Species-energy theory, pulsed resources, and regulation of avian richness during a mountain pine beetle outbreak

Species-energy theory provides a framework through which to link two features commonly noted in local communities: episodic production of resources (i.e., resource pulses) and the regulation of local species richness through time. We examined the pathways through which a resource pulse, a large-scale outbreak of mountain pine beetles, was borne out in six foraging guilds comprising a forest bird community in British Columbia, Canada, 1997-2007. We investigated statistical relationships between richness and abundance in each guild to evaluate the prediction that the outbreak should be manifested in species most reliant upon bark beetles (i.e., the bark insectivore guild). We then employed randomization methods to evaluate whether the beetle outbreak obscured evidence for local regulation for the six foraging guilds. Density and richness of bark insectivores increased over the course of the outbreak. More species of bark insectivores were detected for a given number of individuals following the outbreak, consistent with an increase in the number and types of resources. Richness of bark insectivores showed no evidence of regulation. In contrast, densities of most other foraging guilds were not strongly correlated with the habitat changes resulting from the beetle outbreak and displayed only weak evidence of regulation of richness. We suggest that such weak regulation of richness may be a general feature of forest bird communities. Coupled with long-term data, resource pulses provide exceptional opportunities through which to test predictions of species-energy theory.