When we think of invasive species, we usually think in terms of ecological damage: disrupted food chains, shrinking native populations, even extinctions. But a new study asks a darker question — how much suffering do these invaders cause before death even occurs?
And if the findings are anything to go by, fire ants may rank among the most brutal.
Published in Nature Communications, the study found that invasive ants — especially red imported fire ants — cause some of the most severe suffering recorded in wild animals, often through slow, repeated attacks rather than quick kills.
Researchers from Freie Universität Berlin and the University of Bristol created a new framework called Animal Welfare Impact Classification for Invasion Science (AWICIS) to measure not just biodiversity loss, but the pain and distress experienced by individual animals.
That shift in perspective led to grim findings. When the team analysed past records of invasive birds and ants, 92% of welfare impacts linked to invasive ants fell into the “highest severity” categories.
The victims weren’t just insects. The study documented suffering in animals across six major groups and 27 orders, including turtles, lizards, crabs, bats, seabirds, cats, and dogs.
What makes fire ants especially alarming is how they attack. Unlike predators that kill quickly, fire ants overwhelm victims in massive numbers. , but hundreds or thousands delivered over hours can become lethal.
That prolonged assault is what concerned researchers. For instance, in Hawaii, wedge-tailed shearwater chicks exposed to invasive ants were found with missing toes, malformed bills, and eyes partly sealed shut by skin overgrowth.
Similarly, on Japan’s Minami-Daito Island, a young bull-headed shrike attacked by yellow crazy ants developed severe eye inflammation and died days later. On Island, invasive ants have also been linked to devastating injuries in native red crabs, including blindness.
Researchers noted that the larger the animal, the longer it may take for repeated stings to become fatal — potentially extending suffering.
Traditionally, conservation studies focus on species-level outcomes: Did a population decline? Did a species disappear? But the researchers argue this misses what individual animals endure.
For example, an invasive species may not wipe out an entire population, yet still inflict severe physical harm and prolonged distress on countless animals.
Interestingly, invasive birds appeared far less severe in comparison. Only 9% of welfare impacts linked to introduced birds were classified in the higher severity tiers, mostly involving competition for food or nesting space rather than direct injury.
The researchers also flagged a blind spot: most existing data tracks visible injuries, while subtler signs of suffering — such as elevated stress hormones — are rarely studied.
That means the real toll could be even worse. The new study has nudged conservationists to also consider measuring not just who survives, but what animals go through in the process.
