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When a species is invasive...

 
A large snake in the grass
There is perhaps no better example of an invasive species than the Burmese python, which has taken over much of the everglades, even competing with the alligator for the spot as top predator in the ecosystem.

NPS Photo.

An invasive species is a non-native organism whose introduction causes or is likely to cause economic or environmental harm, or harm to human, animal, or plant health, according to Executive Order 13751. Learn more about invasive species by visiting our About page here or in the navigation.

It is often thought that the terms 'invasive' and 'non-native' can be used interchangeably, but this is not always true.

For a plant or animal to be invasive, it must do harm. Simply being non-native is not cause for concern. The National Park Service actively manages invasive non-native species.

Understanding the difference between invasive and non-native species and when a species is managed is crucial.

 

...and when a species is non-native

 
A stone path leading into a lush garden, tomatoes growing to the right.
Fruits, vegetables and herbs grow along the path through the White House Kitchen Garden in President's Park.

NPS Photo.

Non-native species are those that have occurred outside of their natural range. That natural range could be as far as another country or as near as a different region of the same country.

Unlike invasive species, non-native species may not hinder or prevent the survival of others within the ecosystem. They simply exists where they typically have not. Other terms used for non-native species include, 'exotic' or 'alien' species, but these are discouraged terms, as it can cause confusion.

You might even recognize some non-native species of plants in your own backyard or on your dinner table. Non-native species such as petunias and tomatoes, present no threat to native plants and have been cultivated by humans for centuries.

 

Location, location, location...

In some cases, it is not correct to call an entire species native, non-native, or invasive to the U.S. But for some specific populations of a species, park staff can, depending on the population's location. Rainbow trout provide an interesting example of a species for which management is complex.

 
A rainbow trout held in the hand of a person above water. The pink, iridescent sheen obvious down the fish's side
Rainbow trout in Yellowstone National Park.

NPS Photo.

 

Rainbow trout are native to all of Oregon, most of Washington, much of California and Idaho, some areas of Alaska, and a small portion of northwestern Montana. Rainbow trout are managed as invasive species in many parks, including some waters in Yellowstone National Park, where they are non-native and pose a serious threat to native fish such as Yellowstone and Snake River cutthroat trout. They compete with cutthroat trout for food and habitat, and are also capable of crossbreeding with them.

However, there are NPS waters where rainbow trout are native and managed as such, and other waters where they are non-native but are managed to provide sport fishing opportunities. In some instances, non-native rainbow trout fisheries occur in waters where habitat has been altered so that it is no longer suitable for native species. In other instances, such as the Bighorn River in Montana, no negative impacts by the presence of non-native rainbow trout have been detected.

 

More information about invasive species from the NPS and our partners can be found on the Resources page, here.

Last updated: February 23, 2018