Conservation Paradox: When Invasive Species Are Threatened Themselves

According to the IPBES report from last year, invasive species are among the main drivers of global species extinction. They are responsible for 60% of all documented extinctions. Several of these alien species have also become established in Germany, including the raccoon from North America, the coypu from South America and the raccoon dog from Asia.

However, a recent study by biologists from the University of Vienna and Sapienza University of Rome, published in the journal Conservation Letters, reveals a paradoxical picture: Some of the species introduced by humans, which can become invasive and problematic in new habitats, are themselves threatened with extinction in their original range. Conservation therefore faces the challenge of deciding whether such species should be protected in their new habitats, even though they can have negative effects on native biodiversity there.

How threatened are alien species in their native range?

The researchers used the DAMA database (Distribution of Alien Mammals) to analyze data on 230 globally established alien mammal species and compared them with information from the IUCN Red List. The aim was to identify species that are threatened in their native range but have stable populations in their new habitats.

The result is surprising: 36 of the 230 species studied are threatened in their original range. Of these, 17% are considered critically endangered, 25% endangered and 58% vulnerable. Many of these species were originally introduced within Asia for hunting. Today, their new ranges are found mainly in Southeast Asia and eastern Australia.

Threatened at home, abundant elsewhere—Three examples

The study shows that many threatened species come from the tropical regions of Asia, where they suffer from rainforest destruction and overhunting. At the same time, some of these species have established stable populations in new habitats through human introduction, which could offer them better chances of survival. This conservation paradox becomes clear in the following examples: While these species are severely threatened in their native range, they often find better conditions in their new habitats—albeit with potential risks for local biodiversity.

One example is the European rabbit (Oryctolagus cuniculus), which is threatened in Europe by diseases such as myxomatosis and habitat loss. In Australia, by contrast, where it was introduced in the 18th and 19th centuries as a source of meat and as game, it has multiplied massively and is now considered a pest. In addition to Australia, European rabbits were also introduced to New Zealand, South Africa, North and South America, as well as to numerous Pacific islands, islands off the coasts of Africa and the Caribbean.

Another example is the Celebes crested macaque (Macaca nigra), which originally lived exclusively on Sulawesi in Indonesia. There, it is threatened with extinction due to hunting—whether as a source of meat or because of conflicts with farmers—and the destruction of its habitat. Since 1978, its population in its native range has declined by 85%. On other Indonesian islands such as Bacan, where it was introduced for the pet trade and for cultural purposes, stable populations have developed that are now larger than those in its original range.

The banteng (Bos javanicus), a wild cattle species from Southeast Asia, is also severely threatened in its native range. Populations on Java, Borneo and the Asian mainland have declined dramatically due to poaching and habitat loss. In countries such as Thailand, a decline of 85% was recorded between 1980 and 2000, while the species is nearly extinct in Laos, Cambodia, Vietnam and China. Surprisingly, the world’s largest population of this species is now found in Australia. The animals were introduced to the Northern Territory in the 1840s to provide meat and serve as working animals. After the colony was abandoned, the bantengs became feral and formed stable populations.

A double challenge: saving alien species and protecting native biodiversity

The global assessment of extinction risk has so far considered only populations in species’ original ranges. The new study shows, however, that for 22% of the species examined, the extinction risk would be lower if alien populations were also included. For example, the IUCN status of the European rabbit, which is threatened in Europe, could be downgraded from “threatened” to “least concern” because of the stable populations in Australia.

This finding highlights a central dilemma: On the one hand, alien populations could help threatened species survive. On the other hand, these populations can pose new threats to native species and ecosystems. Such threats include competition, the transmission of diseases or changes in local biodiversity.

Conservation faces the difficult task of weighing the benefits for threatened species against the potential damage to other habitats. Protecting species in their original ranges must remain a priority, as they are often threatened by habitat loss, hunting or other pressures. At the same time, reality shows that some species may have better long-term chances of survival outside their native range. This illustrates how profoundly globalization influences the distribution and protection of species, and what new challenges it creates for conservation.

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Source

• Tedeschi, Lisa & Lenzner, Bernd & Schertler, Anna & Biancolini, Dino & Essl, Franz & Rondinini, Carlo. (2024). Threatened Mammals With Alien Populations: Distribution, Causes, and Conservation. Conservation Letters. 10.1111/conl.13069.