Experts Fill the Gaps as France Ranks the True Ecological Toll of 228 Invasive Plants

Invasive alien plants are quietly reshaping the French landscape, from dense thickets of Japanese knotweed smothering riverbanks to acacia stands transforming soil chemistry in ways that may never be undone. A new study published in the journal NeoBiota has now delivered the most comprehensive evaluation to date of the ecological damage these species inflict in France, and the results carry a sharp warning for policymakers: nearly two-thirds of the country’s worst invasive plants remain unregulated. By adapting two internationally recognised assessment frameworks and folding in the field knowledge of dozens of botanists, the researchers have produced a national impact ranking that could reshape how France, and potentially other countries, prioritise the fight against biological invasions.

The study rests on two complementary methods. The Environmental Impact Classification for Alien Taxa, known as EICAT, was adopted by the IUCN as a global standard for scoring the harm that alien species cause to native biodiversity. Its younger counterpart, EICAT+, introduced only in 2022, applies the same logic in reverse, capturing the benefits that alien species can deliver to ecosystems. Both frameworks sort species into a ladder of categories, from Minimal Concern through Minor, Moderate and Major, up to Massive, the latter reserved for impacts as severe as the irreversible local extinction of a native species. In theory, both methods work at any spatial scale. In practice, they were designed for global assessments, and attempts to apply them at national level have repeatedly collapsed under a familiar problem: not enough data.

That data gap is far from trivial. Previous national-scale assessments in Europe and elsewhere have sometimes found that more than 90 percent of species had to be labelled Data Deficient, meaning managers simply could not tell how dangerous they were. The protocols only permit sourced information, whether published papers or grey literature such as reports and databases, and research on invasive species tends to concentrate on a handful of emblematic culprits. Global assessments, meanwhile, can mislead when transplanted to new contexts. The devastating impacts of the strawberry guava, Psidium cattleianum, on the forests of Mauritius, for example, tell managers in temperate continental France very little about what to expect on their own territory. Overreliance on such distant evidence risks exaggerating threats, misdirecting scarce resources and eroding the credibility of regulation itself.

To break through this bottleneck, the research team assembled a list of 228 invasive alien plants known to be problematic in continental France and Corsica, drawing on regional lists compiled by the National Botanical Conservatories and adding recently emerged invaders. The list included a few pragmatic compromises: two taxa were assessed at subspecies level because one of their subspecies is native to France, and the bamboo genus Phyllostachys was treated as a single unit, since the species and cultivars within it are so difficult to distinguish that mislabelled assessments would have been inevitable. The team then designed a three-step workflow that began with a rigorous sweep of the literature, using targeted searches in both English and French across Google Scholar and Google, plus non-digitised French journals on plants and ecology.

The crucial innovation came in the second and third steps. Forty-four experts and biodiversity organisations were invited to translate their unpublished field observations into standardised impact categories using a purpose-built key, and fourteen ultimately contributed assessments. Rather than leaving expert judgement free-form and subjective, the researchers converted EICAT’s abstract categories into three observable indicators: plant cover, stand size and persistence over time. A plant colonising tens to hundreds of square metres with more than 50 percent cover and displacing at least one native species, for instance, scores Moderate; thousands of square metres sustained over at least a decade, systematically excluding natives, scores Major. Assessors recorded the worst impact observed, excluding anomalies such as dense stands triggered by recent disturbance, and attached confidence scores ranging from possible to certain. Finally, committees of five to twelve botanical experts reviewed every pre-assessment in seven half-day workshops, with unanimous consensus required for any change.

The effect of adding expert knowledge was dramatic. Relying on published literature alone, 39 percent of the 228 taxa had to be classified as Data Deficient for negative impacts, and a staggering 86 percent for positive impacts. After integrating field observations and expert consensus, those figures fell to 5 percent and 37 percent respectively. Expert input did not merely fill gaps; it also refined existing assessments, raising the impact category for 40 taxa and lowering it for 37. In one telling case, the literature had reported a Moderate impact of the Himalayan balsam relative Impatiens balfourii on geophyte plants in southern France, but experts judged the species to be of Minimal Concern. Discussion with the original study’s author confirmed why: the earlier work measured impacts only within stands a few square metres in size, and a decade of re-measurement shows the plant has naturally vanished from many of its former strongholds.

The final verdict on France’s invasive flora is sobering. Of the 228 taxa assessed, 129 were found to cause harmful impacts: 71 at Moderate level, 56 at Major level and two at Massive, the highest category. The two Massive offenders are Robinia pseudoacacia, the black locust, and Acacia dealbata, the silver wattle, both of which alter plant communities and eliminate native populations through a combination of competition, chemical modification of soils and long-term elevation of nutrient levels on poor soils, an effect the authors judged effectively irreversible. The Major category reads like a roll call of familiar garden escapes and aquatic nuisances: Japanese knotweed and its hybrid, giant hogweed, water primroses, parrot’s feather, Cortaderia selloana, cherry laurel, red oak and dozens more. Competition emerged as the dominant mechanism, cited for 212 taxa, followed by structural and physical changes to ecosystems.

Perhaps the most striking finding is how unevenly the burdens and benefits fall across the tree of life. Negative impacts overwhelmingly targeted native plants, with 79 percent of non-negligible assessments concerning flora alone and none concerning fauna exclusively. Positive impacts told the opposite story: 74 percent of them benefited native animals, most often through the provision of food resources, a pattern the authors link to the horticultural origins of many invaders, bred for abundant flowers and fruits. Eight invasive plants were even found to help threatened species, six of them aquatic plants whose meadows provide hunting and nursery habitat for the pike, itself listed as Vulnerable in France. Yet the researchers caution that EICAT and EICAT+ results must never be directly compared or merged into a single score; national priority lists should be built on negative impacts alone, with positive assessments reserved for anticipating the unintended consequences of management actions such as eradication campaigns.

The study also delivers a pointed message to Brussels. Of the 21 plant species on the EU’s list of Invasive Alien Species of Union Concern that were assessed here, nearly all scored Moderate or Major, validating the regulation’s choices. But the reverse comparison is uncomfortable: 43 of the 58 taxa classified as Major or Massive in France, some 74 percent, remain unregulated at EU level, with candidates such as Senecio angulatus and Delairea odorata flagged as deserving of attention. The authors argue that expert-informed EICAT protocols should become a standard national instrument, used as routinely for invasive species as the Red List is for threatened ones, allowing impacts to be compared across countries and even predicting the threat posed by species not yet arrived. With introductions still accelerating worldwide and no sign of saturation, that kind of early, evidence-based triage may be the best defence Europe’s ecosystems have left.

Subject of Research: National-scale assessment of the negative and positive ecological impacts of invasive alien plants in France using adapted EICAT and EICAT+ protocols combined with expert knowledge

Article Title: Integrating EICAT and EICAT+ assessments and expert knowledge to classify invasive alien plants in France, based on their ecological impact

Article References: de Solan, T., Fried, G., Caillon, A., Soubeyran, Y., & Albert, A. (2026). Integrating EICAT and EICAT+ assessments and expert knowledge to classify invasive alien plants in France, based on their ecological impact. NeoBiota, 105, 89-111. https://doi.org/10.3897/neobiota.105.144928

Image Credits: AI Generated

DOI: 10.3897/neobiota.105.144928

Keywords: invasive alien plants, EICAT, EICAT+, France, biodiversity, impact assessment, expert knowledge, NeoBiota, EU regulation, ecology, conservation, data deficiency