Population variability is lower in diverse rock pools when the obscuring effects of local processes are removed1

Ecological theory predicts that species richness should impact population variability. In contrast, most empirical evidence suggests no or only a weak positive relationship between species richness and population variability. We investigated the hypothesis that the obscuring noise of local processes at small scales such as differences in environmental conditions and species composition may mask the effects of species richness on population variability. Using long-term data on invertebrate populations in rock pools, we considered species richness-population variability relationships using three analytic resolutions in which data for the two key variables, species richness and population variability, were averaged for each population at decreasing levels of resolution to successively remove more noise arising from local processes. Of these levels the resolution most useful in making predictions about the effect of species richness on population variability removed the most noise in population responses arising from local processes. Our results show that populations are less variable in species-rich environments, a finding that reiterates the importance of species richness not only for aggregate properties such as biomass stability, but also for individual species abundances. Comparing results at different resolutions also provides a methodology to identify relevant detail in richness-population variability relationships.