How biological invasions are silently remodelling ecosystems

Many of the most damaging invasions do not simply subtract species; they fundamentally remodel the environment, altering habitats, rewiring interactions and shifting processes in ways that species lists alone cannot reveal. Consider the goat, the horse or the deer, introduced to many islands around the world. While their voracious grazing can indeed drive native flora towards local extinction, the legacy is etched deeper into the land. These invasive herbivores compact the earth, accelerate erosion, open up the undergrowth and modify fire regimes, scarring the landscape long after the herds are gone. These systemic changes threaten biodiversity just as profoundly as the loss of any single species. To navigate this complexity, invasion scientists have increasingly turned to the Environmental Impact Classification for Alien Taxa (EICAT). This pioneering framework marked a significant leap forward, offering a transparent, evidence-based method to rank “invaders” by the severity of their toll on native species, from negligible effects to local extinctions. However, EICAT operates with a specific blind spot: it is strictly species-orientated. It assigns a single global severity score to an invader, usually based on the worst-case scenario recorded in its invasive ranges. While powerful for global prioritisation, this approach can overlook the complexities in specific, local ecosystems, each with unique vulnerabilities. As a recent study published in PLOS biology illustrates, there is cause for further investigation. The invisible architecture of invasions In addition, biological invasions generate a spectrum of impacts extending far beyond direct effects on native species seen in typical assessments. In our recent synthesis published in 2025, we catalogued 19 distinct types of environmental impacts. When examining all of the well-documented impacts, it became obvious that most structural changes operate at the level of communities, ecosystems or physical processes. Crucially, twelve of these categories concern scales broader than the actual species: nutrient cycling, habitat structure, or the physical properties of soil and water, for instance whose impacts are therefore underestimated. Three distinctive levels of structural changes can be distinguished: individuals and populations, assemblages of different species, ecosystem functioning, or abiotic conditions – in other words non-living physical or chemical parts of the environment that affect living organisms such as pollination and fire regimes. Nutria are a species renown for its ecosystem engineering skills. Max Saeling Despite being widespread and well documented, these structural changes remain largely unclassified by frameworks that focus ultimately on native species loss. This omission is critical because many invasive species act as “ecosystem engineers,” organisms that do not merely live in an environment but actively modify it, influencing the fate of entire communities. To capture this nuance, from our recent research we developed a complementary assessment tool: EEICAT, the Extended Environmental Impact Classification for Alien Taxa. From the invader to the invasion EEICAT is not a replacement but an evolution. It brings a necessary expansion in impact assessments, while seeking simplicity and integration. It is based on EICAT, but it shifts the unit of assessment from the invasive species to the invasion event. Under this framework, all 19 impact types can now be considered and an invasive population can be assigned one or multiple severity categories, at any ecological level. Under EEICAT, the loose threads of evidence from multiple impacts can now be woven to reflect the unique tapestry of effects on native species, communities, processes and even abiotic conditions. The necessity of this distinction is vivid in aquatic systems invaded by zebra mussels (Dreissena spp.). In too many lakes, these molluscs threaten native mussel populations through competition and biofouling, a classic impact well captured by standard assessments. Yet, simultaneously, they are transforming the environment itself: filtering water, lowering turbidity, altering nutrient cycles, and triggering cascading shifts in vegetation and food webs. EEICAT allows us to map both the direct blow to native biodiversity and the systemic reengineering of the lake, within a single framework. Zebra mussels (Dreissena polymorpha) attached to native mussels. A similar logic applies to the terrestrial realm. The Argentine ant (Linepithema humile) is notorious for displacing native ants, simplifying communities into ghost towns, devoid of the usually numerous native ant species. But its influence ripples further. By disrupting mutualisms between plants and insects, these invaders alter seed dispersal, pollination, invertebrate assemblages, and even soil processes. These indirect, community-level impacts often differ in severity across invasion events, depending on climate, habitat integrity and recipient ecosystems. You can therefore assess several impacts per invasion, and characterise the specificities of each invasion: with a case-based approach. invasive ant supercolony or ants tending aphids. Alexander Wild (reuse prohibited without authorisation) Context is everything The plant kingdom offers some of the clearest arguments for an event-based approach. Acacia species, introduced globally, act as ecological chameleons. In South Africa, they are aggressive suppressors of native flora, or Fynbos, and transformers of soil chemistry through nitrogen enrichment. In Mediterranean Europe, the same Acacia dealbata species, commonly known as mimosa, may exert moderate competitive pressure but still alter fire regimes, litter accumulation and hydrology. EEICAT provides a straightforward way to document these contrasts, where all evidence counts in helping to assess the severity of each particular invasion. The winter flowering Acacia species invading native vegetation in Italy. Marco Bertorello/AFP Reinterpreting the ecological history of biological invasions Importantly, adopting EEICAT does not mean starting from scratch. We can leverage decades of existing impact studies, and even previous EICAT impact assessments could be adapted. The framework simply translates qualitative ecological evidence into a broader set of categories that span biological, community and abiotic levels. It even uses the same five levels of severity, from “minimal concern” to “massive concern,” with the same guiding decision rules. This compatibility allows us to reinterpret the history of invasion ecology through a wider lens. Because EEICAT is case-specific, it enables us to track how a single species behaves differently across regions, or how multiple invaders compound pressure on a single ecosystem. It reveals patterns of cumulative stress and ecosystem vulnerability that global scores simply cannot articulate. Biological invasions are not merely about losing species; they are also about the silent rewriting of ecosystems. From the chemistry of the soil to the rhythms of wildfires, their impacts ripple through the environment long after their arrival. By embracing the Extended EICAT framework, we can finally capture the full scope of how invasive species really impact ecosystems and tailor management strategies to the complex realities of the living world, with each invasion, one by one. Created in 2007 to help accelerate and share scientific knowledge on key societal issues, the Axa Research Fund – now part of the Axa Foundation for Human Progress – has supported over 700 projects around the world with researchers from 38 countries on key environmental, health & socioeconomic risks, like this project led by Franck Courchamp. To learn more, visit the website of the AXA Research Fund or follow @ AXAResearchFund on LinkedIn. A weekly e-mail in English featuring expertise from scholars and researchers. It provides an introduction to the diversity of research coming out of the continent and considers some of the key issues facing European countries. Get the newsletter!