Native, invasive, or both?

If you read this week’s NatNews, you may have noticed that I called brown-headed cowbirds (Molothrus ater) a “native invasive” species. This phrase may seem contradictory, so in this post I will clarify the use of the terms native, exotic, and invasive.

A native species has historically occurred in a given area. For example, white-tailed deer are native to Texas.

A non-native, or exotic, species has been introduced to an area, intentionally or unintentionally, by humans. Axis deer are an exotic species in Texas. Introduced is another term for exotic species.

Invasive refers to a species that undergoes rapid population growth and out-competes other species. Often, exotic species become invasive because they have few or no natural predators in the new area. However, native species can also become invasive if environmental conditions change in their favor. Brown-headed cowbirds do well in open and human-altered landscapes, so their populations have increased in areas where forests were cleared and human development occurred. Honey mesquite (Prosopis glandulosa) is a native but invasive brush that can thrive in poor range conditions and choke out open areas. Wild pigs (Sus scrofa), on the other hand, are an example of an exotic invasive species that has spread rapidly across the U.S. and is not native to this country.

Are all exotic species harmful to the environment? Sometimes, introduced species can make a living without noticeably impacting other species. House geckos (Hemidactylus spp.) are native to Europe and Asia but have become invasive throughout the southeastern U.S., residing primarily on and in houses. The geckos’ penchant for urban areas seems to have limited their impact on native lizards — but research is lacking and house geckos may be impacting native insects. Most often, non-native invasive species have profound negative impacts on the environment, which is one main reason you should never release non-native pets (or any pets) into the wild.

Do all exotic species become invasive? Not necessarily — if you released a couple of polar bears (Ursus maritimus) in central Texas, they would probably not live very long*. However, predicting whether an exotic species will become invasive is very difficult, and people have been surprised in the past by species that unexpectedly became invasive. Therefore, every effort should be made to avoid introducing exotic species into a new area.

* Polar bears are mentioned here only as a dramatic example. Live-capturing polar bears and releasing them into Texas would be illegal, dangerous, expensive, and an overall bad idea that is not condoned by the author.

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