Invasive species & extinction: What role do non-native animals and plants play?

Whether rats on tropical islands, introduced snails in the Pacific, or feral cats in remote bird colonies – invasive alien species (IAS) can throw entire ecosystems off balance and push animal and plant species to the brink of extinction. In 2023, the Intergovernmental Science-Policy Platform on Biodiversity and Ecosystem Services (IPBES) reported that in around 60% of all documented extinctions, invasive animals or plants were involved.

But how big is their real impact? Are they the main drivers of extinction – or usually just one factor among many, alongside habitat loss, overexploitation, and climate change? And should measures against invasive species be given greater weight in the future?

These questions are addressed by an international study from 2026, published in the journal Biosphere. The researchers Kelvin Peh and Zoe Bird systematically evaluated IUCN data and analyzed the role invasive species play in extinction events worldwide. Their conclusion: invasive species are rarely responsible on their own, but they often deliver the decisive final push to already weakened populations. Precisely because they can be targeted directly, they play a key role in conservation.

What are invasive species?

Invasive species are animals, plants, or microorganisms that are brought by humans—intentionally or unintentionally—into new regions and then put pressure on native species. This can happen directly, for example through predators without natural enemies, or indirectly, by competing for food, habitat, or light. In extreme cases, such displacement can lead to extinction.

One important point: not every introduced species is automatically invasive. Only when it spreads widely, alters ecosystems, and causes demonstrable ecological, economic, or health damage is it considered invasive.

For a long time, their importance was underestimated—also because the impacts of many introduced species are insufficiently studied. Often, the consequences only become visible when populations have already shrunk drastically or entire ecological communities tip out of balance.

How was the study conducted?

Earlier research on the relationship between invasive species and extinction mostly focused on one question: how many threatened or extinct species can be attributed to a particular cause? The emphasis was primarily on identifying the “most important” trigger.

But this approach falls short. In reality, threatened species are almost always exposed to multiple pressures at the same time. According to the IUCN, this applies to around 80% of all threatened species. Habitat loss, overexploitation, climate change, and invasive species often act together and amplify one another.

The new study therefore took a different approach. It examined invasive species both as a sole cause of extinction and in interaction with other factors such as hunting, agriculture, or environmental change.

The analysis was based on the IUCN Red List. The study evaluated:
– 981 extinct species worldwide
– of these, 338 linked to invasive species
– 84 species for which invasive species are considered the main cause
– 200 species with multiple causes, including invasive species

For each species, the researchers recorded information on taxonomic group, region, habitat, island or mainland occurrence, and documented threats. In addition, they created so-called ecological networks showing which invasive species are most frequently linked to particular extinction groups. So the goal was not just to count causes, but above all to make patterns and interactions visible.

Study results: what role invasive species really play

Especially at risk: snails, islands, and forests

The evaluation of IUCN data shows that extinctions caused exclusively by invasive species affect specific animal groups. Snails and bivalves were hit especially often: around 51% of all purely invasion-driven extinctions fell into this group. Among the introduced species themselves, too, snails played a central role, followed by mammals and plants.

Geographically, these extinctions were concentrated in Oceania. About 70% of all purely invasion-driven extinctions occurred there, and 91% happened on islands. Affected were mainly forested habitats with many specialized and sensitive species. A well-known example is the Pacific Partula snails, more than 50 species of which were exterminated within a few years by the introduced rosy wolfsnail (Euglandina rosea).

Invasive species rarely lead to extinction on their own

At the same time, the study shows that invasive species lead to extinction on their own in comparatively few cases. Out of 981 documented extinctions, only about 9% were attributable exclusively to them.

Much more often, they acted in combination with other pressures. In around 25% of cases, invasive species played a role together with additional threats. They frequently co-occurred with hunting, logging, agriculture, and the transformation of natural habitats.

In such situations, it is usually impossible to identify a single “main culprit”. Instead, different factors reinforce one another and gradually push populations toward collapse.

Invasive mammals as key drivers

Invasive mammals play an important role. Rats, cats, mice, pigs, or goats were almost always introduced by humans and in many places have become one of the greatest dangers to native species.

Feral domestic cats in particular are now considered a global threat. They are associated with at least 14% of all modern bird, reptile, and mammal extinctions—with especially severe consequences on islands.

Networks of extinction

The study’s network analyses make these patterns visible. In cases where invasive species were solely responsible, snails were central—typically displaced or eaten by other introduced snails.

In extinctions with multiple causes, however, birds were especially affected—often through interaction with invasive rats or cats. Examples include the Stephens Island wren from Stephens Island and the Dieffenbach’s rail from the Chatham Islands.

These complex interactions can also be seen in fish. In Lake Victoria, the introduction of the Nile perch (Lates niloticus), together with overfishing and water pollution, led to the loss of around 300 endemic cichlid species, including Haplochromis vonlinnei.

The decisive final push

Overall, the study shows that invasive species rarely cause extinction on their own. In many cases, however, they provide the decisive final push to weakened populations. Especially on islands and in sensitive groups such as snails, they can have devastating effects.

Most often, together with habitat loss, human use, and environmental change, they accelerate species decline until survival ultimately becomes impossible.

Why plants are especially at risk

Only relatively few plants are officially considered extinct due to invasive species. But concluding from this that plants are less affected than animals would be a mistake.

The problem lies in the pace of disappearance. Many plants don’t go extinct abruptly, but instead gradually lose their ability to regenerate. They may persist for decades in small remnant populations or remain present as seeds in the soil—even when they have long stopped reproducing successfully. From the outside, the species still appears to be there, but ecologically its persistence is already threatened. This phenomenon is known as “extinction debt”: the species is still present, but in the long run it is no longer viable.

Invasive plants and grasses often intensify this process. They displace seedlings of native species and change light, nutrient, and soil conditions without the decline becoming immediately apparent. Old individuals initially survive, but recruitment fails. Only when the last individuals die does the true extent become visible.

Especially in long-lived tree species or species with long-lived seed banks, this process can take decades. A substantial portion of plant species loss may therefore only become detectable after a long delay.

Easter Island provides an example. There, introduced rats prevented the regeneration of palm forests for centuries by eating a large share of the seeds. The ecological collapse only became visible once the old trees died. For a long time, deforestation was considered a result of human overexploitation—but more recent research shows that invasive rats crucially blocked natural regeneration.

The low number of officially documented plant extinctions caused by invasive species should therefore not be interpreted as an all-clear. It likely reflects that many losses only become visible with a long delay—when it is already too late.

Data gaps and blind spots

The study makes clear how patchy our knowledge of global extinction still is. The IUCN Red List is the most comprehensive database available, but it, too, is often based on incomplete information.

Many species were never systematically observed over long periods. Their disappearance therefore often goes unnoticed, and in many cases it is unclear which factors actually led to extinction. Long-term data are frequently lacking.

Invertebrates such as insects, snails, or spiders are especially affected. They make up the largest share of biodiversity, but are studied far less often than mammals or birds. Accordingly, the number of undocumented losses is likely high.

There are also major geographic blind spots. In particular, tropical regions are often poorly studied, even though they are among the most species-rich areas on Earth. In many countries, long-term monitoring programs, trained specialists, and financial resources are lacking.

Another factor is that extinctions are often only recognized with a delay . Many species are considered missing for years before they are officially classified as extinct. During this time, crucial information about possible causes is lost.

In fact, the exact background is known for only about 25% of all documented extinctions. This means certain threats are consistently underestimated, including invasive species, whose impact is often only visible indirectly or with a time lag.

The current study therefore concludes that the contribution of invasive species to global extinction is likely higher than the available data currently suggest. Many losses remain invisible because they are recorded too late—or not at all.

Invasive species can be controlled

Despite all the bleak figures, the study also conveys a positive message: invasive species are among the few major drivers of extinction that can be tackled in a targeted and effective way. While climate change or global land consumption can only be influenced slowly, biological invasions are in many cases directly manageable.

Numerous conservation projects over recent decades show that decisive action can bring measurable success—especially on islands, where invasive species often cause the greatest damage. Programs to eradicate introduced rats, remove feral cats, and control invasive plants have proven effective there.

In addition, quarantine measures and biosecurity programs are gaining importance. They are intended to prevent new invasive species from being introduced in the first place, for example through stricter controls in international trade.

How effective such measures can be is illustrated by one figure from the study: 88% of documented rat eradication programs on islands were successful. In many cases, the introduced animals disappeared permanently from the protected areas.

The ecological effects are often impressive. After invasive species are removed, ground-nesting birds return, reptiles increase again, and rare plants begin to germinate once more. Entire ecosystems can partially recover within just a few years.

The Ogasawara Islands off Japan provide an example. There, feral cats pushed the red-headed wood pigeon (Columba janthina nitens) to the brink of extinction. By 2008, fewer than 80 of these birds remained. Between 2010 and 2013, 131 cats were removed, after which the population recovered to almost a thousand pigeons within three years.

The example shows how quickly threatened species can recover when the most important disruptive factor disappears. Invasive species are therefore not an inevitable fate. With sufficient knowledge, political will, and long-term funding, much of the damage they cause can be limited—and in some cases even reversed.

How Are Invasive Species Controlled on Islands?

On isolated islands, invasive species can often be removed in a targeted and long-term manner. The process usually follows a clearly defined sequence.

First, specialists assess which invasive species are present, how large their populations are, and which native species are particularly at risk. Based on this information, a detailed management plan is developed.

This is followed by active control measures, such as trapping, targeted baiting programs, or the capture of feral animals. The goal is to completely remove the introduced species before they can reproduce again.

In practice, this usually means that captured animals are not relocated but humanely euthanized in accordance with animal welfare standards. Releasing them elsewhere would merely shift the problem and cause new ecological damage. The aim is therefore to minimize suffering while protecting entire species from extinction in the long term.

At the same time, strict quarantine and monitoring measures are introduced to prevent reintroduction. Even after the main eradication phase, islands are often monitored for many years.

Sources: Island Conservation; Department of Conservation (DOC)

What does this mean for conservation?

The study’s findings have consequences for practical conservation. Invasive species are not a marginal issue, but a central factor in global species loss. Worldwide, they now affect more than 60% of all threatened species. This places biological invasions on a par with habitat loss, overexploitation, and climate change, and they must not be treated as a side topic.

The special significance of invasive species becomes visible on islands. They are hotspots of extinction and extremely vulnerable. Many island species live in small, isolated populations, have few defenses against introduced enemies, and have no possibility to escape. Here, consistent measures against invasive species can make the difference between survival and disappearance.

The study also shows that isolated single measures are rarely enough. Habitat protection, regulating use, climate action, and controlling invasive species are interlinked. When these factors are considered together, the drivers of extinction can be addressed effectively.

Successful nature and species conservation requires international cooperation, scientific support, and the willingness to make uncomfortable decisions. But the study offers hope: where invasive species are recognized early and consistently controlled, nature can recover surprisingly quickly.

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Source

• Peh, K. S.-H., & Bird, Z. (2026). The link between invasive alien species and extinction. Biosphere, 2(1), 2. https://doi.org/10.3390/biosphere2010002