The roles of resource availability and competition in mediating dominance of an invasive freshwater snail

If competition structures communities, then highly diverse communities should be biotically resistant to invasion.  However, the importance of biotic resistance in repelling invasions remains controversial.  High resource availability can often neutralize competitive effects from native community members or cause an invasive species to become an even stronger competitor.  Here, we experimentally tested these two mechanisms in promoting dominance by measuring growth rates of invasive (Potamopyrgus antipodarum) and native snails (Pyrgulopsis robusta) and interspecific competition between them at two experimentally-altered resource levels. We found that Potamopyrgus always grew faster than Pyrgulopsis.  In the presence of interspecific competition, Potamopyrgus growth rates were not affected by resource levels or the biomass of Pyrgulopsis competitors.  Alternatively, Pyrgulopsis grew slower at low resource levels and especially when the biomass of Potamopyrgus competitors was high.  These results suggest that competitive effects of Potamopyrgus on Pyrgulopsis were reduced at high resource levels due to faster Pyrgulopsis growth rates; Pyrgulopsis, though, does not strongly compete with Potamopyrgus under any resource scenario. In this system, increased resource availability might not be necessary for Potamopyrgus to dominate because competitive effects from this native community member are weak.