Anolis cristatellus from mesic habitats. Photo by Manuel Leal.
For obvious reasons, there is great concern about how species will cope with climate change–as the world gets hotter, will species be able to survive? Many studies have taken a macroscopic view, examining the geographic distribution of a species to divine what its temperature tolerances are and then projecting where it will be able to occur in the future. Although such approaches are useful as a first pass, direct study of the physiology of species is a much more informative way of determining how a species will be affected. An excellent example of just this approach was published recently by Gunderson and Leal in Functional Ecology (pdf here).
Mesic and xeric habitats. Photos courtesy Alex Gunderson (left) and Manuel Leal (right)
The authors studied the Puerto Rican crested anole, A. cristatellus, which occurs throughout Puerto Rico and the Virgin Islands. They focused on comparing populations living in cooler, wetter (mesic) habitats versus those living in hotter, drier (xeric) places. They found that in the field, mesic populations had an average body temperature of about 29 C, whereas xeric populations averaged 32.5 C. However, using copper models as described in previous posts, the authors determined that a lizard randomly placed in a mesic habitat would have a temperature of about 29 C, whereas the random xeric lizard would be 33.5. In other words, the lizards are not thermoregulating in the mesic forest (lizards and randomly placed copper models have the same temperature), but they are actively altering their habitat use in the xeric areas to use cooler spots and thus keep their temperature lower than if they were sitting in random spots. In support of this conclusion, the mesic lizards were in the sun about as much as expected, but the xeric lizards were in the sun less often than predicted.
Sprint speed of Anolis cristatellus is maximum at about 32 C, but varies little from 29-34, before dropping off steeply. Populations from xeric and mesic habitats have nearly identical performance curves.
The authors also investigated the effect of temperature on physiological performance. Like many researchers before them, they used maximum running speed as an ecologically relevant index of physiological performance. And, like previous work on anoles, they found that sprint speed increased with temperature up to a point, after which it sharply declines. Most importantly, there is a broad plateau at which change in body temperature has little effect on temperature, in this case from roughly 29-34 C. The result is that all cristatellus populations, both xeric and mesic, are currently well adapted to the environments in which they occur.
But what about when it gets warmer? Inspection of the graph says it all: even if the mesic lizard habitat gets a few degrees warmer, lizard body temperatures will still be on the plateau where performance is maximal. But the xeric lizards, already to the right of the hump, have little wiggle room–if they get warmer, their performance will decline dramatically. Gunderson and Leal analyzed this in greater detail. Assuming that the predicted 3 C raise in temperature over the next century uniformly increases temperature by that amount throughout the habitat, the proportion of sites within xeric habitats that will cause lizards to have suboptimally hot body temperatures will rise from 7% today to 41% in a century, leading to an average decrease in physiological performance of 30%. By contrast, the proportion of suboptimal sites in the mesic habitat will not change and predicted physiological performance should actually slightly increase (3.5%).
It’s not clear how a decrease in individual performance like that projected for xeric habitats would translate into population effects, but it can’t be positive. Of course, another outstanding question is whether anole physiology might be able to evolve–and quickly–to match the new conditions.
Clearly, this work is just a starting point to understand how a warming world would affect its ectothermic denizens. Detailed physiological and field study–though time and labor intensive–is the best means of predicting how species will be affected, and the study by Gunderson and Leal is a first rate example of how such work can be conducted.
Alex R. Gunderson, Manuel Leal (2012). Geographic variation in vulnerability to climate warming in a tropical Caribbean lizard Functional Ecology DOI: 10.1111/j.1365-2435.2012.01987.x
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James Lazell
Very interesting. This will be especially important on small dry islands like many VI. I wonder what will happen to Anolis ernestwilliamsi on Carrot Rock, BVI. Let’s decrease the human population and stop all this environmental wrecking.