Unique among the several hundred species of the genus Anolis native to the Neotropics, one, the invasive Brown Anole (Anolis sagrei) has successfully colonized the United States. This species was repeatedly introduced into the United States from Cuba and the Bahamas beginning in the 1880s in the Florida Keys. By the 1940s, the invader had established itself throughout southern Florida and today it is distributed throughout the Florida peninsula. The invader also has turned up in Florida’s northern counties and in coastal southern Georgia, Louisiana, Texas as well as in Hawaii. Winter minimum temperatures are believed to be the principal factor limiting A. sagrei’s distribution; but survival in areas that regularly experience sub-freezing temperatures suggests that the species is successful at finding overwintering sites that afford local protection. There also may be some inherited adaptation to lower temperatures in some of the more northern populations. Something similar has been substantiated for A. cristatellus, the Puerto Rican anole. Individuals of this species established in Miami were shown by Leal et al. (2012) in cold tolerance experiments to tolerate temperatures several degrees lower than Puerto Rican anoles. This cold response represents an evolutionary adaptation expressed in only 35 generations, the duration of the anole populations in the United States.
One consequence of this species’ aggressive range expansion, which might have been accelerated by global warming, is an increase in competition with the green anole (A. carolinensis) the only anole native to the United States. Eight other anole species have established reproducing populations in the United States with the help of human activity, but none approaches the brown anole in numbers or range.
Populations of A. carolinensis have declined in central and southern Florida from the effects of habitat destruction and the conversion of native vegetation to artificial plantings (usually non-native tropical species) and landscaping where brown anoles far outnumber green anoles. Direct negative interactions between the two species are widely suspected. They are similar in size ( a total length of about 8 inches in adult males) suggesting competition and A. sagrei adults have been observed eating young of the native anole but there is little evidence of the reverse. Both species, however, have been observed eating each other’s eggs. A. sagrei is a habitat generalist in native and non-native lands, occupying everything from mangrove forests to golf courses. In native vegetation, some observations reveal that where the two species come into contact, A. sagrei has appropriated perch sites close to the ground that A. carolinensis formerly utilized before the invader established itself. Although the full implications of this displacement are not known, the net effect appears to be a decline in densities of A. carolinensis where the two species co-occur.
Among the differences in the two anole species, A.carolinensis is relatively short-legged and A. sagrei long-legged. The latter uses speed more to capture prey and to avoid predators to which they may be especially vulnerable given their frequent dew-lap displays and tendency to perch on a tree trunk near the ground where they engage in “sit-and-wait” predation. A. sagreii are highly territorial and spend much time surveying from their perch for conspecifics or just displaying. The short-legged green anoles are more likely to spend time on twigs rather than large diameter trunks or branches and to be slower runners that methodically search for prey.
Unlike A. carolinensis, A. porcatus adapts readily to human environments but presumably co-existed with A. sagrei on the Caribbean islands well before the heavy influence of humans there. The factors restricting A. porcatus in the United States are not known. Neither is the mechanism by which A. porcatus (or some predecessor taxon) colonized the United States from Cuba. The hurricane-driven rafting of downed trees or logs across water bodies is known to occur; and with the distance between Cuba and the United States across the Florida Straits as short as 50 km, (93 miles), such a process is conceivable although unproven. An even shorter distance lies between the Bahamas and easternmost Florida. However, studies of the ability of brown anoles to stay afloat in seawater for hours (up to 24 hours) with their heads held above the water suggests that this species is able to float between islands on the prevailing currents (Schoener and Schoener 1984).
Apparently, the North American population of A. carolinensis can be traced genetically back to a source population in Cuba of the closely-related A. porcatus (the Cuban Green Anole) which is restricted in the United States to small populations in Miami-Dade County. A. carolinensis and A. porcatus are difficult to distinguish morphologically, and doing so, may require having a lizard in hand (A. porcatus appears slightly bulkier and more intensely colored). Nevertheless, they comprise two distinct species.
In a twist of fate, A. carolinensis is also becoming a pest outside its native range. It is now established on several Pacific Islands, in Hawaii where it once again encounters A. sagrei, and on the southern Japanese island of Ogasawara where it is believed to have contributed to the extinction of several endemic species of beetles. The invader also is guilty of reducing populations of odonates (dragonflies and damselflies), various pollinators and other beetle species on the Japanese island (issa.org). In Hawaii, the two exotic anoles not only compete with each other but also with another exotic–the gold dust gecko which was introduced from Madagascar (Losos 2011).
Recently, it has been found that A. sagrei has transmitted worm parasites to Hawaii where they have infected local anoles (tsusinvasives.org). A study of parasitic worms from 10 anole species in the northern Lesser Antilles found parasites from nine different genera- 6 nematodes (tiny roundworms), two digeneans (flatworms), and an acanthocephalan (a worm with spines used to attach to the gut wall of its host) (Dobson et al. 1992).
On islands of the Puerto Rican Grand Bank,which includes the islands of Puerto Rico and most of the Virgin Islands, an acanthocephalan parasite studied by Falk (2013) was found to parasitize both anoles and termites. Infection of termites rendered them easier to capture by anoles; but in doing so, the anoles themselves become infected. The worm encysts in the lizard’s body cavity where it remains in the larval stage until the anole is ingested by a predator.
But not just any predator. If the anole is eaten by a pearly-eyed thrasher (a bird native to the Caribbean islands) or by a mongoose (an introduced predator not native to the Caribbean), the ingested worm persists in the larval phase in the new host. Only in the domestic cat, which lives as a feral animal on many Caribbean islands, does the worm reaches adulthood and reproduces. One cat was found to bear 87 adult acanth worms! The parasite’s life cycle renews when termites ingest worm-laden cat feces. The obvious question arises as to what native anole predator served as a host for the parasite before cats were introduced to the islands by humans. Bats are a possibility. They are known to eat anoles and are the only native mammal on the islands. Is it possible, however, that the parasite cycle just described was established only after cats arrived? This remains unanswered; but the novel interactions of exotic and native species over several hundred years, especially where parasites are involved, seems time enough to possibly lead to new selection pressures and adaptations.
Dobson et al 1992. The parasites of Anolis lizards in the northern Lesser Antilles. Oecologia 91:110. (researchgate.net).
Falk, Bryan, 2013. Thorny-Headed Worm Paasites in Anoles: Host Manipulation, Complex Life Cycle And Recent Expansion. anoleannals.org
Leal, M. and A. Gunderson 2012. Rapid Change in the Thermal Tolerance of a Tropical Lizard. The American Naturalist 180:815.
Losos, J. 2011. Brown Anoles on Hawaii and Battle of the Intercontinental Convergents. anoleannals.org
Schoener, A. and T Schoener 1984. Experiments on dispersal: short-term flotation of insular anoles, with a review of similar abilities in other terrestrial animals. Oecologia 63:289-294.