Anoles of South America Part 2: Diversity South of the Andes

Anolis phyllorhinus. [Photo from iNaturalist by Christopher Borges](https://www.inaturalist.org/observations/145912245)

In a previous post, I reviewed the diversity of Anolis lizards north of the Andes in South America. In this post I continue on that theme with a review of the diversity of anoles in the Amazon and south of the Amazon throughout South America. Much of this review must include the caveat that this understanding is ever-changing as new taxa are described.

I start with a list of native anoles south of the Andes in South America, which is a surprisingly short list considering the large area included and that this list also includes the Amazon basin.

Widespread throughout both Amazon and Atlantic rainforests
A. punctatus, A. fuscoauratus, A. ortonii

Widespread, but limited to the Amazon
A. trachyderma, A. transversalis, A. chrysolepis, A. tandai, A. bombiceps

Occurs in Amazon and/or llanos northwest of Amazon
A. scypheus, A. inderenae, A. vanzolinii, A. huilae, A. ruizii

Limited distribution in northern llanos/Amazon and north of Andes
A. planiceps, A. auratus

Range restricted in western and southwestern Amazon
A. phyllorhinus, A. cuscoensis, A. dissimilis, A. lososi, A. boettgeri, A. soinii, A. fitchi, A. podocarpus, A. orcesi

Widespread in sub-Amazonian (southeastern) dry forests of South America
A. meridionalis, A. brasiliensis

Limited distribution in sub-Amazonian (southeastern) Atlantic rainforests of South America
A. nasofrontalis, A. pseudotigrinus. A. neglectus

There are roughly 102 anole species that occur in all of South America. However, 67 or so of these occur north and west of the Andes (the so-called trans-Andean region) leaving perhaps 35-40 species south of the Andes (cis-Andean). The numbers don’t quite add up because a small handful of species (e.g., A. planiceps, A. auratus) occur in both regions depending on how these regions are defined geographically. So, despite the great diversity of anoles in the Caribbean and Central America, or even the respectable diversity of anoles north of the Andes, the cis-Andean region is relatively diversity poor. Causes for this lack of cis-Andean anole diversity includes the lack of meaningful geographic barriers that may make speciation less common in anoles. Alternatively, these animals are less commonly used as study subjects so subtle molecular diversity may not yet have been identified in these lineages. Either way, despite occurring throughout the Amazon basin, the llanos of Colombia and Venezuela to the northwest of the Amazon, the dry transitional forests and grasslands of Brazil to the southeast of the Amazon, as well as in the coastal rainforests of Brazil along the Atlantic coast, only several dozens of species of anoles occurn south of the Andes mountains, many of which are range restricted in a few small areas up against the mountains themselves.

Most of the anoles in this area of South America are found in the Amazon region, which makes sense given the vast extent of the rainforest ecosystem in the northern half of the continent. Despite this, there are currently only 28 or so species of anoles in the entire Amazon observed on iNaturalist.org, all but three of which are considered native (non-introduced) species. Over half of these species (16 or so) are endemic to the Amazon region (broadly defined) and found nowhere else [note, in this context, I use iNaturalist’s definition of “Amazon region” which includes the dry edges [llanos in the northwest, cerrado in the southeast] and not just the rainforest ecosystem proper, including extending all the way to the Orinoco in the north). So, clearly the Amazon is the center of diversity for cis-Andean anoles even as the number of species (25 or so native species) is not especially impressive given that there are currently over 300 species of lizards described from the Amazon. Perhaps it is this lizard diversity itself that accounts for the lack of species richness in anoles in that many of the niches occupied by anoles elsewhere (e.g., crown giants) are likely occupied by competing species (e.g. Tropiduridae, Polychrotidae). Or if not competitors, perhaps the great diversity of predators is what accounts for the lack of anole diversity (iNaturalist currently includes observations of over 300 species of snakes in the Amazon alone). Either way, anoles are clearly not the dominant taxon south of the Andes that they appear to be elsewhere. However, they do still occur nearly everywhere in South America.

The natural distribution of anoles in South America reaches south to about the latitude of São Paulo (23.6°S), or just outside the Tropic of Capricorn. This corresponds roughly to the latitude of Miami just outside the Tropic of Cancer in the northern hemisphere. However, the native Anolis carolinensis in the north has a natural range at higher latitudes, suggesting the southern extent of the anole radiation is not as great as it is in the north. As with that northern range margin for the clade, the diversity of anoles dwindles greatly along the Atlantic coast near the range edges with only perhaps one native species occurring south of Rio de Janeiro (22.9°S) (Anolis porcatus is introduced in São Paulo, but introduced species will be discussed in a future post). Even inland, there is only perhaps one species of anole that ranges as far south as Paraguay or far northern Argentina. So, even though widespread, the pattern of diversity is such that anoles appear less widespread in southern hemisphere than in the northern hemisphere.

Perhaps, one of the more interesting patterns to emerge when considering anole diversity in South America is the slow, but steady description of new species, many of which are range limited to small areas in the southern and western parts of the Amazon. The greatest number of species in the Amazon can be described as range restricted with perhaps nine species occurring in small local enclaves or areas. These include some of the more enigmatic species such as Anolis phyllorhinus with the long fleshy proboscis in the males. Thus, as with the trans-Andean anoles, the geographic complexity of the Andes has likely created conditions for some version of allopatric species to take place, whether it be true allopatry or some type of peripatry on a range’s periphery. I fully expect that additional species will be described from the Andean foothills or the western and southwestern Amazon.

Finally, it is interesting to consider the near complete replacement of species on either side of the Andes with very little crossover; clearly the Andes are a fairly effective barrier to gene flow for anoles. A few species appear to occur on both side of the Andes in the area south and east of Bogotá, Colombia. However, this crossover appears to be limited in terms of diversity and extent of species. Instead, the biggest exchange of species appears to have occurred around the northern terminus of the Andes in Venezuela where two species in particular span the cis- and trans-Andean regions: Anolis planiceps and A. auratus. Anolis planiceps occurs along the Caribbean coast of Venezuela as well as south into Colombia, Guyana, and Roraima, Brazil. However, Anolis auratus as currently defined occurs much more broadly ranging from Costa Rica and Panama, throughout cis- and trans-Andean regions of Colombia and Venezuela, along the Atlantic coast to at least Amapá, Brazil, and south into the states of Roraima and Pará, Brazil. In Brazil in particular A. auratus seems to be limited to areas of geologic uplift that have created the grassland habitat with which this species is associated. The highlands to the north and south of the Amazon river were created when the Guiana Shield and Brazilian Shield, respectively, were forced upward by geologic uplifting. Hence, the Amazon river roughly runs in the low areas in-between the uplift of these two large blocks on the South American continent. Just south of the Amazon River, an isolated population of Anolis auratus occurs in a place called Alter do Chão (translation from Portuguese is literally ‘altar of ground’) where a small piece of the raised uplift creates savanna-like grassland/forest habitat surrounded by lowland Amazon rainforest to the south and east and bordered by the Amazon and Tapajós Rivers to the north and west, respectively. The presence of these lizards in as a small and isolated enclave 1000 km from their nearest neighbors generates more questions than answers. How long have they been there? How did they get there? How distinct are they? How stable is this population? These are all good and unresolved questions remaining to be addressed biologically.

I saved the above observation for last to make the point that on the surface the Amazon may appear to be a vast tract of uniform habitat with little barriers to gene flow leading to biological uniformity, but the reality is that this view is overly simplistic in the context of how species are actually distributed on the landscape. Rather, it is likely the lack of available data that prevent us from truly understanding the biogeography of lizards in the Amazon and other areas of South America. This makes the continual addition of new species and biogeographic patterns useful and exciting given the advances in molecular biology that continue to further our understanding. Hence, even in the relatively anole-poor regions of South America, anoles continue as a model system for understanding patterns of biological diversity.