Spreading the risk: Small-scale body temperature variation among intertidal organisms and its implications for species persistence

Abstract

The intertidal zone of wave-swept rocky shores is a potentially useful system in which to monitor, experimentally manipulate, and possibly forecast the ecological consequences of impending changes in environmental temperature. However, the spatial and temporal complexity of the shoreline's thermal environment challenges ecologists' ability to progress from site-specific measurements to general predictions. In this study, we conducted intensive field experiments to quantify inter-individual variation in body temperature among organisms and surrogate organisms at a typical intertidal site, and we used these measurements to characterize micro-scale variation in potential thermal stress. Within the 336-m extent of our site, local variation was substantial in several metrics of extreme and cumulative thermal exposure. The within-site variation in extreme temperatures rivaled (and in some cases greatly exceeded) variation among sites along 14° of latitude (1660 km of Pacific shoreline). Application of our within-site data to a simple theory of risk suggests that small-scale spatial variation in temperature can reduce the chance of local extirpation that otherwise would accompany an increase in average habitat temperature or an increase in the frequency of extreme thermal events. The magnitude of this “buffering” effect also depends heavily on inter-individual variation in thermal tolerance, a factor that has received relatively little attention from physiologists. We hope that by highlighting the important role of within-site variability (both of temperature and tolerance) in the persistence of intertidal populations, this study will foster further research into the biophysical, physiological, behavioral, and genetic interactions underlying ecological dynamics on wave-washed shores.