Brown Anole

They are different species from different places. The green anole, Anolis carolinensis, is native to the southeastern United States, can change between bright green and brown, and its dewlap is pink. The brown anole, Anolis sagrei, is native to Cuba and the Bahamas, is always brown or grey and cannot turn green, and its dewlap is orange-red with a yellow border. Where both occur, the brown anole stays low, on the ground and on trunks and fences up to head height, while the green anole is now usually seen higher up in shrubs and trees. The brown anole is generally more abundant and more tolerant of disturbed and urban ground.

The evidence is strong. Researchers tracked green anole populations on small islands in a Florida lake after brown anoles were introduced to some of the islands and not others. On the islands with brown anoles, green anoles moved their perches higher within months, and after about 15 years, roughly 20 generations, the green anoles on those islands had measurably larger toe pads with more of the microscopic clinging structures, better suited to gripping the narrower, smoother branches of the upper canopy. Green anoles on the brown-anole-free islands did not change. It is one of the best real-world demonstrations of natural selection acting fast in response to a competitor.

In their introduced range they are an established part of the fauna now and are not something a homeowner can realistically remove. They eat garden pests and are themselves food for birds, snakes, and larger lizards, so they slot into the food web. The main ecological concern is their effect on native anoles and, in some places, on native insects and spiders, and the risk of them being carried further afield in nursery plants. For most people they are simply the small brown lizards doing push-ups on the patio, and they are harmless, not venomous, and unable to give more than a tiny nip.

Several traits line up. They breed continuously through a long warm season, laying a steady stream of single eggs rather than one big clutch, so a population rebuilds fast from a few founders. They tolerate hot, dry, open, disturbed ground, including pavement edges, parking lots, and nursery yards, which suits movement by the horticulture trade. They are aggressive and eat a wide range of small prey, including other lizards' young. And they reach very high densities, which lets them dominate the low perches and starve out or displace competitors before those competitors can respond.