Of Ecomodes And Ecomorphs: I. Are The Data Available To Categorize The Habitat Use Of All Anoles?

Anolis lividus. Trunk anole? Trunk-ground? Trunk-crown? Photo by Jonathan Losos

Mainland anoles exhibit a great diversity in habitat use and morphology, a topic we have discussed previously on AA. For this reason, an analysis of patterns of evolution in habitat use across all anoles, not just mainland species, would be very welcome. Nicholson et al. step into the breach by presenting habitat categorizations for a large number of mainland species, as well as for most West Indian species, and then analyzing habitat evolution on their preferred phylogeny. Along the way, they coin a new term, “ecomode,” argue that the ecomorph concept is fatally flawed and should be discarded, and present a scenario for patterns of ecological diversification in both mainland and island anoles. Although I applaud the effort to understand ecological evolution in mainland anoles and welcome the attention this paper brings to an important and little-studied question, I find the conclusions unconvincing. In this post, I discuss whether the data are sufficient to create categories of habitat use and confidently assign species to them; in subsequent points I will discuss the analysis of habitat use evolution and Nicholson et al.’s critique of the ecomorph concept.

What is an “ecomode”? The term is not explicitly defined in Nicholson et al., but it appears to refer to different categories of habitat use. The problem with creating such categories and assigning species to them is two-fold. First, most anole species use a variety of different habitats. I like to say that you can find almost any anole anywhere sometimes. More specifically, most anole species use the trunks of trees, often at different heights, and most can be found on the ground occasionally. How, then, do you distinguish a trunk anole from a trunk-ground or a trunk-crown anole, or a trunk-ground from a grass-bush? Second, how can one make sure that a given species fits into a single category? Perhaps some species have a broader niche that encompasses multiple ecomodes, or perhaps a species slices up the environment in an entirely different way (e.g., a trunk-bush or twig-ground species)?

Previous workers (including me) have been able to define ecomorphs and categorize species for two reasons. First, the ecomorph categories are defined not just on the basis of habitat use, but also by reference to morphology and behavior. Indeed, the morphological differences between ecomorphs are quite clear, and they correlate strongly with habitat use and behavior. One may quibble with a few assignments (e.g., is A. opalinus a trunk-crown or trunk anole?), as I discuss in Chapter 3 of Lizards in an Evolutionary Tree, but for the most part, assignment to ecomorph category is clear-cut (including the category of “non-ecomorph” for the minority of West Indian species that fail to meet the morphology/behavior/ecology criteria of any of the ecomorph categories).

The second reason we can make these assignments is because we have quantitative data that can be statistically analyzed. By contrast, the Nicholson et al. assignments are subjective decisions based on a reading of the literature, often relying on short summaries in broad regional reviews such as Savage’s (2002) The Amphibians and Reptiles of Costa Rica and Henderson and Powell’s (2009) Natural History of West Indian Reptiles and Amphibians. Use of these summaries is problematic for two reasons. First, although some mainland species have been studied extensively and quantitatively (e.g., the work of Vitt, Fitch, and Andrews), the habitat use of many species is not well studied. As a result, evaluating some summaries can be difficult because one does not know the extent and quality of the underlying data—in some cases (not Savage or Henderson and Powell), I suspect summary statements are not based on any hard data at all, but just qualitative impressions. In addition, even when species have been studied extensively, going from an encapsulated summary of such studies to an ecomode categorization is often not straightforward. For these reasons, the Nicholson et al.’s assignments of species to specific habitat use categories in many cases may not be reliable.

West Indian Non-Ecomorph Species

I will illustrate these problems by first discussing Nicholson et al.’s treatment of West Indian non-ecomorph species. For these species, there are a number of errors resulting from trying to interpret summary information provided in overview volumes. In addition, because anoles use a variety of habitats, it can be difficult—especially with qualitative descriptions—to categorize a species. How does one decide whether an anole that uses tree trunks should be categorized as a trunk, trunk-ground, or trunk-crown anole? And even grass-bush anoles occasionally are found on tree trunks. Moreover, it is not even clear that the non-ecomorph species can be pigeon-holed into any one of these categories, as opposed to the possibility that these species carve up the environment in some other way that either combines the habitats of the ecomorphs or slices them in some different way.

Here are some examples:

Consider the mid-sized anoles of the Lesser Antilles (e.g., aeneus, lucius, roquet, trinitatis, marmoratus, oculatus, sabanus). These are species with very similar habitat-use; basically, they occur on trees perching on average at eye level, but sometimes found high up in the tree, sometimes low or on the ground, on building walls, bushes—in other words, a very broad habitat use. Using Henderson and Powell (2009), which provides an unsynthesized compilation of every published literature report, Nicholson et al. categorize some species as trunk-ground, others trunk-crown, and others as trunk anole; oddly, marmoratus is listed as “trunk/ground/grass though “grass” is not a recognized ecomode. How they arrive at these different categorizations is not clear, but seems to reflect slight differences in the literature summaries for these species; I suggest that most readers, reading the accounts for each species (or seeing the species in the field), would be much more impressed by the similarities in their reported habitat use than the differences. Moreover, with the possible exception of A. gingivinus, none of these species, nor some non-Lesser Antillean species such as A. conspersus, corresponds to the very ground-directed habitat use of West Indian trunk-ground ecomorphs, even though several are categorized as trunk-ground anoles. The bottom-line is that it is difficult to partition trunk-using non-ecomorph species into different categories; the data as amalgated by Henderson and Powell are not sufficient to make such distinctions. Moreover, it is possible that the species may not even fit into a single one of these categories, but rather may have a broad habitat use encompassing several more specialized ecomodes.

A trunk anole? Photo of a captive juvenile from http://www.gherp.com/kingsnake/scientific/lizard/Chamaeleolis%20chamaeleonides%20lgcc.jpg

In addition, there are a number of cases in which the assignments are simply incorrect, due to reliance on inadequate information in the literature. For example, three Cuban Chamaeleolis species are categorized as trunk anoles, the other as a crown-giant. However, these categorizations are based on very brief statements by Schettino Rodriguez (The Iguanid Lizards of Cuba, 1999), themselves based on who-knows-what data. In fact, the only detailed ecological study of Chamaeleolis shows that they definitely are not trunk anoles, but primarily use relatively narrow branches and probably should be considered twig anoles (Leal and Losos, J. Herpetology, 2000).

Anolis christophei is listed as semi-aquatic, presumably because one obscure paper says that it enters the water, but this species’ habitat use is in no way similar to that of the other semi-aquatic anoles of the West Indies or mainland. If anything, it is a trunk anole.

Anolis fowleri is considered a twig anole, probably because Williams (chapter in Lizard Ecology: Studies of a Model Organism,1983) labeled it as such, but no habitat use data have ever been collected for non-sleeping A. fowleri so we have no idea what it does (many anoles of all types can be found sleeping on twiggy surfaces).

Nicholson et al. categorize A. argillaceus and A. centralis as trunk anoles, but their close relative A. pumilus as a grass-bush anole. However, Henderson and Powell (2009) describe the habitat use of A. centralis thusly: “on fences, trees, in shrubby areas, and on spiny bushes and neighboring xeroyphytes; an individual on the rocky bed of a deeply shaded stream,” which sounds as much like a bush anole as a trunk anole. Moreover, Schettino Rodriguez (1999) also describes A. centralis as a bush anole, with a description very similar to that of A. pumilus. In reality, the habitat use of these species probably lies somewhere between that of grass-bush and trunk-ground ecomorphs; forcing them into one category or another is problematc and, regardless, putting them into different categories is misleading.

The bottom line is that—for these species which I know very well—I would not put a lot of stock in the habitat categorizations of these species.

Mainland Species

But what about the mainland species? These are taxa I don’t know as well, but I suspect the situation is not much better. First, although some mainland species have been studied extensively, for many—perhaps most—the available data are quite limited. Moreover, just as with the West Indian non-ecomorph species, categorizing species that occur somewhere on the trunk into one of their ecomodes is not straightforward, particularly when the data are few and qualitative, and when it’s not even clear that the species correspond to any of the ecomodes.

Here are some examples:

Nicholson et al. put A. altae in the trunk-ground ecomode and A. intermedius in the trunk ecomode, based on Savage (2002). Savage says that intermedius is: “essentially [an] arboreal species that perches and forages between 0.3 and 2 m above the ground on low trees, bushes, or trunks of taller trees.” But Pounds’ (1988) highly detailed study found that intermedius often uses the ground and that the habitat use of altae and intermedius were basically indistinguishable. It would seem both should be considered trunk-ground anoles in the Nicholson et al. framework.

For another example, compare A. frenatus, categorized as a crown-giant by Nicholson et al., and A. woodi, categorized as a trunk anole. For both species, Savage (2002) is cited as the source of information. Here’s what that reference has to say. For frenatus: “It is a thermoconformer that uses tree trunks for perches, but actively feeds on the leaf litter once it finds a prey item. Although [data from elsewhere] indicate that D. frenata perches 0.3 – 2.1 – 6 m above the ground, observations in Costa Rica suggest that large males frequently perch at the 4 to 5 m level.” And for woodi: “This common diurnal scansorial lizard forages on tree trunks; perch sites occur from near ground level to a height of 4.5 m.” Based on these descriptions, these two species seem to be very similar and I cannot understand why they are categorized differently.

Anolis lemurinus. Photo from http://www.ohio.edu/people/brune/belize-dec04/norops_23dec04_3b.jpg

On the other hand, consider another species categorized as a trunk anole, A. lemurinus. Savage describes it as: “a strictly diurnal species that perches on tree trunks 0.5 to 3 m above the ground,” but Leenders (2001) in his book A Guide to the Amphibians and Reptiles of Costa Rica has a very different take on what he calls the Canopy Anole: “Canopy anoles rarely venture out of the canopy to the forest floor. The crown of a large rainforest tree may harbor a substantial population…which travel nimbly through the maze of branches by jumping from one limb to limb. This species often exposes itself when basking on tree trunks, branches or leaves…” And just to make things more complicated, Stafford and Meyer in A Guide to the Reptiles of Belize (2000) say this: “generally encountered in lower understory vegetation and on the forest floor. Most commonly observed perched face downward on vertical stems, tree trunks and buttress roots up to 2 m or so above the ground, rarely higher.” Nicholson et al. have a “polymorphic” category which they apply to four species which don’t seem to fit any category; these differing views, and my own observations, suggest that this is where lemurinus belongs.

In addition, I note that the “grass-bush” categorization used for mainland species in Nicholson et al. is generally different from what is meant by that term for West Indian ecomorphs. In the latter case, grass-bush anoles normally use dense vegetational matrices, either grasses or thick, low-lying shrubs. By contrast, as applied by Nicholson et al. to taxa such as A. limifrons and A. polylepis, this category refers to species that use narrow diameter vegetation and leaves low to the ground, regardless of whether that vegetation occurs in a dense vegetational matrix, which in most cases it does not. Most notably, West Indian grass-bush anoles are rarely found on broad tree-trunks, but species like limifrons and polylepis regularly use such habitats. Given this discussion, I don’t understand Nicholson et al.’s distinction between the “ground bush” and “grass bush” ecomodes.

Anolis carpenteri. Photo by Fred Janzen.

Finally, as with the West Indian anoles, there are some categorizations that I consider to be inaccurate (and note that I have no opinion on the many mainland anoles with which I’m not familiar). First, A. carpenteri is considered a trunk-ground anole, based on Savage’s description “although the lizards are sometimes found on boulder-strewn stream banks and in vegetation tangles in swamp forests, they are usually seen on low perches (0.5 to 1 m above the ground) or occasionally on the ground…may spend more time [than limifrons] higher up on tree trunks.” It’s hard to picture the habitat use from this description, but in my experience, carpenteri is always found on vegetation well off the ground and in no way does it seem like a trunk-ground anole. My guess is that it is probably most common high in the vegetation, where it is very hard to spot from the ground. In this, I agree with Corn, who possibly has seen more carpenteri than anyone else and says this in his unpublished thesis (1981, U. Florida, p.19): “I would suggest that the apparent rareness of A. carpenteri reflects its preference for lichen-covered perches, probably at moderate heights….like several other rain forest reptiles (for example, see Corn, 1974), only encountered when disturbed from its high arboreal habitat.”

Second, the phenacosaurs A. inderanae and A. nicefori are labeled as “ground bush,” whereas A. heterodermus is considered a twig anole. However, the source cited for this information, Miyata (1983) never refers to inderenae or nicefori. Very little is known about the habitat use of inderenae or nicefori, but the very brief statements in Rueda and Hernández (Trianea, 1988) and Barros et al. (Breviora, 1996) and seem very similar to Miyata’s description for heterodermus, calling into question the disparate assignments of Nicholson et al.

Why does this all matter? Does it really matter if some–possibly substantial–proportion of species are misclassified into the wrong ecomode category, or if these categories are not, in fact, a good way of describing variation among species in habitat use? I would argue that it does, because these categories, now that they have been presented and every species assigned to one, will be used by researchers to examine questions such as: does morphology or life history or any other variable differ among ecomodes? Or, do mainland vs. island members of the same ecomode differ in these sorts of variables? For this reason, I recommend that researchers not use these categorizations without independently examining the underlying data and coming to their own decisions.

On the other hand, in fairness to Nicholson et al., the main conclusion they draw from their analysis is that anoles are ancestrally crown dwellers and that their evolutionary history, writ large, is a story of movement down the tree toward the ground. For this particular conclusion, perhaps it is not important if one can’t confidently distinguish trunk anoles from trunk-ground anoles or ground bush from grass bush anoles; put another way, it’s unlikely that the imprecision of these categorizations will lead to many major mischaracterizations with reference to degree of arboreality, say, incorrectly labeling a ground anole as a trunk-crown species. Thus, although the analysis would be more stronger with more precise, quantitative data, for the purposes of the conclusions of this paper, I suspect the authors’ categorizations are not greatly misleading.

The ultimate message, however, is that more data are needed on the natural history of mainland species. By compiling such data sets, we will be able to really get at questions of habitat evolution and the causes for differences that exist between island and mainland species.

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Jonathan Losos

Professor of Biology and Director of the Living Earth Collaborative at Washington University in Saint Louis. I've spent my entire professional career studying anoles and have discovered that the more I learn about anoles, the more I realize I don't know.

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