In my three previous posts [1,2,3], I have discussed Nicholson et al.’s ecomode concept and their conclusion from it that the ecomorph concept should be rejected. Here I conclude my discussion by addressing two other related points raised in Nicholson et al., whether differences in forest structure are responsible for different evolutionary patterns in the islands and on the mainland, and their critique of my 1992 paper on the sequence of ecomorph evolution.
Are Differences in Forest Structure Responsible for Different Evolutionary Patterns in Mainland and Island Anoles?
Nicholson et al. state (pp. 54-55): “In discussing differences between island and mainland anoles, Losos (2009) considered, but dismissed, forest structure as a driving factor in shaping anole assemblages, suggesting that, to anoles, a tree is a tree…[W]e are impressed with the complex nature of the moist, wet, and rain forests of Central and South America (Solé et al. 2005) that are home to the majority of anole species. The heavily fluted bark of Neotropical rainforest canopy trees such as Lecythis must require substantially different limb and toe pad shapes in anoles that use these trees than those that use the smooth bark of canopy trees such as Pterocarpus. The facts that bark texture is likely to be much more diverse in mainland than island forests, and that trees with appropriate bark texture are likely to be so much more widely dispersed in mainland than island forests, must play an important role in making morphology of mainland anoles so much less predictable than it is for island anoles. The fact that island forests are dominated by a relatively few short, smooth-barked tree species must limit the number of morphs that anoles can attain, must increase the density that anole populations can maintain, and must increase the interactions among sympatric species above that experienced by mainland anoles. Additionally, the differences in the structure of understory shrubs associated with mainland areas possessing an ancestral fauna that includes grazing mammals, compared to island areas that lacked such grazers (Dirzo and Miranda, 1990), must affect habitat available for adaptive radiation in anoles. In short, we see little evidence that the assembly rules proposed for anole communities on Caribbean islands will ever be discovered as applicable to mainland anoles, because the factors shaping vegetation structure are so different between island and mainland forests.”
And by the end of the paper (p.68), the idea has been transformed into a firm conclusion: “We note that evolution of ecomodes appears to be widely constrained within anoles and does not necessarily lead to constrained morphology within an ecomode because variation in forest structure across the geographic range of anoles is so great.”
It is certainly plausible that differences in vegetation structure between mainland and island forests are responsible for different patterns of ecomorphological evolution in the two regions. But what is the evidence for this? I have actually looked for comparisons of structure between mainland and island forests and have not found any relevant literature. The authors only cite two papers and neither documents differences between mainland and island forests: Solé et al. (2005) is about differences between canopy and understory at Barro Colorado Island, and Dirzo et al. (1990) is a comparison of mainland sites with and without large mammal herbivores (note: these references were presented by Nicholson et al. to document appropriate points about mainland forests; I am not claiming they were inappropriate citations, only that application to Caribbean forests is entirely an extrapolation of the authors). The authors may well be correct that mainland and island forests differ, but they do not provide any evidence to support this claim. Moreover, even to the extent that mainland and island forests do differ in structure, the effect such differences have had on anole evolution is entirely conjectural (e.g., perhaps different bark texture would select for differences in toepad structure, but to date, there are no data relevant to such a claim).
Indeed, one may question how likely it is that differences in tree structure actually affect anole morphological adaptation. Continue reading