Beta diversity patterns and their origins

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Accurately predicting how perturbations caused or exacerbated by humans - including climate change, habitat loss, and exotic species invasions - will affect the structure and functioning of communities and ecosystems is essential to devising effective adaptive management strategies. Key to achieving accurate predictions is an adequate understanding of the factors that govern variation in the taxonomic, functional, and phylogenetic composition of ecological communities. In particular, better information is required about the conditions that promote the importance of local assembly processes over contingencies related to the regional species pool. For example, if local communities are simply random subsets of the regional species pool, then local processes need not be invoked to explain variation in composition observed among communities. Insights into this issue can be gained by analyzing variation in community composition (called "beta diversity") using recently devised null models. These null models help to distinguish patterns that are consistent with a predominant role for local, deterministic drivers of community structure, from those consistent with stochastic assembly from the regional species pool. The few studies that have applied these methods thus far have yielded important and unanticipated discoveries, including, for example, the observation that tropical forests exhibit no more variation in taxonomic composition than temperate forests once differences in regional (gamma) diversity are accounted for. This finding suggests that much of the beta diversity that has been documented across the globe in a variety of study systems may not, as traditional thought, arise due to variation in local assembly processes. Instead, it might ultimately originate in historical / biogeographical processes that influence the characteristics of regional species pools. I propose to apply these null model methods broadly, across a variety of study systems that vary in key characteristics, including dispersal capacities of the taxa (e.g. terrestrial plants versus freshwater diatoms) and ecosystem type (e.g. terrestrial versus aquatic). The specific study systems include vascular plants of British Columbia forests, diatoms of the freshwater lakes of North America, macroinvertebrates in U.S. rivers, and zooplankton in U.S. lakes. I then propose to explore the implications that stochastic assembly and regional pool contingencies have for paleolimnology, an applied field with strong and persistent roots in classic beta diversity theory. Paleolimnologists use spatial and temporal patterns of beta diversity among freshwater diatoms (and other organisms) to infer environmental change. Although highly successful, the paleolimnological method is founded on outdated ideas about community assembly. My second objective is to conduct the first rigorous assessment of how the precision of paleolimnological transfer functions vary according to different assumptions about the roles of local determinism versus stochastic assembly and regional pool contingencies. The ultimate goal of this work is to improve upon current quantitative methods that largely ignore the potential for systematic variation in the importance of stochastic and deterministic assembly along environmental gradients.