Explaining Species Diversity By Linking Local and Large Scale Processes

Large-scale processes are known to be important for patterns of species richness, yet the ways in which local and larger scale processes interact is not clear. I first examined published experiments that manipulated dispersal among local communities using meta- analyses. I show that local communities often readily increase diversity, but that there may be declines at larger spatial scales. I then used metacommunities consisting of microbial aquatic communities to examine how processes at different scales affect local and metacommunity richness. Specifically, I manipulated the potential dispersal rate, whether dispersal was localized or global, and variation in initial community composition. I showed that a low dispersal rate and intermediate distance dispersal enhanced local richness. Initial assembly variation had no effect on local richness, while a lack of dispersal or global dispersal reduced local richness. I also show that predation undoes any diversity increases associated with dispersal. At the metacommunity scale, richness was enhanced throughout the time course of the experiment by initial compositional variation and was reduced by high or global dispersal. Also predation identically structured local communities, and thus reveals large impacts at the metacommunity scale. I further show that these organisms exhibit competition- colonization tradeoffs, and examine how local scale disturbances can structure species diversity. If species are evenly distributed along this tradeoff, then diversity is maximized at intermediate disturbance rates. However if the tradeoff is colonist-skewed then diversity increases with disturbance, and declines is the tradeoff is competitor-skewed. But patterns of diversity at scales larger than the local community always show that diversity is maximized at intermediate disturbances, regardless of the distribution of species along the competition-colonization tradeoff. These results indicate that the effects of dispersal on species richness have a complex relationship with scale and are not solely divisible in to "regional" versus "local" scales. Finally, predictions of how dispersal structures communities appear dependent on local-scale processes, species interactions and historical assembly and disturbance frequency.