Living Beyond the Edge: Impacts of Climate Change on Rock Lizards at the Niche Margin

ABSTRACT Ectotherms are particularly threatened by climate change because they are strictly reliant on environmental conditions for homeostasis. Increasing environmental temperatures may approach the species' critical thermal maximum, with deleterious effects on individual thermoregulation capacities. This study tests the hypothesis developed in a recent work that under ongoing global warming populations living in sites at the warm edge of the species' thermal niche will suffer a disruption of the thermoregulation process, with detrimental effects at the individual and population level. We collected individual measurements and temperature data for Mediterranean endemic rock lizards, across the entire distribution range of the species and during two different sampling periods ~20 years apart to compare thermoregulation coefficient (C), body condition index (BCI) and population size under different climatic conditions. We found that C and BCI vary across space and time following a linear pattern along the thermal niche gradient (Niche Margin Effect, NME) until a threshold temperature, beyond which the NME is disrupted. This threshold temperature indicates the warm edge of the species' thermal niche. A slightly higher temperature marks the threshold at which we observed significant population declines over the 20‐year study period in the warmest sites. This suggests a lagged response of population trends to climate warming. This study suggests a mechanism of disruption of homeostatic processes when the warm margin of the thermal niche is reached and indicates that individual parameters such as thermoregulation coefficient and body condition, rather than demographic trends, are key indicators for an early detection of population extinction risk. The multipopulation approach implemented in our study allows to identify the niche edge that underlies species' vulnerability to global warming, and to identify populations suffering negative effects of climate change before demographic collapse. This might allow to plan appropriate mitigation measures and management strategies to avoid local extinctions.