The way to hell is paved with good intentions. In the hope of saving certain endangered species and ecosystems, some conservation projects propose the   introduction of large plant-feeding mammals into areas outside their natural range. Yet, however commendable in appearance, this strategy may be a bad good idea, as three researchers at the University of Fribourg have shown. Generally, the negative effects of introduced plant-feeding mammals on native biodiversity clearly outweigh the observed benefits.

Is it possible that introducing a species into an environment outside its natural range can have positive effects on biodiversity? This is the question that Zoé Bescond-Michel, Giovanni Vimercati, and Sven Bacher from the University of Fribourg’s Department of Biology tried to answer. “Given the declines in animal populations, especially among the large plant-feeding mammals like elephants, camels or the American and European buffalo, we are seeing a growing interest in introducing them outside their native range for conservation purposes,” Dr. Vimercati explained. For the proponents," he continues, "these introductions also promise to restore natural processes that have been lost over the past centuries—such as seed dispersal or nutrient cycling—thereby revitalizing ecosystems degraded by human activities”. For experts on introduced species, however, caution is the prevailing catchword here because there is the danger of merely setting a foreign fox to mind the geese, so to speak. There are numerous examples of introduced species, deliberately released or not, that have become uncontrollable, even harmful to native species.

Objective standardized risk assessment
Scientists noted that until now, studies have focused on the negative impacts that species introduced outside their native range had on biodiversity. They concluded that a more nuanced view is needed. Prof. Bacher made clear that “the majority of scientists in the field recognize there can be positive effects for native species. A newly introduced species, for example, might adopt the ecological role of a formerly extinct species, or indeed serve as a crucial food source for native predators. However, we currently do not know to which extent these potential positive effects exist.”

To gain a clearer picture, the three scientists from the University of Fribourg analyzed more than 2000 positive and negative impacts of large plant-feeding mammals introduced for various reasons outside their native range around the world. For the comparison, they used two analytical frameworks they developed together with an international team of scientists over the last decade. The first, known as EICAT (the Environmental Impact Classification for Alien Taxa) assesses the negative ecological impacts of introduced species on biodiversity, and is officially adopted by the International Union for Conservation of Nature (IUCN) as global standard. The second, EICAT+, adopts EICAT’s main elements to examine the degree to which these species may help to reduce native biodiversity decline.

A cure often worse than the disease
The results of these analyses have just been published in the journal Nature Communications. For Ms. Bescond--Michel, they sound a real warning, “Studying cases of introducing large plant-feeding mammals like deer, horses, and buffalo outside their original range, we noticed that these actions, despite the best intentions, often do more harm than good to local biodiversity.” Worse still, the scientists found only one sole species, the Indian hog deer (Axis porcinus), that showed no negative impact in the scientific literature! Ms. Bescond-Michel added, “What we realized is simple. We noticed that only one out of five impacts of the introduced large herbivorous mammals has been positive.”

The scientists discovered in particular that insular ecosystems and species located high up in the food chain proved the most vulnerable to the impacts resulting from the introduction of herbivores. However, they also saw that there are benefits for certain native species, although those benefits often come at a cost to other native species. To mention just one example, in certain areas the deer species that were introduced (Cervus elaphus, Cervus nippon, and Muntiacus reevesi) were indeed able to benefit some rare native plants by feeding on their natural competitors. But Prof. Bacher immediately raised a caveat, “This positive effect, however, is produced to the detriment of other native species the grazing introduced animals prefer.”

Consequences for conservation strategies
Even when the team of scientists tried to take the positive effects into account, they remain rare and weak. Moreover, the magnitude of the benefits decreased over time; not so for the negative impacts. They remained undiminished and on average higher than the benefits. Because of this asymmetry, the three researchers now invite their peers to reconsider the arguments for favoring introducing nonnative species in the course of rewilding projects or to escape climate change.

The authors also recommend carefully assessing the risks and benefits before any interventions are attempted. Giovanni Vimercati stressed in conclusion, “If certain endangered species are introduced outside their natural range for conservation purposes, they may play an ecological role there, but they may also become invasive in turn and threaten native species that are already in a fragile state. An ounce of prevention is worth a pound of introduction.”

Bescond--Michel, Z., Bacher, S. & Vimercati, G. Harms of introduced large herbivores outweigh benefits to native biodiversity. Nat Commun 16, 8260 (2025). https://doi.org/10.1038/s41467-025-63807-2