The role of habitat-specific demography, habitat-specific dispersal, and the evolution of dispersal distances in determining current and future distributions of the ant-dispersed forest herb, Hexastylis arifolia
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One of the most recognized ecological principles is that the distribution of species is closely associated with the distribution of suitable habitats. This principle is still highly valued both within and outside the scientific community, despite the documentation of many exceptions. The main goal of my dissertation has been to describe, explain, and model the distribution of a small ant-dispersed forest herb, Hexastylis arifolia (Aristolochiaceae) in relation to the distribution of its suitable habitat. I investigated how the distribution of suitable habitat and seed dispersal interact in determining the distribution of H. arifolia. I measured habitat-specific demography and habitat-specific seed dispersal of H. arifolia in occupied and unoccupied sites over several spatial scales. I used these measurements to model the potential of this forest herb to respond to change in distribution of suitable habitat. It is often hypothesized that seed dispersal by animals is a mutualistic interaction, at least in the context where the species evolved. I surveyed the literature for competing hypotheses regarding the evolution of dispersal by ants and tested two hypotheses with the data I had collected. The current distribution of H. arifolia does not reflect the current distribution of suitable habitat. H. arifolia is often absent from apparently suitable habitat near and far from established populations. In addition, due to a very low mortality rate, it can persist in remnant populations in apparently unsuitable habitat even in the absence of local recruitment or immigration. Seed-dispersing ants were present within and outside the distributional range of H. arifolia. Seed dispersal distances were extremely short and, combined with a very low fecundity, they limit the rate at which populations of H. arifolia can spread into new suitable habitats. Overall, the distribution of H. arifolia seems to be dispersal-limited. The results of this study suggest that seed-dispersal by ants evolved under conditions that benefited plants by reducing predation, reducing competition, and allowing colonization of suitable habitat at a small scale. However, this mutualism will provide little benefit to the plants if the distribution of suitable habitat will continue to change at the rates that are currently observed.
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