COS 42-9
The effects of trout on the species abundance distributions of Sierra Nevada aquatic communities
Species abundance distributions (SAD)s are a community level metric and their shape describes the commonness and rarity within a community. While the shape of the SAD is a result of biotic and abiotic processes, there is considerable debate whether a specific process will be reflected in the shape of the SAD. To this end, we examined how benthic macroinvertebrate community SADs responded to introduced trout in lakes in California's Sierra Nevada. In particular, we examined 42 paired trout and troutless streams, 10 paired trout and troutless lakes, and 379 and 361 independent trout and troutless lakes, respectively. For each lake and/or stream, we fit a flexible, neutral SAD (a discrete gamma distribution) and compared the shape parameters of this distribution between trout and troutless communities. Moreover, for the paired lakes and streams, we performed a strong test of SAD similarity in which we “trained” our SAD parameters on troutless communities and then “tested” these predictions on the trout communities. If trout were not having a significant effect on the SADs, we would expect the fit of the SAD parameter value calculated from our “training” troutless communities to accurately predict the SADs of the trout communities.
Results/Conclusions
We found that the parameters of the SADs (i.e. the shape of the SAD) differed significantly between trout and troutless lakes and streams. As predicted by theory, the mode of the trout SADs shifted to the left indicating increased rarity in communities with trout. Moreover, when we “trained” our SAD on the troutless SADs we found that these SADs did not adequately predict the SADs of equivalent paired trout communities. While our analyses show that SADs of benthic macroinvertebrate communities do reflect the effect of introduced trout, the majority of the variation in the shapes of our SADs can be explained by the difference in species richness between trout and troutless lakes. SADs are a result of ecological processes, but our work supports current theory suggesting most ecological processes are not explicitly reflected in the shape of the SAD. Instead, many ecological processes are reflected in the SAD implicitly by their effects on species richness and total community abundance.