The role of host plant transitions in the performance of a locally adapted specialist butterfly

Background/Question/Methods The potential for geographic range shifts, where individuals move poleward to track changes in climate, is a pressing issue in ecology given the rapid rate of anthropogenic climate change. However, non-climatic limitations to range shifts are often missing in models that predict how species will respond to climate change. Herbivorous insect populations respond relatively quickly to changes in temperature and often have greater dispersal capacity than their host plants. Therefore, specialized insects could become spatially separated from their preferred host plants under a shifting climate. We tested the performance of a specialist herbivore (Eyrnnis propertius) on different host species (Quercus spp.) that occur within its range to determine if host switching associated with differential population migration could have negative consequences for insect populations. Specifically, we focused on two host plants occupying different positions within the range of E. propertius: Quercus garryana is the only host species occurring in the northern portion of the species' range, and Q. agrifolia is a dominant host for southern populations.
Results/Conclusions Populations of E. propertius that feed on the deciduous Q. garryana experienced increased mortality when we fed them the more-southerly, and evergreen, Q. agrifolia. We also found that Q. garryana and Q. agrifolia differ significantly in moisture content, nitrogen and carbon concentration, and leaf toughness. Despite its decreased quality, however, Q. agrifolia provides a year-round food source that could be important to E. propertius if phenological shifts occur under climate change. Experiments performed during the spring of 2008 will test the performance of populations from Q. agrifolia regions on the northerly host, Q. garryana.