Background/Question/Methods Climate is often assumed to be the primary constraint on species’ poleward range limits, and many species have historically or recently shifted their ranges in response to climatic warming. Therefore, ecologists rely heavily on climatic envelope mapping to predict future range change. To test the role of climate in setting the range limit of species, we performed an experiment inside and outside the northern coastal distributional boundary of two contrasting butterfly species, E. propertius (a host plant specialist) and P. zelicaon (a host plant generalist) on Vancouver Island, British Columbia. This range boundary is characterized by a pronounced rise in mean annual precipitation and, for one of the two species, an abrupt decline in host plant availability. We performed field transplant experiments during the summer of 2007, rearing butterfly larvae on enclosed, potted host plants inside and outside their range. Caterpillar performance was measured up through the end of July.

Results/Conclusions Our results indicate that climatic factors do not preclude the persistence of these butterfly species northward of their current range boundary, at least at the caterpillar stage. Mean growth rate of larvae reared outside the range did not significantly differ from larval growth rate within the range. Instead, host plant limitation may be key. For the specialist species, a lack of host plants at higher latitudes will prevent it from tracking climate change. Therefore, assisted migration of the host plant may provide an unconventional, though potentially necessary, strategy to enable range expansion. Our study focused strictly on larval growth rate as an indicator of fitness, but other life-stages may serve as the limiting factor on range expansion. For instance, overwintering conditions are known to constrain the northern range edge of many taxa. Likewise, phenological timing of spring butterfly eclosion and first flowering date of critical nectar resources may decouple outside of the range, potentially reducing reproductive success. These additional studies are being currently pursued.