Geographic distribution, habitat association, and host quality for one of the most geographically restricted butterflies in North America: Thorne's hairstreak (Mitoura thornei)

Abstract

Mitoura thornei, Thorne's hairstreak butterfly, is endemic to a single mountain in southwestern North America. The small geographic range of this species coupled with threats to its host plant, Hesperocyparis forbesii, motivated a study of habitat association and mapping of both butterfly and host distributions. Specifically, the following questions were posed. How much of the range of the host plant is occupied by M. thornei? What biotic and abiotic features are associated with the presence and abundance of M. thornei? And, how does tree age affect larval performance? These questions were addressed with a combination of field observations and experiments. Hesperocyparis forbesii size and density were found to be the most important factors associated with the presence and absence of M. thornei at the scale of individual sampling plots, whereas H. forbesii density was found to be the most important factor at the scale of host plant stands. Experiments with larvae showed no effect of tree age on survival, but did show a reduction in larval performance for individuals reared on foliage from the oldest trees. From a conservation perspective, our most important result is the widespread occurrence of M. thornei throughout the study area. Ecologically, the distribution of M. thornei points towards much that has yet to be learned about the use of space in a small, locally rare insect. In general, our results illustrate the challenges of understanding habitat association for a geographically restricted species, and the utility of studying different life history stages, both in the field and in the laboratory.