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

Abstract

ABSTRACT: Although climate envelope models are used increasingly to predict the response of spe-cies to climate change, these models may perform poorly when species are comprised of locallyadapted populations with differing environmental tolerances. Despite this concern, little is knownabout how tolerance traits vary across a species’ geographic range. In this study, we tested whetherthe upper thermal limits of a direct-developing intertidal snail, Nucella canaliculata , varied amongpopulations 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 laboratoryenvironment to minimize the potential influence of field acclimatization and other non-geneticeffects. Laboratory assays of acute lethal temperature tolerance (LT 50 ) 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 mosaicof potential thermal stress in rocky intertidal habitats along the northeastern Pacific coast. In partic-ular, some northern sites experience longer exposures to stressful midday low tides than southernsites, due to variation among regions in the timing of low tides. Persistent regional differences in tidalregimes, climate, and other environmental factors may act as selective forces that influence the phys-iology of intertidal species with broad latitudinal ranges. The resulting geographic distribution ofthermally tolerant genotypes may be spatially complex, and may thus alter predictions regarding theeffects of climate change on local extinctions and species’ geographic range shifts.KEY WORDS: Thermal tolerance · Thermal stress · Biogeography · Local adaptation · Climatechange ·