Here be dragons: Proximate mechanisms mediating temperature and biogeography in alligator lizards
Temperature defines a major axis of the fundamental niche, and thus affects where species can persist. A goal of modern biology is to predict how thermal variation associated with climate change will affect species’ distributions. Many authors advocate the use of mechanistic models that account for physiology; however, there is no consensus as to which organismal traits, life-history stages, or aspects of the thermal environment are most limiting. To begin bridging this knowledge gap, we examined the effects of thermal variation on the physiology of two congeneric lizards, northern and southern alligator lizards (Elgaria coerulea and E. multicarinata). These lizards are ecologically similar, but northern alligator lizards exist at higher elevation and latitude (i.e. colder environments) than do southern alligator lizards. Even so, previous work suggests that mean environmental temperature does not differentially affect adults of these species. We therefore examined the effects of mean temperature on developing embryos and extreme temperatures on adults. We first used controlled incubation experiments to estimate the thermal reaction norm for development in southern alligator lizards and compared this to previous work on northern alligator lizards. We then examined the physiological effects of acute heat stress on adults of both species.
Our results suggest that variation in the thermal sensitivity of multiple life history stages (embryos and adults) contributes to biogeographic differences in alligator lizards. Southern alligator lizard embryos could not tolerate developmental temperatures as cold as those tolerated by northern alligator lizards. In fact, the average developmental temperature experienced by northern alligator lizards in the field was cold enough to induce high mortality in southern alligator lizards. By contrast, northern alligator lizard adults displayed physiological stress (e.g., high corticosterone levels) at warm temperatures tolerated by southern alligator lizards. With these data we can begin to understand the proximate mechanisms governing how the thermal environment affects landscape-level biogeography in alligator lizards. Differences in northern/high elevation range limits likely result from variation in developmental thermal tolerance, whereas differences in southern/low elevation range limits result from variation in extreme heat tolerance of adults. Generally, mechanistic niche models consider the effects of changes in average conditions on adults. Such models would not accurately predict the effects of changes in environmental temperature on alligator lizards. Rather, developmental tolerances and sensitivity to extreme events appear most limiting.