The introduction and proliferation of species outside their historical ranges is recognized as a major threat to biological diversity. The problem of “invasive species” has become well known outside the scientific community, although many people are hard-pressed to identify even the most conspicuous invasive species in their area. For example, most people I talk to while I’m doing my research here in South Florida are surprised to learn that the brown anole (Anolis sagrei) – easily the most common lizard in developed areas of Florida – isn’t native here, but an invasive species from Cuba.
But not every species that arrives in a new habitat is “invasive,” and a plethora of terms are used, often casually, to describe species living outside their native range. All of these terms, and the imprecision with which they are often used, can confuse scientists and non-scientists alike. Here are a few of the most common: invasive, exotic, non-native, non-indigenous, introduced, established, and naturalized. And let’s not even get into the more emotionally loaded terms like “weedy,” “nuisance,” and “noxious.” What is the scientific consensus on these terms? Well, it turns out that the scientific community hasn’t quite settled on good definitions for all of them, but I’ll do my best to make some basic distinctions between them.
Exotic, non-native, and non-indigenous refer to species occurring outside their historical ranges. These terms don’t necessarily imply that these species have self-sustaining populations in their new range, only that they occur there. Introduced has a similar meaning, but with the added connotation of an anthropogenic (human-caused) colonization event. Established and naturalized describe species that have reached stable, sustainable population sizes in their new range – these species don’t require periodic or continuous replenishment to perpetuate themselves.
Invasive is the trickiest term. Some use it interchangeably with “exotic,” but this definition is too broad. Most scientists agree that the term invasive should be reserved for those species that have become established not only in human-altered habitats (like urban and suburban environments), but also in less disturbed natural habitats. Some authors also require that to be invasive, a species must have some negative effect on natural habitats. Most non-native species, however, have not been studied well enough to determine whether they have negative impacts on native plants and animals.
In South Florida, exotic and invasive species are everywhere. The brown anole that I study is certainly an invasive by any definition – since becoming established in peninsular Florida in the 1940s, it has spread throughout the state and beyond, it occurs in most habitats, and it competes with the native green anole (A. carolinensis), which seems to be suffering from the interaction.
On the other hand, there are many South Florida exotics that don’t seem to be invasive yet, and may never be. Last week I photographed a beautiful pair of red-masked parakeets (Aratinga erythrogenys) at a nest hole in South Miami. These parrots are native to Ecuador and Peru, and almost certainly arrived in Miami through the pet trade. They have been established in Miami since the early 1980s, and their populations seem to be self-sustaining if not growing. Their distribution is limited to suburban areas, however, where they often visit fruit trees and bird feeders for sustenance. They have not spread into undisturbed habitats, and they don’t seem to be a threat to native species (although they may compete for nest holes with native cavity-nesting birds). So we could call them a “non-invasive exotic.”
It’s important, I think, to recognize that not all non-native species are invasive. Unfortunately, however, we just don’t know enough about most exotics to classify them as “invasive,” “potentially invasive,” or “non-invasive.” Since most of these species initially become established in human-dominated areas, non-scientists have an important role to play in monitoring the arrival and spread of new species, as well as the interactions between these new arrivals and native species.