Headlines mentioning invasive species often conjure up images of ecological disaster and vanishing native wildlife.

For example, the sinister-looking snakehead fish mentioned in a USA Today story grows up to 3 feet in length, breathes air and survives on land. Common to parts of Asia, it’s recently been reported by the U.S. state of Georgia, where it out-competes vulnerable native species for food and habitat. State officials encourage killing snakehead fish if caught to prevent further spread.

How are these species introduced to new environments? Are they always detrimental to a new environment?

What Is an Invasive Species?

This particular type of species causes problems when it is introduced to a new habitat outside its natural area or distribution, according to the International Union for Conservation of Nature (IUCN). The IUCN holds a global registry and monitors invasive species impact on ecosystems as part of its mandate to safeguard the natural world.

A great example is the cane toad. Introduced to Florida from Central and South America in the 1930s, cane toads were meant to act as natural predators for cane crop pests. However, they adapted a little too well to their new environment, and, as The Smithsonian reported, they soon outgrew their welcome. In Florida and also in Australia, these animals multiplied quickly and are now seen as pests, clogging drains with sheer size and numbers. The invasive toads also have good appetites for smaller native frogs and reptiles, further shrinking already endangered populations. If a dog ate a cane toad, they would die of toxins emitted by the toad.

Invasion Through Travel

Another well-known example of invasion, and probably one that seems very commonplace now, are the great murmurations of European starlings flying at dusk over North America. According to the New York Invasive Species website, theater fans in the 1860s who were missing an authentic Shakespearean experience introduced all the birds mentioned in the bard’s plays to New York City’s Central Park. Now starlings are found all over the U.S. where they damage crops extensively and compete with native species for valuable food resources.

Another infestation with subsequent invasive species impact on ecosystems comes from Scotch Broom. According to the Nature Conservancy of Canada, the plant made it across the Atlantic as packing material around whiskey cargoes. The seeds grew well in the Pacific Northwest and spread across British Columbia, choking out native plants, reducing bird and butterfly habitats and increasing fire risk with its oily roots.

Invasion Through Abandonment

“If you love something then set it free” is not a good rule for managing alien species. There are a number that have successfully established themselves far from home simply due to being abandoned.

Terrapins in your local pond? You can thank the “Teenage Mutant Ninja Turtles” for that. Popularized in cartoon strips, comics and films, red-eared slider terrapins briefly became popular pets until owners abandoned them after finding that they got larger, smellier and more aggressive with age. The California Department of Fish and Wildlife reported that they are major sources of salmonella infections in people; they also seriously impact western pond turtle populations through disease, competition for food and general aggression.

In Death Valley, animal abandonment has led to herds of burros that the National Parks Service considers a nuisance; they trample plant life and destroy stream beds. Undark explains how mine closures meant that these pack animals were no longer required for labor, so owners simply set them free.

Invasion as Salvation

Usually invasive species are not generally considered beneficial to an ecosystem, even if some like the cane toad were brought in for good purpose. But is this always the case?

Not always. Take the samurai wasp, for example. It’s a natural predator of the U.S. stinkbug, and at one time scientists thought that they could use it for biological control of the smelly pests. So, they made plans to bring the tiny wasp into areas of the U.S. that needed help, only to find that the insect alien had already landed and was targeting stink bugs.

There’s also evidence that invasive burros might actually be making life better in Death Valley. A research study shows that far from destroying the landscape around stream beds and water sources, the “damage” done to the landscape and vegetation by burro hooves actually helps the ecosystem.

Burros dig deep underground to tap into groundwater for survival. These wells can be up to 5 feet deep and researchers found that they not only provide fertile growing conditions for native willow and cottonwood seedlings, but they also make water available to other desert species. Since the burros travel in large groups, they also cut tracks into dense vegetation and open routes for other animals to access the new springs.

The burros act as highly efficient water resource managers, creating rather than destroying opportunities for natives in an arid landscape.