Anolis cristatellus basking in the sun. Photo by Janson Jones.
In recent years, concern has arisen about how tropical ectotherms will cope with rising temperatures. For a variety of reasons, tropical species are considered particularly vulnerable, and coarse scale modelling exercises suggest that many populations and species may face extinction in the near future. Some of the most influential studies, such as Sinervo et al.’s mammoth 2010 paper (already cited more than 200 times!), have focused on lizards.
The field of thermal ecological physiology made great advances in the 1970’s and 80’s and a, perhaps the, major player in the work was research on lizards. And amongst this work, studies on Anolis played a particularly prominent role (reviewed in Chapter 10 of Lizards in an Evolutionary Tree). Hence, it is no surprise that a reconsideration of lacertilian prospects, based on detailed understanding of how lizards interact physiologically with their environment, is stemming from in-depth studies on anoles.
Most modelling studies are based on a coarse-grained (1 km2 resolution), remote sensing scale analysis of global temperature variation, with the assumption that relatively little variation in thermal environment occurs within each block. Recent papers focusing on anoles in Puerto Rico (Leal and Gunderson, 2012) and offshore islands in Honduras (Logan et al., 2013) have tested this idea and found it wanting–in open areas and, to a lesser extent, within forests–considerable thermal heterogeneity occurs. Moreover, many anole species thermoregulate behaviorally–i.e., they aren’t passive samplers of the environment, their body temperature a simple reflection of the ambient, but rather they move in and out of sun and shade, and thus can determine their temperature, mediating what is available in the environment. Thus, even if the environment gets warmer, lizards may have the option simply to switch to increased use of the cooler micro-environments, maintaining the same body temperature.
Plateau in peak sprinting performance in relation to body temperature in Anolis cristatellus. Increases in body temperature over the range of ca. 31-36 C will have little effect. Figure from Gunderson and Leal (2012).
A third point is relevant as well. Physiological performance is generally temperature-dependent, but often a broad plateau exists in which maximal performance varies over a broad range of body temperatures. Hence, populations may be buffered from effects of increased temperatures if the resulting increase in body temperature does not push them off the plateau.
Both studies ask the simple question: if global temperatures go up, will lizards in open and forested habitats experience an increase or a decrease in the quality of the thermal environment, quantified in terms of how readily they are able to achieve their optimal temperature (using sprint speed as a proxy).
Anolis bicaorum from Utila, one of the forest species in the Logan et al. study. Photo by J. Losos.
The results show interesting similarities and differences. Both studies agree that forest lizards, contrary to some predictions, will generally not be bothered by increasing temperatures. The reason is that in such shady habitats, lizards generally attain body temperatures on the low end of the optimality plateau, and hence even if global warming increases temperatures in forests, it will not push them off the cliff at the other end.
Where the studies differ is in the results for the open habitat species. On the Honduran islands, Anolis allisoni will actually benefit. By contrast, in Puerto Rico, xeric-habitat populations of A. cristatellus will suffer as availability of micro-environments in which they can attain optimal temperatures will decline and the proportion of such sites which would cause them to be too hot will increase. The reason for this difference is simple. Anolis allisoni in Honduras today tends to occupy habitats toward the cooler end of their thermal plateau, and thus has room to get warmer without harm; by contrast, xeric A. cristatellus are near the hot end already and have little wriggle room to accommodate increased warming.
carolinensis clade anoles, such as A. allisoni, like it hot. Photo by J. Losos.
Why this difference? The obvious possibility concerns the taxonomic-evolutionary level at which comparisons were made. Logan et al. studied two different clades. Their open habitat species, A. allisoni, is from a clade that is a sun-loving, warm-adapted species, whereas their trio of forest species are all members of the A. lemurinus group, all of which are found in forest interiors. Hence, they are dealing with taxa well adapted to one habitat or the other. By contrast, Gunderson and Leal studied populations of the same species inhabiting very different habitats. It may well be that these populations are not ideally adapted to their respective habitats–perhaps too much gene flow between them prevents adaptation, or maybe (less likely if you ask me) the populations haven’t had enough time for such adaptation to evolve.
The differences between these two studies are intriguing, and call it for further work. Fortunately, anoles are the perfect clade for such a study given their enormous diversity in habitat use, behavior and thermal physiology demonstrated by these anoles. The bigger message, though, is clear: coarse-grained, global scale studies are helpful as a useful first step, but to really understand how lizards and other organisms will adapt to changing climates, we need detailed, species-specific studies of their organismal biology and natural history.
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Harry Greene
And of course climate change might affect the forest lizards via some factor other than temperature, e.g., insect diversity and abundance, habitat structure, predation, etc.
James (Skip) Lazell
…Yes, and climate change may well affect the forest (or any vegetation type) itself and change habitat so much that the lizards will be affected too…. What was a forest may become a cactus patch.
Alex Gunderson
Great points!
Harry, it’s true that climate change will almost certainly have indirect effects on populations (e.g., via responses of prey, predators and competitors) that are not explicitly incorporated into studies of direct effects like those outlined above. But I think that direct effects are the place to start for number of reasons, one of which is that direct effects will often modulate the indirect effects. For example, if a population is predicted to experience decreased locomotor capacity under warming (a direct effect), it may be more susceptible to predation or it may tip the balance in favor of an interspecific competitor. We argue in Gunderson and Leal 2012 that A. cristatellus in the xeric habitat will likely experience a double-whammy of direct and indirect effects due to warming: it’s physiological capacity is predicted to decrease, and it is sympatric with a more heat tolerant congeneric competitor that occupies the same structural niche (A. cooki).
Skip, other indirect effects, like changes in the structure of the forest community, can also have an impact, particularly through changes in rainfall patterns. In our case, minimal changes in annual rainfall are predicted for PR (IPCC 2007). So drastic changes in the physiognomy of the forests appears unlikely. Of course, those predictions may change!