After presenting the concept of “ecomodes” (equivalent to habitat specialist types) as an alternative to “ecomorphs,” Nicholson et al. argue that the ecomorph concept should be abandoned. My previous two posts have discussed the ecomode idea and what it can tell us about the evolution of habitat use in anoles (1,2). In this post, I analyze their chain of reasoning that leads to the call to discard ecomorphs.
The argumentation in Nicholson et al. undergoes a curious transformation. At the outset they note, rightly enough, that a number of workers have found that the ecomorph concept developed for Greater Antillean anoles does not apply to mainland species, but by the end of the paper, they conclude that the ecomorph concept is fatally flawed and must be discarded in its entirety. How do they make this leap?
Let’s start by defining what we mean by “ecomorph.” In his classic 1972 paper, Ernest Williams defined an ecomorph as “species with the same structural habitat/niche, similar in morphology and behavior, but not necessarily close phyletically.” This definition has been quoted repeatedly and is the essence of how the term has been used throughout the literature on ecomorphs and their evolution. Thus, the trunk-crown ecomorph is composed of those species that are similar in morphology, habitat use, and behavior; moreover, to constitute an ecomorph, a set of species must come from multiple lineages, instead of composing a single clade. (It’s worth noting that the term “ecomorph” was coined by Williams in reference to anoles and has since been widely applied to other taxa, as discussed in a previous post).
Now, let’s trace the Nicholson et al. argument:
First, they state (p.50): “the consistent morphological features of island ecomorphs do not emerge from studies of mainland species (Irschick et al. 1997; Pinto et al. 2008; Pounds, 1988; Schaad and Poe, 2010). Worse yet, mainland species that conform to morphological definitions of island ecomorphs do not necessarily occupy the forest stratum to which that ecomorph is supposed to be adapted (e.g., Dactyloa frenatus, Irschick et al. 1997; Losos et al. 1991: Norops altae, Savage, 2002; Schaad and Poe, 2010). The lack of uniform morphological features suggests that selection pressures associated with particular parts of the environment differ between Greater Antillean islands and mainland forests or other types of islands (Schaad and Poe, 2010).”
This is all correct; a number of studies have shown that the relationship between morphology and ecology is different for mainland and Greater Antillean species, and thus the Greater Antillean ecomorph categories—based as they are on a particular association of morphology, habitat use, and behavior—do not fit well for most mainland species. The ecomorph concept was developed and applied to island species; it does not apply—nor has anyone claimed that it does—to most mainland anoles.
However, at the end of the ecomode section of the paper, the authors conclude (p.55): “attempts to force mainland anoles into ecomorphological categories based on data from island anoles seems pointless to us.” Where did this come from? Who’s trying to force mainland anoles into island ecomorph categories? Surely, this is a valid hypothesis worth testing, and it has been tested by a number of papers cited by Nicholson et al. (e.g., Irschick et al., 1997; Schaad and Poe, 2010) and in each case, these studies have concluded that the ecomorph categories don’t apply to most mainland anoles. On the other hand, a small minority of mainland species may fit into the Greater Antillean ecomorph categories. For example, the Ecuadorian A. proboscis is extremely similar in morphology, habitat use, and locomotor behavior to Greater Antillean twig anoles, and hence it would seem appropriate to consider it a twig anole. But this conclusion was made only after detailed data on the mainland species was collected and statistically evaluated. In summary, no one is “forcing” mainland anoles in the Greater Antillean ecomorph mold; some studies have tested this hypothesis and rejected it as a general proposition; detailed field studies, however, can test–and potentially–confirm the idea for particular species.
Finally, in the one paragraph summary of the entire paper on p.69, the authors provide a new critique: “we reject the ecomorph concept as currently recognized, largely because the pattern of ecomorph evolution documented so well for Jamaica by Losos (1992b) appears to be found nowhere else in anole evolution”—in other words, only on Jamaica do the ecomorph species form a monophyletic group relative to species on other islands—“and because a high proportion of island—and especially the mainland—anoles fail to fit any ecomorph class, despite exhibiting clear habitat selection.”
This argument by Nicholson et al. does not lead to a rejection of the ecomorph concept; rather, it is irrelevant to it. Recall again the definition of an ecomorph, species similar in ecology, behavior, and morphology. This definition says that members of the same ecomorph must come from multiple lineages, but does not say that members of different ecomorphs on the same island must form a monophyletic group. Moreover, nowhere does this concept say that all West Indian species must be a member of these six ecomorph categories, although, indeed, 80% of West Indian species are, including almost all common species in lowland communities. Finally, the anole ecomorph concept was developed and applied explicitly to the islands of the Greater Antilles. That the concept doesn’t work elsewhere in no way denies the reality of the ecomorphs on those islands.
In other words, Nicholson et al. are rejecting an ecomorph concept entirely of their own invention and greatly distinct from that used by all other workers in the field. In contrast, the ecomorph concept proposed by Williams; tested, confirmed and refined by many subsequent anole workers; and widely applied to many other taxa, remains alive and well.