Geographic variation in the upper thermal limits of an intertidal snail: implications for climate envelope models

Although climate envelope models are used increasingly to predict the response of species to climate change, these models may perform poorly when species are comprised of locally adapted populations with differing environmental tolerances. Despite this concern, little is known about how tolerance traits vary across a species' geographic range. In this study, we tested whether the upper thermal limits of a direct- developing intertidal snail, Nucella canaliculata, varied among populations distributed along the northeastern Pacific coast. Snails from 7 sites in central California, northern California, and Oregon (USA) were reared through 2 generations in a common laboratory environment to minimize the potential influence of field acclimatization and other non-genetic effects. Laboratory assays of acute lethal temperature tolerance (LT50) indicated that newly hatched N. canaliculata from central California were less heat tolerant than their conspecifics from Oregon. These differences in upper thermal limits likely have a genetic basis and are consistent with a mosaic of potential thermal stress in rocky intertidal habitats along the northeastern Pacific coast. In particular, some northern sites experience longer exposures to stressful midday low tides than southern sites, due to variation among regions in the timing of low tides. Persistent regional differences in tidal regimes, climate, and other environmental factors may act as selective forces that influence the physiology of intertidal species with broad latitudinal ranges. The resulting geographic distribution of thermally tolerant genotypes may be spatially complex, and may thus alter predictions regarding the effects of climate change on local extinctions and species' geographic range shifts.