Translocation experiments with butterflies reveal limits to enhancement of poleward populations under climate change

Abstract

There is a pressing need to predict how species will change their geographic ranges under climate change. Projections typically assume that temperature is a primary fitness determinant and that populations near the poleward (and upward) range boundary are preadapted to warming. Thus, poleward, peripheral populations will increase with warming, and these increases facilitate poleward range expansions. We tested the assumption that poleward, peripheral populations are enhanced by warming using 2 butterflies (Erynnis propertius and Papilio zelicaon) that co-occur and have contrasting degrees of host specialization and interpopulation genetic differentiation. We performed a reciprocal translocation experiment between central and poleward, peripheral populations in the field and simulated a translocation experiment that included alternate host plants. We found that the performance of both central and peripheral populations of E. propertius were enhanced during the summer months by temperatures characteristic of the range center but that local adaptation of peripheral populations to winter conditions near the range edge could counteract that enhancement. Further, poleward range expansion in this species is prevented by a lack of host plants. In P. zelicaon, the fitness of central and peripheral populations decreased under extreme summer temperatures that occurred in the field at the range center. Performance in this species also was affected by an interaction of temperature and host plant such that host species strongly mediated the fitness of peripheral individuals under differing simulated temperatures. Altogether we have evidence that facilitation of poleward range shifts through enhancement of peripheral populations is unlikely in either study species.Lepidoptera ͉ range center ͉ range expansion ͉ range periphery T he biological impacts of climate change are likely to be multifaceted, involving behavioral change, evolutionary change, and local and global extinction, but a well-documented response is geographic range change. Given a species that is completely occupying its thermal niche, warming should open poleward (or upward) territory to population establishment (1, 2). In most cases we would expect establishment to be driven by populations at the poleward (or elevational) periphery of a species' range. If these peripheral populations are preadapted to warmer conditions, due to gene swamping from the center of the range or historical selection under warmer conditions, we would expect them to increase with warming and thereby enhance the colonization process (3, 4). The assumptions underlying this ''peripheral enhancement,'' however, have not been tested despite their significance in determining geographic range change under climate change.A number of factors could prevent peripheral population enhancement. Resource availability and quality in peripheral locales could limit the growth of poleward populations or the colonization of poleward locales. In herbivorous insects, for example, interactions with host plant...

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