Bristol Study Reveals Invasive Species Cause Widespread Animal Suffering Worldwide

A groundbreaking study from the University of Bristol's Bristol Veterinary School has illuminated a previously underappreciated dimension of biological invasions: their profound effects on animal welfare. Published on 5 May 2026 in the prestigious journal Nature Communications, the research introduces a novel framework that quantifies how invasive species inflict suffering on both native and introduced animals across the globe. Led by Research Fellow Thomas Evans and Professor Michael Mendl, this work bridges invasion biology and animal welfare science, urging a reevaluation of how we approach environmental management.

The paper, titled "Quantifying and categorising the animal welfare impacts caused by biological invasions," demonstrates that while biological invasions are notorious for disrupting ecosystems and driving species extinctions, their toll on individual animal well-being—injuries, chronic stress, and behavioral disruptions—has been largely overlooked. This oversight matters because understanding welfare impacts can sharpen conservation priorities and bolster biosecurity measures, particularly in regions like the United Kingdom where invasive species pose ongoing threats to biodiversity and agriculture.

The Innovators Behind the Study: Bristol Veterinary School Excellence

Thomas Evans, now affiliated with Freie Universität Berlin and the Leibniz Institute of Freshwater Ecology and Inland Fisheries while maintaining ties to Bristol, brings expertise in biodiversity and welfare impacts of invasions. Professor Mike Mendl, a veteran in animal behaviour and welfare at Bristol Veterinary School, has long championed integrating welfare science into broader ecological contexts. Their collaboration exemplifies the interdisciplinary prowess of UK higher education institutions.

Bristol Veterinary School, part of the Faculty of Health and Life Sciences, stands as one of Europe's leading centres for veterinary education and research. Located at the Langford Campus, it offers state-of-the-art facilities for clinical training, wildlife health studies, and behaviour research. This study underscores Bristol's commitment to real-world applications, training future veterinarians and ecologists to tackle pressing global challenges like invasive species management.

Biological Invasions: A Global Ecological Challenge

Biological invasions occur when non-native species are introduced to new environments, often via human activities such as trade, travel, or accidental transport. In the UK, examples abound: the grey squirrel (Sciurus carolinensis) outcompetes native red squirrels, while the American signal crayfish (Pacifastacus leniusculus) devastates freshwater ecosystems by preying on native species and spreading crayfish plague.

These invasions cost economies billions—estimated at £4.5 billion annually in the UK alone through agricultural damage and control efforts—while eroding biodiversity. Yet, the study's innovation lies in shifting focus from population-level effects to individual-level suffering, revealing hidden costs that traditional metrics miss.

Unveiling AWICIS: The Game-Changing Framework

At the heart of the research is the Animal Welfare Impact Classification for Invasion Science (AWICIS), a structured tool designed to systematically evaluate welfare changes in animals affected by invasions. Drawing from the established Environmental Impact Classification for Alien Taxa (EICAT) but tailored for welfare, AWICIS categorises impacts across five domains inspired by the Five Domains Model: nutrition, physical environment, health, behavioural interactions, and mental state.

The framework identifies 11 impact mechanisms, including predation, competition, hybridisation, and disease transmission. Severity is graded from i (negligible) to v (more severe than in the absence of invasion), distinguishing short-term (under one hour) from prolonged effects. Evidence draws from physical signs (e.g., injuries), behavioural indicators (e.g., stereotypic preening), and physiological markers (e.g., elevated stress hormones like corticosterone).

AWICIS is publicly available via Figshare, empowering researchers worldwide. Its application to existing literature on bird and ant invasions proves its practicality, with assessors achieving 87-93% agreement in categorisation tests.

Severe Welfare Toll from Invasive Ants

Analysis of ant invasions revealed stark results: 92% of impacts classified as 'more severe' (levels iv-v). Predation dominated (85%), affecting animals across six classes and 27 orders—from birds and reptiles to mammals. Native species bore 95% of the brunt, though introduced pets like cats and dogs suffered too, such as Florida keratopathy from ant stings.

Iconic examples include the red imported fire ant (Solenopsis invicta), whose aggressive venom attacks cause corneal damage, missing limbs in seabirds, and heightened stress in lizards. Yellow crazy ants (Anoplolepis gracilipes) similarly inflict widespread harm. Even where native ants exist, invasives prove more virulent, amplifying suffering through sheer numbers and toxicity.

Photo by Roman Kraft on Unsplash

• Physical domain: Injuries from stings/bites (86% of ant impacts).
• Behavioural: Avoidance or aggression alterations.
• Mental: Chronic stress inferred from physiological changes.

Contrasting Impacts of Introduced Birds

In contrast, bird invasions showed milder effects: only 9% 'more severe.' Competition (61%) and predation (25%) prevailed, impacting 28 orders, primarily Passeriformes (songbirds). Islands amplified severity (33% more severe), lacking native predators—think introduced rats or cats preying on seabirds.

Philornis downsi flies parasitising Galápagos birds exemplify: nestlings suffer wounds, deformities, and death. Yet, no severe impacts on introduced birds themselves were noted, highlighting asymmetry. Confidence was high (50%), underscoring reliable biodiversity data as a welfare proxy.

Global Patterns, Underreporting, and Data Gaps

Impacts skew towards high-income regions (North America, Europe, Oceania) and islands, likely due to better monitoring. Underreporting plagues introduced animals and low-income areas. Physiological data remains scarce (only 2% for ants), limiting subtlety detection—e.g., competition-induced anxiety.

The study mines 1,000+ biodiversity papers, proving invasion literature as a welfare goldmine. For UK relevance, consider Asian hornets (Vespa velutina) threatening honeybees or Himalayan balsam altering riverbanks, potentially stressing aquatic life.

Policy and Conservation Implications for the UK

In the UK, the Great Britain Invasive Non-Native Species Strategy (2023) targets prevention and rapid response. AWICIS enhances this by quantifying welfare, aiding prioritisation—like banning high-risk ants at borders. It supports the 'One Health' approach, linking animal, human, and environmental welfare.

The GB Strategy emphasises coordinated action; Bristol's framework could inform risk assessments under the Invasive Alien Species Regulation.

Bristol Veterinary School: A Hub for Welfare Innovation

Bristol Veterinary School excels in blending veterinary medicine with ecology. Its Wildlife Health and Conservation Medicine courses equip students to address invasion welfare. Facilities like the Behaviour and Welfare Lab enable cutting-edge research, fostering PhD opportunities in interdisciplinary fields.

Evans and Mendl's work aligns with Bristol's Cabot Institute for the Environment, tackling climate-invasion synergies. For aspiring researchers, Bristol offers funded positions via UKRI, positioning it as a leader in ethical environmental science.

UK Higher Education's Broader Contributions

Beyond Bristol, UK universities drive invasive research. The Royal Veterinary College (RVC) advances welfare science via its Animal Welfare Science cluster. Plymouth University studies marine invasives like slipper limpets. Oxford's WildCRU integrates welfare into conservation, influencing global policy.

Funding from NERC and Defra supports networks like INVASIVESNET, training early-career scientists. Careers in this field—lecturer roles, research assistants—thrive amid rising biosecurity demands.

RVC's welfare research complements AWICIS, promoting holistic approaches.

Photo by Sue Winston on Unsplash

Future Directions: Research and Career Opportunities

Challenges persist: physiological monitoring in wild settings and low-income data gaps. Solutions include tech like camera traps and hormone assays. UK universities gear up with AI-driven invasion forecasting.

For students, MSc/PhD programs at Bristol, RVC, or Exeter offer pathways. Job markets expand in policy (Environment Agency), NGOs (RSPB), and academia.

• Key skills: Field ecology, stats (R/Python), welfare ethics.
• Opportunities: Postdocs via Horizon Europe, lecturing amid faculty shortages.

Charting a Welfare-Informed Path Forward

Bristol's AWICIS marks a pivotal advance, embedding animal sentience into invasion management. As UK higher education confronts biodiversity crises, such innovations from Bristol Veterinary School exemplify leadership. By prioritising welfare, we safeguard ecosystems and ethics, paving the way for compassionate conservation.

Explore research careers shaping this field at leading UK institutions.