The Effects of Dispersal on Macroecological Patterns
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Ecologists have long sought to uncover the mechanisms behind large-scale, macroecological patterns in the distribution and abundance of species. Macroecological patterns are often attributed to the dynamics of dispersal (e.g. dispersal limitation or widespread dispersal). However, few studies actually measure dispersal to determine if dispersal rates are commensurate with the observed macroecological patterns. In this dissertation, I use population genetic analyses across many species to obtain community-level estimates of dispersal rates for two different ecological systems: birds on islands and trees in tropical rainforests. These independent estimates of dispersal then allow me to determine if macroecological patterns in these two systems can be attributed to dispersal dynamics.
In chapter two, I explore the contrasting macroecological patterns of two groups of Lesser Antillean birds. The groups' differing macroecological patterns could be due to differences in dispersal, but other authors have advocated different mechanisms. Population genetic analyses show that the two groups do differ significantly in rates of inter-island dispersal, indicating that dispersal dynamics can explain their contrasting macroecological patterns. In chapter three, I turn my attention to tropical tree communities. In contrast to studies of birds on islands, studies of trees in tropical rainforests may suffer from misidentification of individuals in the field. Using a phylogenetic approach, I determine errors rates in identification, and then assess the effect of these errors on macroecological patterns and other ecological analyses of tropical tree communities. I find that error rates are substantial, but that they have little effect on macroecological patterns. In contrast, species-level ecological analyses can be dramatically affected by these errors.
In chapter four, I return to the influence of dispersal on macroecological patterns, this time in tropical tree communities. One notable macroecological pattern in Amazonian tree communities is a high correlation in the relative abundances of species shared across communities, which could indicate high rates of dispersal between communities. However, population genetic analyses show that dispersal is severely limited between communities. Thus, some factor besides dispersal, such as differences in competitive ability or susceptibility to disease, must be driving species to achieve similar relative abundances in geographically separated communities. In contrast, I show that dispersal limitation is the likely cause of another macroecological pattern frequently observed in tropical tree communities: the decline in the compositional similarity of communities with distance. However, this is not steady-state dispersal limitation in an equilibrium framework as is conventionally thought. Instead, the dispersal limitation appears to be historical in nature, which implies a heretofore unnoticed role for historical contingency in the assembly of Amazonian tree communities.
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