Invasive species are often threatened in their native habitat

Non-native species introduced by humans are among the main causes of global species decline—they were partly responsible for 60% of the species that have become extinct worldwide in recent decades. Non-native mammals in Central Europe include species such as the brown rat, the mouflon and the mink.

Now a study led by biologists from the University of Vienna and La Sapienza University in Rome shows that some of these species introduced by humans are themselves endangered in their native areas. The study was published in the current issue of the journal Conservation Letters.

Globalization of the Earth contributes to the introduction of many animal and plant species into new parts of the world. Invasive species can displace native species through competition, or they transmit new diseases. At the same time, however, some of these non-native species are threatened with extinction in their native range.

This creates a conservation paradox—because the question now is, should non-native populations of species that are endangered in their native range be protected or combated? However, it was not previously known how many non-native mammal species this paradox actually applies to. In the new study, the scientists have now quantified this in order to get one step closer to an answer to this paradox.

A total of 230 non-native mammal species have been introduced into new areas by humans and have settled there permanently. “We were interested in how many of these species are threatened in their own homeland,” explains Lisa Tedeschi from La Sapienza University and the University of Vienna, the lead author of this study.

The scientists were able to show that 36 of the non-native mammal species are threatened in their original homeland and thus fall under this conservation paradox. “This high number surprised us very much, as we assumed that invasive species were also common in their area of origin,” Tedeschi continues.

One important natively threatened mammal species is the crested macaque, whose numbers have declined by 85% in its native range on Sulawesi since 1978, while it has spread to other islands in Indonesia and has stable populations there. The wild rabbit is threatened in Europe, while in other parts of the world, such as Australia, it has very large introduced populations that are far larger than those in Europe.

Most of the natively threatened species originate from tropical Asia; in many cases their decline is a result of massive rainforest destruction and overhunting. Human-introduced populations could therefore help prevent these species from becoming extinct.

Globalization: Nature conservation faces a difficult task

When assessing the global extinction risk, occurrences of a species that do not live in their native range are currently not taken into account. In the current study, however, the researchers were able to show that the endangerment situation of some species would improve if non-native occurrences were also taken into account.

“For 22% of the species analyzed, the global extinction risk would be reduced if non-native occurrences were also included in the assessment,” explains biodiversity researcher Franz Essl from the University of Vienna, one of the main authors of the study. According to the scientists, this result underlines the considerable importance of non-native populations for the survival of endangered species—especially when there is a high level of threat in the native range.

However, including non-native populations of these species in the threat assessment also entails risks—for example, less attention is paid to protecting the endangered populations in the native area. In addition, non-native populations can have negative effects on other species.

“The main focus must continue to be on protecting species in their native area. However, it is likely that in the future there will be more species that are threatened with extinction in their native areas and have a better chance of survival in the new distribution area.

“This presents nature conservation with the difficult task of weighing up opportunities and risks,” concludes Franz Essl. “This is also a fingerprint of the globalization of species distribution.”