Neighborhood Nuisance
We've all heard stories of scary urban pets: huge snakes lurking in the plumbing or alligators in the sewers. While many of these are urban legends, the scenarios are not that far-fetched. Reptiles seem to be over-represented in the released or escaped pets category. It is not uncommon for snakes returned to the wild to survive for at least a while, probably because they tend to find a quiet place to hide and can go a long time between meals. Many snakes find their way to the wild just because they are escape artists, but some are likely released because they have become large, unmanageable, and it is nearly impossible to find a new home to take them. Every so often news stories crop up of constrictors being spotted in unlikely neighborhoods (and often suspected in the disappearances of neighborhood pets).
Other exotic animals such as non-domestic cats are also troublesome since they could pose a risk to other pets in the neighborhood, or even people.
Ecosystem Damage: Invasive Species
Given the right animals in the right environment, released exotic pets do sometimes find success and colonize in the wild. However, this is often an ecological or agricultural disaster. There are numerous examples of "Invasive Species" -- cases where an introduced plant or animal has become established to the extent where it "takes over" an ecosystem, diminishing native populations. Invasive species can cause problems by preying on native species, competing for limited resources, or by introducing parasites and diseases not normally found in the area. Only a few of the most dramatic cases of invasive species can be attributed to the pet trade, but there are a few isolated locations where released animals have become established and caused damage. Red eared sliders are quite adaptable at colonizing lakes or ponds, and they often thrive at the expense of other species. Parakeets have successfully colonized some areas to the detriment of native species and agricultute. Giant African Land Snails are considered a huge risk of becoming an invasive species due to their voracious appetites and amazing reprductive rates. Warmer climates are often more hospitable to invaders. For example, iguanas and Burmese pythons appear have established populations in Florida. Iguanas are doing a fair amount of damage to the local vegetation as well as becoming a nuisance, and burmese pythons are feeding on local wildlife and becoming a major concern in the Everglades. Another example is the situation with chameleons in Hawaii (PDF format).
Regardless of whether these exotic pets returned to the wild are a danger or a mere nuisance, many people do not agree that these animals should be kept as pets in the first place, and the return of these pets to the wild is ammunition in the fight to restrict the keeping of exotic pets. Please be responsible, and never return your exotic pets to the wild!