Geographic distribution, habitat association, and the importance of host quality for one of the rarest butterflies in North America: Thorne's hairstreak (Mitoura thornei)
AuthorLucas, Amy Marie
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<italic>Mitoura thornei</italic>, Thorne's hairstreak butterfly, is endemic to a single mountain in south-western North America. The extremely small geographic range of this species coupled with threats to its host plant, <italic>Hesperocyparis forbesii</italic>, motivated a study of habitat association and fine-scale mapping of both butterfly and host distributions. Often the study of habitat associations for rare or threatened species is complicated by habitat loss. In the case of <italic>M. thornei</italic>, the range is naturally small, which provides an opportunity to investigate small scale variation in vegetation and geographic features as they may affect larval and adult butterflies. Specifically, the following questions were posed: How much of the range of the host plant is occupied by <italic>M. thornei</italic> within the focal geographic area? What biotic and abiotic features predict the presence and abundance of <italic>M. thornei</italic>? How does tree age affect larval performance? These questions were addressed with a combination of field observations and laboratory experiments involving caterpillars. <italic>H. forbesii</italic> size (diameter at breast height) was found to have a significant association with the presence and absence of larval <italic>M. thornei</italic>, while slope, density of herbaceous plants and density of <italic>H. forbesii</italic> were found to be the factors most strongly associated with adult <italic>M. thornei</italic>. Laboratory experiments with larvae showed no effect of tree age on larval survival, but a slight reduction in adult size for individuals reared on foliage from the oldest trees. From a conservation perspective, the most important result is the widespread occurrence (greater than previously reported) of <italic>M. thornei</italic> throughout the study area. However, I caution that spatial factors (such as fragmentation, isolation and perimeter to interior ratios) could be important for <italic>M. thornei</italic>, though these factors were not addressed directly in this study. In other words, while the presence of the host plant is essential, we do not yet understand how the spatial arrangement of this resource might affect the butterflies. In general, results illustrate the challenges of understanding habitat association for a geographically-restricted species, and the utility of studying larval and adult life history stages, both in the field and in the laboratory.