Anoles are probably best known for the ecomorph story: the presence of specialized species adapted to the same sets of structural microhabitats on different islands. Anoles in the Greater Antilles have contributed hugely to our understanding of both the evolutionary history and the contemporary ecology of communities of specialists.
While they are better known for specialization of species in communities, anoles have also contributed to our understanding of within-species ecological diversity. Around the same time that Ernest Williams was developing the ecomorph concept, Roughgarden (1972) used data from Lesser Antillean anoles to introduce a new framework for investigating the extent to which a population’s niche width (i.e. the diversity of habitats it uses or prey it eats) is determined by variation among individuals versus variation within individuals. For example, individuals in a population of Anolis roquet differ in the size of prey they consume, mainly because larger individuals can catch and ingest larger prey items. While Roughgarden’s early work set the stage for an explosion of studies of individual specialization over the past decade or two (reviewed in Araújo et al. 2011), surprisingly little work has been done to revisit individual specialization within species of anoles. In particular, we don’t know enough about how much individuals specialize in important aspects of microhabitat that differentiate ecomorphs, especially perch height and perch diameter.
Anole Annals contributors Ambika Kamath and Jonathan Losos have helped to fill this gap with a study just published online in Evolution. Ambika and her team spent a summer observing microhabitat use of a population of brown anoles (Anolis sagrei) in a forested park in Gainesville FL. They marked lizards with colored beads, and repeatedly recorded individual lizards’ perch height and diameter, compiling a total of over 1000 observations of 80 anoles. They grouped perch heights and perch diameters into classes, then compared the distribution used by each individual to the distribution used by the whole population (or to the distribution available to that individual) using a proportional similarity index. The mean value of this index gives a measure of the overall degree of individual specialization in a population, as lower overlap values tell us that individuals are specializing on a subset of the available perches.
The results showed that individual brown anoles do in fact use only a subset of the perches available to them. On average, the perch use of an individual was around 40% dissimilar from that of the population as a whole, indicating fairly strong (and statistically significant) individual specialization. In fact, they found that the degree of individual variation relative to the population of A. sagrei was comparable to the degree of specialization by species of different ecomorphs in the diverse Soroa community in Cuba! While the degree of specialization was similar, the population of A. sagrei did not show the same relationships between habitat and ecomorphology (limb length and toepads) that are seen across species in the Greater Antilles, indicating that this within-species specialization is not a perfectly scaled-down version of specialization by species within communities. The authors suggest a possible explanation: within a species, reproductive success may be more important than fit between morphology and habitat, so factors such as dominant males getting access to higher perches may override any tendency for individuals to find the habitats that best match their morphological traits.
Anoles are exceptionally well-suited to studying specialization of both individuals within populations and of species within diverse communities, and this study is a nice step in that direction. Still unknown is whether the extent of individual specialization in this population is representative of populations in the species’ native range. The native green anole A. carolinensis is the only other anole in A. sagrei’s invaded range in northern Florida, whereas in the Soroa community, A. sagrei coexists with at least nine other anole species (Rodríguez–Schettino et al. 2010). Anolis sagrei seems to use a greater diversity of perch heights and diameters in Florida than in Cuba – maybe the absence of many other competing species has opened up new microhabitats to the brown anole. If individuals are constrained in how generalized they can become, this ecological release can occur via increased individual specialization – this idea is called the Niche Variation Hypothesis and has been debated for several decades (Van Valen 1965). Anoles provide plenty of opportunities to study whether the extent of individual specialization changes as a result of the colonization of new habitats or the arrival of new competitors.
NOTE: Ambika Kamath provides the backstory on how the study came to be on her blog.
Read the paper:
Araújo, M.S., D.I. Bolnick, and C.A. Layman. 2011. The ecological causes of individual specialisation. Ecol. Lett. 14: 948–958.
Rodríguez–Schettino, L., J.B. Losos, P.E. Hertz, K. de Quieroz, A.R. Chamizo, M. Leal, and V. Rivalta González. 2010. The anoles of Soroa: Aspects of their ecological relationships. Breviora 520: 1–22.
Roughgarden, J. 1972. Evolution of niche width. Am. Nat. 106: 683–718.
Van Valen, L. 1965. Morphological variation and width of ecological niche. Am. Nat. 99: 377-390.