Conservation paradoxes: 36 species which are invasive in some areas but still need protecting elsewhere.

An invasive alien species is one which becomes established in the wild, accidentally or deliberately, outside its native range in a way that negatively impacts local biodiversity. They can cause substantial ecological damage, and are classed as one of the five global drivers of biodiversity loss by the Inter-Governmental Science-Policy Platform on Biodiversity and Ecosystem Services.

In addition to ecological damage, invasive alien species can generate significant economic costs. These can arise from damage and disturbance caused by the invasive populations, or from management costs related to containing, controlling or removing them. Measures to control invasive alien species in the EU are guided by Regulation 1143/2014, which lists species of concern. As of 2022, this included 47 animals and 41 plants, although the list has subsequently been updated to a total of 114 species.  The total economic impact of invasive species within the EU has previously been estimated at around EUR12 billion per year, and the latest update should revise this estimate upwards.

While a given species many be considered invasive outside of its native range, that does not affect its status within its range, and so it is possible for a species to be both a conservation priority and an invasive alien elsewhere. This can give rise to dramatically different management priorities and actions for the same species in different locations.

The new research explored this phenomenon. Unlike previous studies, it focused on invasive species with quantified associated costs as presented in the open-access database InvaCost, which records the damage and management costs of individual invasive species by country between 1960 and 2020. The team compared this list with the IUCN Red List, which assigns species a threat category, ranging from least concern to extinct, based on the status of the wild populations within its native range.

Of the 355 species of mammals, birds and plants considered by the researchers, five mammals and five plant species were also deemed to be threatened with extinction, which means they were categorised as either vulnerable, endangered or critically endangered, by the IUCN. They term this scenario as a conservation paradox because these species require both conservation measures within their native range and population control outside it. Among such examples, the researchers highlighted the European rabbit (Oryctolagus cuniculus), which is classified as endangered while also being among the most costly invasive species globally, according to InvaCost.

They also researched species that could be considered conservation priorities because they are highly distinctive, due the evolutionary history or the ecological functions they represent, even if they are not currently threatened. They argued that prioritising threatened species – only – is a reactive approach to conservation, and that a proactive approach could be more efficient:  attempting to prevent rather than cure the problem. This could involve identifying distinctive species which make an especially large contribution to biological diversity and focusing on maintaining their healthy populations. This approach, they said, could help to stop species from becoming threatened, and hence avoid the greater costs and challenges associated with recovery efforts.

Following this rationale, the team identified nine invasive species that were distinctive on an evolutionary basis (with few close evolutionary relatives) and 17 that were functionally distinctive (with notable diet, activity or body mass traits). The researchers did not find any of these conservation challenges to be distinctive in both ways, but they identified one that was both a paradox - classified as threatened - and a challenge, being evolutionarily distinctive: the koala (Phascolarctos cinereus). They also identified the American mink (Neovison vison) and the common starling (Sturnus vulgaris) as being both functionally distinctive and among the costliest invasive species.

Among a total 355 species they considered, the researchers found 36 to be either conservation paradoxes or conservation challenges, or both. They say that in such cases, control measures in locations where the species is invasive could have implications for conservation management elsewhere. For example, there may be potential to use individuals captured from invasive populations to establish or supplement wild populations within the native range. This could be an alternative to taking individuals from populations within the range that are already under threat. Research on invasive populations could also provide information about the species to support conservation decision making. This makes it important, say the researchers, for management authorities in the respective territories to communicate and coordinate their actions.

The study focused on mammals, birds and plants as there were only sufficient data to conduct the analysis on these groups. The researchers point out that the InvaCost database is not a comprehensive account of all invasive species costs as these have only been recorded for less than 10% of known invasive species, with the remainder likely to have incurred some significant costs that have not been reported. They also noted that this study only considered the species’ global conservation status, and there may be more paradoxes representing species that are threatened only in some parts of their range. They said that other kinds of conservation paradox or challenge could also be studied, in particular for invasive species which generated economic benefits as well as costs, as in the case of koalas generating tourism income. 

Source:

Robuchon, M., Bernery, C., Cardoso, A. C., Dia, C. A. K. M., Courchamp, F., Diagne, C., Gervasini, E., Heringer, G., Pavoine, S., Renault, D., Rezende, V., Vaissière, A.-C., Bellard, C. (2025) Conservation paradoxes and challenges in invasive alien species with economic costs. Biological Conservation, 305:111041. https://doi.org/10.1016/j.biocon.2025.111041