Thermal tolerance limits as indicators of current and future intertidal zonation patterns in a diverse mussel guild

Abstract

Climate change has driven shifts in species distributions along latitudinal and elevational gradients, and such shifts are likely to continue as warming accelerates. However, little is known about the response of strongly interacting species, including whether multiple, interacting species are likely to shift concordantly or whether climate change will promote community disassembly. In rocky shore ecosystems, mussels are dominant foundation species that provide habitat and increase diversity of associated species. The New Zealand mussel guild is uniquely diverse as four species can be found within 1 m² of shoreline. We integrated comparative ecophysiology and population ecology to evaluate whether air temperature sets elevational range limits and to quantify mussels’ warming tolerances. Air temperature appears to set upper intertidal limits across mid-intertidal species, based on findings that (1) lethal thermal limits coincided with temperatures experienced at upper tide-height limits, (2) species with higher thermal tolerances occurred higher on shore, and (3) lethal tolerances were highest at our warmest site. Based on predicted body temperatures in year 2100, mid-elevation habitat-forming mussels are likely to experience an increase in the frequency of thermal events causing 50% mortality at their upper elevation limit. Such events are predicted to occur 3.0–4.4 times more frequently in 2100 than present at a warmer site and to increase from 0 to 0.4/0.1 days per year for Perna/Aulacomya, but not Mytilus, at a cooler site. These results indicate that the mussel species’ ranges are all likely to contract at warmer sites in the future, decreasing habitat for associated organisms.