When I was a kid growing up in Tampa, I used to amuse myself by watching and catching, and keeping as pets, two types of lizard of the same genus: the Carolina Anole, and the Cuban Anole. http://floridagardener.com/critters/anoles.htm

The former is a domestic critter, a green, elegant reptile maxxing out about 8 or 10 inches, it is the common "chameleon" you used to be able to buy in pet stores. The Cuban Anole is an import, it supposedly came to the states from the Caribbean on a banana boat. It is a bit smaller, more drably colored, but stockier, more robust and more active and aggressive. Both coexist fairly well, they don't fight, cross-breed, or eat each other, and there certainly are enough bugs around for them to feed on. Both species suffer predation from birds and cats, and the occasional garden snake. The only resource they may compete for is display space. In other words, they occupy the same ecological niche.

Both species require shrubbery to hide in, and open, sunlit spaces nearby with high visibility (like fence railings) where they can strut around and attract mates and challenge rival males (they get into neat fights, and are quite randy fellows in mating season). Consequently, ideal habitat for them is not the wilderness, but suburban gardens. They are everywhere in the city, much less common out in the bush.

About 50 years ago, both species were equally common. But over the years, the
domestic species has become scarcer and scarcer, while the import has increased greatly in numbers. The Cuban Anole has definitely been displacing the Carolina Anole. Today, the latter is so rare they are just about non-existent in town, although I occasionally run into one out in the country. The country is not ideal lizard habitat, there are more predators, and bare, sunny spaces where you can strut your stuff, flash your dewlap, and cruise for chicks are few and far between. Although there are less Cubans in the bush than in town, they greatly outnumber the Carolina boys there. It is clear that the Cuban lizard is taking over. I wouldn't be surprised if the Carolina Anole wasn't already on the way to extinction, or at least, to becoming a rare and ecologically insignificant species, at least in South and Central Florida. Hopefully it will survive further north, it appears to be more tolerant of cold weather.

Maybe something like this is happening to the dominant mammals in the American Southwest.