Anole Annals

Large prey taken by brown anoles (top two photos) and Swinhoe’s tree lizard (bottom two).

Starting in the 1970s, Caribbean anoles became a model system for studying community ecology, especially interspecific competition. Such studies generally focused only on anole species. Though seemingly chauvinistic, this anolocentrism is reasonable in many localities, where resource competition probably is primarily between anole species (although there was a boisterous debate in the 1980s on the extent to which anoles and insectivorous birds might compete).

However, this is not always the case. In Central and South America, for example, the much greater non-anole saurifauna than on Caribbean islands makes it likely that anoles may experience much greater resource competition with non-anole lizards, as well as other taxa. And the same may be true for anoles introduced to far-flung regions.

Take, for example, the brown anole in Taiwan, which occurs with the native Swinhoe’s tree lizard. Like brown anoles, the agamid is found on the ground and low on tree trunks, and thus might be considered a trunk-ground anole. Being only slightly larger than brown anoles, the tree lizard probably eats much the same food. Gerrut Norval posted a while back on the amazingly large prey that brown anoles and tree lizards eat in Taiwan, and now he and colleagues have published a paper documenting the extensive diet overlap between the species (Gerrut previously provided a post on the background to this study, including some interesting information and photographs on the research methods). Very likely they are strong competitors, although Norval et al. argue that the size discrepancy means that the effect is asymmetric. However, at least in some areas, brown anoles have much higher densities, meaning that their aggregate effect on tree lizards may be just as great as the reverse.

Brown anoles are most dense in hot, open areas, whereas the tree lizards reign supreme in shaded habitats, suggesting that environmental effects mediate the outcome of interspecific interactions between the two species. In addition, this difference indicates that reforestation efforts would be a good conservation move to stem the effect of the brown anole invasion.

Do events unfold in a predictable sequence when organisms undergo adaptive radiation? Anoles have diversified in many ecologically important characteristics as they have radiated both in the Caribbean and on the mainland. As one of our best-understood cases of extraordinary evolutionary diversification, they make a great system in which to ask how ecological diversity builds up during adaptive radiation.

The idea that anoles radiated in stages dates to at least 1972, when Ernest Williams derived some hypotheses from his observations of Puerto Rican Anolis in particular, drawing upon earlier work by Stanley Rand and Rodolfo Ruibal. Williams noticed that the most closely related species on Puerto Rico tend to belong to the same ecomorph class and occupy similar structural habitats (e.g. branches, trunks or twigs), but occur in different thermal habitats (e.g. closed forests or hot open areas). He proposed that anoles on Puerto Rico diversified first in structural habitat, and later in thermal habitat, a pattern that might scale up to the entire adaptive radiation of Anolis. While this idea has been discussed many times, and helped to inspire more general hypotheses about stages of radiation (e.g. Streelman and Danly 2003), until now it had not been tested using modern analytic techniques that incorporate phylogenetic information for many species.

Figure 11 from Williams (1972), modified to show only the 8 species in the main Puerto Rico radiation.

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Over at The Lizard lab, Martin Whiting discusses a recent paper published in Biological Conservation on the conservation status of reptiles. Basically, a cast of thousands assessed a random sample of 16% of the world’s reptile species, categorizing them into the IUCN’s categories of conservation concern, which range from “least concerned” to “critically endangered” and, of course, “extinct.”

Martin nicely summarizes the paper in his post, but I’ll reprint his conclusion summary paragraph here: “59% of species were Least Concern, 5% were Near Threatened, 15% Threatened (Vulnerable, Endangered or Critically Endangered) and 21% were Data Deficient. To put this another way, one in five species are threatened with extinction and another one in five are data deficient. The paper identifies freshwater habitats, oceanic islands and tropical regions as containing the highest proportion of threatened species. Habitat loss and direct harvesting are two key threats to reptile populations and these are depicted in Figure 3 from the paper” (above).

Of course, from the AA perspective, the question is: what about anoles? The results were, to me at least, surprising. Of the 65 species surveyed, 29.3% were in one the three threatened categories, nearly twice as many as the global average! I would have guessed the opposite–most anoles seem to being doing reasonably well. But, then I rationalized, it must be the mainland anoles, because Caribbean anoles are generally doing fine. Again wrong! 11/28 (39%) Caribbean anoles are in these categories (including the only two “critically endangered species, A. juangundlachi (known from one specimen, if I recall correctly) and A. roosevelti), compared to 8/37 (22%) for mainland species. One non-surprise is that all 10 “data deficient” species are from the mainland; however, even when they are removed, the percentage threatened in the mainland (30%) is still less than in the Caribbean. At least for the Caribbean species, the biggest predictor seems to be range size, as all threatened species either have small distributions or occur on small islands. I am less familiar with some of the mainland species, but think the same may be true for those. I’ll append the list below.

One last note: the paper truly has an extraordinary number of authors who contributed to this massive compilation. One amusing consequence is that the list of authors’ affiliations at the start of the paper is three pages long!

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