Alien species: unloved but not necessarily harmful

Alien (that is, nonnative or exotic) plant and animal species are frequently deemed harmful to local biodiversity by conservationists. But viewing all alien species as harmful would amount to painting with a very broad brush. A new methodology in classifying species called EICAT+ aims to include the observed positive impact of these alien species. Giovanni Vimercati of the University of Fribourg’s Department of Biology had a hand on developing this revised approach.

The tiger mosquito, the red-eared slider turtle, the quagga mussel, common ragweed, and Japanese knotweed are among some of the alien species, whether animal or plant, that are worrying specialists, often to the point of making the front page of newspapers. They are indeed called invasive and are often of great concern because they generally represent a threat to local fauna and flora, as can be seen with the North American spinycheek crayfish, which has almost completely replaced its indigenous cousins in Europe. In 2020 the International Union for Conservation of Nature (IUCN) adopted a system for classifying nonnative species which it calls EICAT (Environmental Impact Classification for Alien Taxa). Researchers at the University of Fribourg also contributed significantly to the original system. EICAT allows scientists to simply and objectively rank alien species in terms of the nature and scale of their impact to indigenous biodiversity.

Adding nuance to the picture
This new standard, however, only considers the negative impacts of species. “Yet we have to bear in mind that certain nonnative species can have positive effects on local biodiversity,” explained Dr. Giovanni Vimercati, a researcher in Sven Bacher’s group at the Biology Department of the University of Fribourg. “They can, for example, provide food or furnish habitat to an indigenous species in decline.” Dr. Vimercati went on to give the example of the Aldabra giant tortoise (Aldabrachelys gigantea), a species that is indigenous to Seychelles but was introduced to the Mascarene Islands east of Madagascar. “Although this species is not native to the Mascarene Islands, it plays an important role in seed dispersal, a function which was lost when the local native species became extinct.”
In order to factor in this type of positive impact, which had been overlooked until then, Dr. Vimercati and and his colleagues from the University of Fribourg (Sven Bacher, Anna Frances Probert, and Lara Volery), along with several international experts, have developed a new classification tool, EICAT+. This tool should allow environmental protection specialists, along with policy-makers, to better gauge the effects alien species have on the local environment.

EICAT+ evaluates the impact of nonnative species using five semi-quantitative scenarios that can describe and measure the extent of observed positive effects on biodiversity. “You might, for example, classify an alien species moderately positive if it helps to increase the population of an indigenous plant or animal. This impact could even be called major or massive if it goes so far as to stop the extinction of the indigenous species,” Dr. Vimercati stressed.
EICAT+ makes it possible to grasp the underlying mechanisms and see if their effects on indigenous flora and fauna prove reversible once the nonnative species have been removed. It can be applied at a range of spatial scales that run from the local to the global, as well as to all taxa, in other words, to all species (animals, plants and fungi). “This new tool allows us to fill in gaps in our knowledge and expand scientific understanding of the consequences of biological invasions,” Dr. Vimercati was pleased to point out. “Thanks to EICAT+, we will be able to foresee possible negative effects resulting from measures to control or eradicate alien species.”
In the future, initiatives to counter the spread or proliferation of certain alien species will be more nuanced since they will take into account the impacts, both positive and negative, on local biodiversity. Specialists will even be able to use EICAT+ to evaluate the extent to which plant and animal species coming from somewhere else enable them to achieve their goals of preserving the environment. A real paradigm shift!

Freely available paper in Plos Biology