Avian community responses to ecological disturbance and recovery at Mount St. Helens, WA, USA
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Community assembly occurs through the complex interaction of local and regional processes which contribute to the differential colonization and extinction of species within a local site. Understanding these processes is of fundamental importance to ecology because it enables predictions for the trajectory of recovery in ecological systems following disturbance. In this dissertation I combined field studies of the Mount St. Helens bird community with historical and regional data to better understand the processes contributing to local assembly in a mainland community following large scale volcanic disturbance.
First, I applied a novel spatial approach to examine avian colonization patterns at Mount St. Helens and approximate the geographic extent of the region influencing local community assembly in the first thirty years of recovery. Despite the prevalence of regional sources, avian colonization of Mount St. Helens has occurred slowly over thirty years. By approximating `minimum source regions' for local communities across time, I developed a new approach for examining the spatiotemporal dynamics of colonization and found that species from a broad geographic area extending beyond the Cascade mountains have colonized Mount St Helens. I then focused on the primary successional habitat of the Mount St. Helens Pumice Plain to examine what ecological processes have contributed to avian community assembly. Testing multiple assembly hypotheses I found evidence of niche-based assembly through nestedness and habitat filtering but no support for competition-based assembly rules. Finally, I addressed the specific mechanism of local recruitment in maintaining populations of birds on the Pumice Plain by monitoring nest success across species. I found observed nest success lower than previously recorded in other habitats for several common ground-nesting and shrub-nesting species. I determined that in the absence of sufficient local recruitment, repeated colonization from the surrounding region may contribute to the persistence of some species on the Pumice Plain. Overall, my results found evidence of habitat filtering rather than interspecific competition in limiting early assembly and supported the importance of continued colonization processes drawing from a range of regional habitats.