the Snake River and examine the interactions between Valvata and P. antipodarum.  Demographic information was collected monthly (May through November) from four Valvata colonies in the Vista reach of the Snake River (Rm 713) with a venturi suction dredge.  Intra- and interspecific interactions between Valvata and P. antipodarum were explored in a field experiment where snail growth was measured across varying snail densities and relative abundances of the two species.  A two-way ANOVA was used to determine the relative strengths of intra- and interspecific interactions.  Demographic data revealed a semelparous lifecycle for Valvata with P. antipodarum moving in and out of colonies throughout the year.  Results from the field experiment suggest density influences the direction of interactions between species.