We've already grown used to seeing them around: green parakeets, black rats, grey squirrels, mouflon. These are just some of the alien animal species – that is, species widespread and well established in areas outside their original distribution range – so common that they have now become naturalised in Europe. So common, in fact, that everyone, scientists included, assumed they were abundant in their native ranges too. But that's not the case. A new study, conducted by the Università Sapienza di Roma and the University of Vienna and published in Conservation Letters, has shown that out of more than 200 non-native mammal species, as many as 36 are threatened – some critically – in their original habitats. For the researchers, this is a genuine conservation paradox, one that raises a profound dilemma, ethical and otherwise: is it right to protect these species within their native ranges, despite the harm they may cause elsewhere?

The threats posed by alien species

The figures surrounding the global displacement of species are staggering: counting mammals alone, humans are estimated to have introduced 230 alien species into new areas of the world, where they have established themselves permanently. According to the Intergovernmental Science-Policy Platform on Biodiversity and Ecosystem Services (Ipbes), more than 37,000 species have been introduced globally into non-native ranges. And the rate of introduction reaches as high as 200 new species every year. The trajectory of alien species can be one of growth: after an initial period of adaptation, many species become invasive.

This happens when they manage to reproduce at high rates and even come to outnumber the native species originally found in the area. In Italy, this is the case with the tiger mosquito, for instance, or the notorious blue crabs, coypus and American pond turtles, now virtually impossible to eradicate. Again according to Ipbes, alien species also carry an enormous economic cost for the global economy, estimated at 423 billion dollars a year, spent on monitoring, prevention and damage remediation. Above all, as if the management challenges were not enough, invasive species are among the leading causes of global biodiversity loss: data from international reports show they have contributed to 60 per cent of native species extinctions recorded in recent decades.

Threatened in their native range

The new study, led by Lisa Tedeschi of the Department of Biology and Biotechnologies Charles Darwin at the Università Sapienza di Roma, analysed the distributions, introduction pathways, threats and conservation strategies of more than 200 non-native mammal species introduced by humans and permanently established around the world. The results were striking: among the 230 non-native mammals, 36 are threatened in their area of origin. Seventeen per cent are listed on the IUCN Red List (International Union for Conservation of Nature) as "critically endangered", 25 per cent as "endangered" and as many as 58 per cent as "vulnerable".

The case of the crested macaque

An emblematic example of a mammal threatened in its original range is the crested macaque (Macaca nigra), whose population in Sulawesi, Indonesia, has collapsed by 85 per cent since 1978, yet remains widespread and stable on other Indonesian islands.

And of the wild rabbit

A similar case is that of the wild rabbit, which is endangered in Europe, yet has very large populations in other parts of the world – such as Australia, where numbers vastly exceed those found in Europe. The analysis also showed that the majority of species threatened within their native range are found in the tropical regions of Asia, where massive rainforest destruction and intensive hunting represent the primary drivers of biodiversity loss.

The right of alien species to exist

The outlook is not encouraging, and the situation is set to worsen unless the rate of deforestation slows or solutions are found to the devastating consequences of the ongoing climate crisis. The researchers also sought to analyse the influence of "foreign" populations on the conservation status of a species. "We reassessed," the paper reads, "the hypothetical IUCN Red List category for each species, including also the alien portion of its range." When evaluating the global extinction risk of species, populations living outside the native range are currently not taken into account. This is an ill-advised approach, because including alien populations improved the extinction risk categories for eight (that is, 22 per cent) of the 36 threatened mammals. The most significant changes occurred for species that jumped two or more IUCN category levels, as in the case of the European rabbit (Oryctolagus cuniculus), which moved from EN (endangered) to LC (least concern), the Javan rusa (Rusa timorensis) and the Sambar deer (Rusa unicolor), both of which moved from VU (vulnerable) to LC.

For these and other species, alien populations living far from home may therefore serve as a reservoir for future conservation actions, potentially preventing extinction in the event of habitat fragmentation or severe climate change. The Barbary macaque (Macaca sylvanus), for example, is experiencing a rapid population decline in Algeria due to drought, and the alien population could represent a stronghold for conservation. An ark of life that we cannot afford to lose.