Scale and ecological and historical determinants of a species' geographic range: The plant parasite Phoradendron californicum Nutt. (Viscaceae)

Abstract

Geographic ranges of species are fundamental units of study in ecology and evolutionary biology, since they summarize views of how species' populations and individuals are organized in space and time. Here, I assess how abiotic and biotic factors limit and constrain species' geographic range, structure its distributions, and change in importance at multiple spatial and temporal scales. I approach this challenge using models and testable hypothesis frameworks in the context of ecological, geographic, and historical conditions. Concentrating on a single species, the desert mistletoe, Phoradendron californicum, I assess the relative importance of factors associated with dispersal, host-parasite-vector niche overlap, and phylogeographic patterns for cpDNA within a 6 mya timeframe and at local-to-regional geographic extents. Results from a comparison of correlative and process-based modeling approaches at resolutions 1-50 km show that dispersal-related parameters are more relevant at finer resolutions (1-5 km), but that importance of extinction-related parameters did not change with scale. Here, a clearer and more comprehensive mechanistic understanding was derived from the process-based algorithm than can be obtained from correlative approaches. In a range-wide analysis, niche comparisons among parasite, hosts, and dispersers supported the parasite niche hypothesis, but not alternative hypotheses, suggesting that mistletoe infections occur in non-random environmental subsets of host and disperser ecological niches, but that different hosts get infected under similar climatic conditions, basically where their distributions overlap that of the mistletoe. In a study of 40 species, including insects, plants, birds, mammals, and worms distributed across the globe, genetic diversity showed a negative relationship with distance to environmental niche centroid, but no consistent relationship with distance to geographic range center. Finally, P. californicum's cpDNA phylogenetic/phylogeographic relationships were most probable under a model of geologic events related to formation of the Baja California Peninsula and seaways across it in the Pliocene and the Pleistocene; however, fossil record, niche projections to the LGM, and haplotype distribution suggested shifting distributions of host-mistletoe interactions and evidence of host races, which may explain some of the genealogical history of the cpDNA. In sum, the chapters presented here provide robust examples and methodologies applied to estimating the importance and scale at which different sets of abiotic and biotic factors act to structure a species' geographic range.