Type: report

Article abstract: This report covers the length of the project, from the summer months of 2009 to the end of summer 2012. Activities are summarized in the context of the primary goals of the project, which were completed. Our three primary objectives were (1) to document the extent of Thorne's hairstreak (TH) presence within the study area of Otay Mountain; (2) to characterize habitat association within that geographic range; and (3) to conduct larval experiments addressing the importance of tree age on the physiological performance of caterpillars. With reference to objective 1, we found the distribution of TH on Otay Mountain to be more extensive than previous reports had suggested. We also examined TH presence in the interior versus perimeter versus exterior of host plants stands. With reference to objective 2, variables characterizing vegetation and the environment were thoroughly documented but found to explain very little of the variation in TH presence/absence and abundance. Finally, larval experiments were able to definitively reject the hypothesis that older foliage might be important for larval growth. The implications of these findings for the conservation of TH are discussed. In brief, we come to the following main conclusions. (1) The widespread range of the butterfly within the study area has positive implications for persistence, though it should be remembered that the entire study area is not itself large and is prone to wildfires. (2) TH appear to associate with the host trees under a range of environmental conditions on Otay Mountain, but this should not be taken to mean that a monoculture of the trees would be sufficient for TH population persistence; to the contrary, patch edge use by TH strongly suggests that variation in patch size, area, and configuration are important and desirable targets for management and conservation. (3) Finally, we note that these findings point the way towards research that could be conducted with TH or (more likely) with closely related species to address the importance of habitat configuration and heterogeneity on population dynamics.