Invasive "Alien" Species Are the Primary Cause of Recent Global Extinctions

Local species don't stand a chance.

The Earth is in the midst of a sixth mass extinction, but not all of it can be directly blamed on climate change. Plants and animals that have been artificially introduced by humans into new regions are wreaking havoc on the native wildlife. In fact, new research in the journal Frontiers in Ecology and the Environment shows evidence that these invasive “alien” species are more responsible for global extinctions than any other factor.

A species can make another go extinct when an imbalance in their populations leads one to hoard local resources, leaving the other without food or a habitat to thrive in. In a paper published on Sunday, an international team of researchers writes that 25 percent of plant extinctions and 33 percent of animal extinctions involve non-native species. By comparison, native animals are responsible for less than 5 percent of plant extinctions and 3 percent of animal extinctions.

Previously, scientists have argued that it didn’t matter whether a species is native or non-native when it comes to driving extinctions: what an organism does, they reasoned, is more important than where it comes from. But according to the authors of the new study, that argument may be been totally wrong.

“The impacts of native species in driving extinctions are much less widespread and prevalent as compared with those of alien species,” they write. The team, led by Tim Blackburn, Ph.D., a professor of invasion biology at University College London, based this conclusion on data from the 2017 version of the Red List of Threatened Species, a database maintained by the Union for Conservation of Nature.

The brown tree snake (Boiga irregularis), native to Australia, Indonesia, and some other Pacific islands, is responsible for more than 50 percent of Guam's native bird and lizard species, not to mention 2 out of 3 of Guam's bat species.

Flickr / Pasha Kirillov

How Invasives Cause Extinction

In many cases, non-native species are introduced by humans, either by accident — often hidden in cargo ships — or on purpose, often to control a native population. Once they take hold, they wipe out the locals with ease because their natural predators aren’t there to interfere.

“Alien taxa are not random samples of species,” they write. “Anthropogenic mechanisms tend to select species whose attributes are conducive to invasion success, and such taxa are often introduced to areas lacking the co-evolved enemies that limit their abundance in their native range.”

The success of these invasive species, unfortunately, put them into direct competition with native species.

For instance, when the brown tree snake (Boiga irregularis) was accidentally introduced to Guam aboard a military cargo ship after World War II, it quickly took hold because Guam offered many prey animals to eat and no predators to threaten it. Brown tree snakes have now forever changed the ecological landscape of Guam. Several species of bats, lizards, and birds, some of which were found nowhere else on Earth, have been completely wiped out from the island. Most cases involving a successful invasive species have similar consequences.

The purple sea urchin, which used to be kept in check by sea otters, killed off the kelp that Steller's sea cows fed on.

Flickr / brewbooks

Invasives Versus Aliens

Sometimes, climate change and habitat degradation can drive an existing species into a neighboring region. These cases don’t have the same impact as species that are introduced by humans to regions far from their native environment. The former are far less destructive than the latter.

“Native species undergoing outbreaks — even those expanding their range into adjacent territory — are less likely than alien species to encounter resident natives that lack evolutionary experience with them,” the authors write. In contrast, long-distance transportation introduces species that evolved with totally different biological needs and therefore are more likely to overwhelm the locals.

Even in the rare cases in which native species causing extinctions, human interference is still the root cause. For example, humans overhunted sea otters, which normally keep populations of purple sea urchins in check. Without the otters to eat them, the sea urchin population exploded and voraciously ate massive amounts of kelp, leaving nothing for the kelp-grazing Steller’s sea cows, which eventually went extinct.

Nevertheless, the most damaging form of human interference observed in the new paper is the introduction of new species. Unfortunately, once the damage becomes noticeable, it’s usually too late to be reversed.

Abstract: Native plants and animals can rapidly become superabundant and dominate ecosystems, leading to claims that native species are no less likely than alien species to cause environmental damage, including biodiversity loss. We compared how frequently alien and native species have been implicated as drivers of recent extinctions in a comprehensive global database, the 2017 International Union for Conservation of Nature (IUCN) Red List of Threatened Species. Alien species were considered to be a contributing cause of 25% of plant extinctions and 33% of animal extinctions, whereas native species were implicated in less than 5% and 3% of plant and animal extinctions, respectively. When listed as a putative driver of recent extinctions, native species were more often associated with other extinction drivers than were alien species. Our results offer additional evidence that the biogeographic origin, and hence evolutionary history, of a species are determining factors of its potential to cause disruptive environmental impacts.
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