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. 2026 May 11;40(5):e70328. doi: 10.1111/cobi.70328

Reevaluating introduced herbivores in conservation

Jonas Trepel 1,✉, Robert Buitenwerf 1, Kristy Ferraro 2, Elizabeth le Roux 1,3, Erick Lundgren 4, Brian Maitner 5, Tanya Strydom 6, Jens‐Christian Svenning 1, Matthew R Kerr 1,✉
PMCID: PMC13613557  PMID: 42109002

The impacts of non‐native species are the subject of enduring controversy in conservation research and policy. This debate has renewed relevance in light of rewilding approaches that introduce or reintroduce herbivores to restore trophic complexity. However, because large herbivores inherently have strong effects, recent work has warned that the “harms of introduced large herbivores outweigh their benefits” (Bescond‐Michel et al., 2025; Nogués‐Bravo et al., 2016; Nores et al., 2024). We argue that such conclusions are conceptually and methodologically flawed, particularly because they conflate the predictable consequences of herbivory with harmfulness attributed to a species' non‐native status. Without assessing whether effects of non‐native herbivores differ from those of native herbivores, these approaches risk depicting the fundamental process of herbivory as problematic. This can misinform conservation and restoration efforts at a time when herbivores, native or introduced, are increasingly recognized as essential for sustaining biodiversity and ecosystem function (Lundgren et al., 2018; Malhi et al., 2016; Pringle et al., 2023; Smith et al., 2016; Svenning et al., 2024; Trepel et al., 2024).

CONCEPTUAL FLAWS IN ASSESSING THE IMPACT OF LARGE HERBIVORES

The International Union for Conservation of Nature's (IUCN) Environmental Impact Classification for Alien Taxa (EICAT and EICAT+) (Kumschick et al., 2020; Vimercati et al., 2022) uses vote‐counting methods to categorize non‐native species based on their effects on native species' fitness and abundance (Kumschick et al., 2020). We argue that using these classifications to determine whether “harms” outweigh “benefits” (Bescond‐Michel et al., 2025) lacks ecological foundation. All terrestrial animals eat, move, and reproduce. Inevitably, eating reduces the abundance of food resources, and movement alters surroundings. Yet, EICAT classifies these predictable ecological processes as “negative impacts”. By this logic, every living animal has negative effects on the species it consumes.

Moreover, there is little focus in the literature on the positive and neutral impacts of non‐native species (Vimercati et al., 2022), and studies on the impacts of introduced species frequently frame non‐native species as harmful regardless of their actual impacts (Pereyra et al., 2024). Even though EICAT+ allows recording of positive effects, it is difficult to see how these biases will not result in a circularity: using reports that are biased toward negative impacts of introduced herbivores to confirm that introduced herbivores have negative effects.

Treating ecological processes as inherently harmful ignores their role in maintaining biodiversity. Ecological theory predicts that so‐called negative effects are often precisely what sustains biodiversity. In fact, processes, such as predation, herbivory, and physical disturbance, are core stabilizing mechanisms (Chesson, 2000) that prevent competitive exclusion and enable diverse assemblages to persist (Janzen, 1970; Koerner et al., 2018; Olff & Ritchie, 1998). By interpreting these impacts as harmful, EICAT moralizes natural processes and conflates ecology with value judgments, preventing robust scientific inference about the impact of non‐native species. Applied universally, EICAT would indicate that all herbivores are harmful, a highly misleading outcome given the fundamental role of herbivory in shaping ecosystems and sustaining biodiversity.

ILL‐SUITED METHODOLOGY TO SUMMARIZE THE EFFECTS OF NON‐NATIVE HERBIVORES

To verify claims that introduced herbivores are harmful—and thus unsuitable for inclusion in trophic rewilding and ecosystem restoration—comparisons with appropriate controls (impacts of native herbivores) are required. After all, native species already have a variety of positive and negative impacts on other species. For example, native rhinoceros reduce vegetation cover but thereby help smaller grazers and reduce wildfire severity (Waldram et al., 2008); native elephants damage trees but increase understory biomass and open habitats (Coverdale et al., 2016); and native, large herbivores in East Africa suppress small mammal populations but decrease tick densities and disease risks (Titcomb et al., 2017).

Undoubtedly, introduced herbivores can contribute to species endangerment, particularly on islands, and these cases are important conservation concerns. Yet, similar outcomes can occur with native herbivores. For example, native giraffes can drive extirpations of tree species in South Africa (Bond & Loffell, 2001). Most of the “negative” evidence compiled in a recent summary (Bescond‐Michel et al., 2025) reflects nothing more than introduced herbivores reducing the abundance of their food resources. Moreover, a recent systematic meta‐analysis shows no difference in the effects of native and introduced large herbivores on plant abundance or diversity (Lundgren et al., 2024).

SHIFTED BASELINES

We suggest that the conflation of herbivory with ecological harm reflects shifting ecological baselines (Figure 1). For example, assuming that non‐native species “destabilize eco‐evolutionary dynamics” (Bescond‐Michel et al., 2025; Rejmánek & Simberloff, 2017) ignores the fact that most contemporary ecosystems lack fundamental components of these dynamics: large herbivores. Large mammalian herbivores have shaped ecosystems worldwide for >40 million years, driving essential ecological processes through consumption, breaking and trampling of plants, seed and nutrient dispersal, and provision of food for predators and decomposers (Malhi et al., 2016).

FIGURE 1.

FIGURE 1

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A community at three different stages: (left panel) evolutionary baseline, which includes large and megaherbivores; (middle) current state after the extirpation or extinction of most large herbivore species; and (right panel) the state after the introduction or reintroduction of large herbivores.

The loss of herbivores has resulted in large shifts in ecosystem structure (Svenning et al., 2024). Since the global expansion of Homo sapiens, megafauna declines and extinctions due to human activities have led to a shifted baseline, wherein defaunated ecosystems are seen as ecologically normal and intact (Søndergaard et al., 2025).

CONCLUSION

By definition, all herbivores reduce the abundance or performance of some plants simply through consumption. Although introduced herbivores can have strong effects that require management, conflating herbivory with ecological harm can obstruct efforts to restore and protect the processes that sustain biodiversity. In most contexts, the ecological roles of non‐native large herbivores (and many other species) do not differ functionally from native species (Davis et al., 2011; Lundgren et al., 2020, 2024; Sagoff, 2005, 2009; Strauss et al., 2006; Wallach et al., 2015, 2023; Wooster et al., 2024): both generate a mixture of positive and negative effects that sustain biodiversity and ecosystem processes. Recognizing this is critical in the Anthropocene, where many large herbivores are extinct, confined to refuge populations, or persist at reduced densities. Ironically, 50% of introduced megafauna are threatened or extinct in their native ranges (Lundgren et al., 2026), further emphasizing the need to develop robust, ecologically grounded frameworks to evaluate their impacts, rather than relying on normative judgements. In conclusion, we argue that restoration and rewilding efforts should be guided by ecological impact and functional considerations not by a priori exclusions based on species’ origin.

AUTHOR CONTRIBUTIONS

All authors contributed to the conceptualization, writing, and editing of the manuscript. T.S. designed the figure.

ACKNOWLEDGMENTS

We thank N. A. Mungi for feedback on a previous version of the manuscript and Z. Bescond‐Michel, S. Bacher, and G. Vimercati for collegial and constructive discussions on the topic. We acknowledge the Danish National Research Foundation for economic support via the Center for Ecological Dynamics in a Novel Biosphere (ECONOVO) (grant DNRF173) and the Natural Science and Engineering Research Council of Canada (NSERC).

Trepel, J. , Buitenwerf, R. , Ferraro, K. , le Roux, E. , Lundgren, E. , Maitner, B. , Strydom, T. , Svenning, J.‐C. , & Kerr, M. R. (2026). Reevaluating introduced herbivores in conservation. Conservation Biology, 40, e70328. 10.1111/cobi.70328

Article impact statement: Herbivory is a fundamental ecological process; treating it as harm misinforms conservation decisions about introduced herbivores.

Contributor Information

Jonas Trepel, Email: jonas.trepel@bio.au.dk, Email: jonas.trepel@gmail.com.

Matthew R. Kerr, Email: matthew.kerr@bio.au.dk.

REFERENCES

  1. Bescond‐Michel, Z. , Bacher, S. , & Vimercati, G. (2025). Harms of introduced large herbivores outweigh benefits to native biodiversity. Nature Communications, 16(1), Article 8260. [DOI] [PMC free article] [PubMed] [Google Scholar]
  2. Bond, W. J. , & Loffell, D. (2001). Introduction of giraffe changes acacia distribution in a South African savanna. African Journal of Ecology, 39(3), 286–294. [Google Scholar]
  3. Chesson, P. (2000). Mechanisms of maintenance of species diversity. Annual Review of Ecology and Systematics, 31, 343–366. [Google Scholar]
  4. Coverdale, T. C. , Kartzinel, T. R. , Grabowski, K. L. , Shriver, R. K. , Hassan, A. A. , Goheen, J. R. , Palmer, T. M. , & Pringle, R. M. (2016). Elephants in the understory: Opposing direct and indirect effects of consumption and ecosystem engineering by megaherbivores. Ecology, 97(11), 3219–3230. [DOI] [PubMed] [Google Scholar]
  5. Davis, M. A. , Chew, M. K. , Hobbs, R. J. , Lugo, A. E. , Ewel, J. J. , Vermeij, G. J. , Brown, J. H. , Rosenzweig, M. L. , Gardener, M. R. , Carroll, S. P. , Thompson, K. , Pickett, S. T. A. , Stromberg, J. C. , Tredici, P. D. , Suding, K. N. , Ehrenfeld, J. G. , Philip Grime, J. , Mascaro, J. , & Briggs, J. C. (2011). Don't judge species on their origins. Nature, 474(7350), 153–154. [DOI] [PubMed] [Google Scholar]
  6. Janzen, D. H. (1970). Herbivores and the number of tree species in tropical forests. The American Naturalist, 104(940), 501–528. [Google Scholar]
  7. Koerner, S. E. , Smith, M. D. , Burkepile, D. E. , Hanan, N. P. , Avolio, M. L. , Collins, S. L. , Knapp, A. K. , Lemoine, N. P. , Forrestel, E. J. , Eby, S. , Thompson, D. I. , Aguado‐Santacruz, G. A. , Anderson, J. P. , Anderson, T. M. , Angassa, A. , Bagchi, S. , Bakker, E. S. , Bastin, G. , Baur, L. E. , … & Zelikova, T. J. (2018). Change in dominance determines herbivore effects on plant biodiversity. Nature Ecology & Evolution, 2(12), 1925–1932. [DOI] [PubMed] [Google Scholar]
  8. Kumschick, S. , Bacher, S. , Bertolino, S. , Blackburn, T. M. , Evans, T. , Roy, H. E. , & Smith, K. (2020). Appropriate uses of EICAT protocol, data and classifications. NeoBiota, 62, 193–212. [Google Scholar]
  9. Lundgren, E. , Svenning, J.‐C. , Schlaepfer, M. A. , Wallach, A. , Andersson, A. , Marris, E. , Rohwer, Y. , & Ramp, D. (2026). Many pasts, many futures: Navigating the complexities of species reshuffling to help prevent extinctions. Cambridge Prisms: Extinction, 4, Article e2. [DOI] [PMC free article] [PubMed] [Google Scholar]
  10. Lundgren, E. J. , Bergman, J. , Trepel, J. , le Roux, E. , Monsarrat, S. , Kristensen, J. A. , Pedersen, R. Ø. , Pereyra, P. , Tietje, M. , & Svenning, J.‐C. (2024). Functional traits—Not nativeness—Shape the effects of large mammalian herbivores on plant communities. Science, 383(6682), 531–537. [DOI] [PubMed] [Google Scholar]
  11. Lundgren, E. J. , Ramp, D. , Ripple, W. J. , & Wallach, A. D. (2018). Introduced megafauna are rewilding the Anthropocene. Ecography, 41(6), 857–866. [Google Scholar]
  12. Lundgren, E. J. , Ramp, D. , Rowan, J. , Middleton, O. , Schowanek, S. D. , Sanisidro, O. , Carroll, S. P. , Davis, M. , Sandom, C. J. , Svenning, J.‐C. , & Wallach, A. D. (2020). Introduced herbivores restore Late Pleistocene ecological functions. Proceedings of the National Academy of Sciences, 117(14), 7871–7878. [DOI] [PMC free article] [PubMed] [Google Scholar]
  13. Malhi, Y. , Doughty, C. E. , Galetti, M. , Smith, F. A. , Svenning, J.‐C. , & Terborgh, J. W. (2016). Megafauna and ecosystem function from the Pleistocene to the Anthropocene. Proceedings of the National Academy of Sciences, 113(4), 838–846. [DOI] [PMC free article] [PubMed] [Google Scholar]
  14. Nogués‐Bravo, D. , Simberloff, D. , Rahbek, C. , & Sanders, N. J. (2016). Rewilding is the new Pandora's box in conservation. Current Biology, 26(3), R87–R91. [DOI] [PubMed] [Google Scholar]
  15. Nores, C. , Álvarez‐Laó, D. , Navarro, A. , Pérez‐Barbería, F. J. , Castaños, P. M. , Castaños de la Fuente, J. , Morales Muñiz, A. , Azorit, C. , Muñoz‐Cobo, J. , Fernández Delgado, C. , Granado Lorencio, C. , Palmqvist, P. , Soriguer, R. , Delibes, M. , Vilà, M. , Simón, M. , Cabezudo, B. , Galán, C. , García‐Berthou, E. , … López‐Bao, J. V. (2024). Rewilding through inappropriate species introduction: The case of European bison in Spain. Conservation Science and Practice, 6(12), Article e13221. [Google Scholar]
  16. Olff, H. , & Ritchie, M. E. (1998). Effects of herbivores on grassland plant diversity. Trends in Ecology & Evolution, 13(7), 261–265. [DOI] [PubMed] [Google Scholar]
  17. Pereyra, P. J. , de la Barra, P. , Amione, L. L. D. , Arcángel, A. , Marello Buch, B. M. , Rodríguez, E. , Mazzolari, A. , Maldonado, M. A. , Hünicken, L. , & Wallach, A. D. (2024). Systematic and persistent bias against introduced species. Bioscience, 74(1), 44–53. [Google Scholar]
  18. Pringle, R. M. , Abraham, J. O. , Anderson, T. M. , Coverdale, T. C. , Davies, A. B. , Dutton, C. L. , Gaylard, A. , Goheen, J. R. , Holdo, R. M. , Hutchinson, M. C. , Kimuyu, D. M. , Long, R. A. , Subalusky, A. L. , & Veldhuis, M. P. (2023). Impacts of large herbivores on terrestrial ecosystems. Current Biology, 33(11), R584–R610. [DOI] [PubMed] [Google Scholar]
  19. Rejmánek, M. , & Simberloff, D. (2017). Origin matters. Environmental Conservation, 44(2), 97–99. [Google Scholar]
  20. Sagoff, M. (2005). Do non‐native species threaten the natural environment? Journal of Agricultural and Environmental Ethics, 18(3), 215–236. [Google Scholar]
  21. Sagoff, M. (2009). Environmental harm: Political not biological. Journal of Agricultural and Environmental Ethics, 22(1), 81–88. [Google Scholar]
  22. Smith, F. A. , Doughty, C. E. , Malhi, Y. , Svenning, J.‐C. , & Terborgh, J. (2016). Megafauna in the Earth system. Ecography, 39(2), 99–108. [Google Scholar]
  23. Søndergaard, S. A. , Fløjgaard, C. , Ejrnæs, R. , & Svenning, J.‐C. (2025). Shifting baselines and the forgotten giants: Integrating megafauna into plant community ecology. Oikos, 2025(5), Article e11134. [Google Scholar]
  24. Strauss, S. Y. , Lau, J. A. , & Carroll, S. P. (2006). Evolutionary responses of natives to introduced species: What do introductions tell us about natural communities? Ecology Letters, 9(3), 357–374. [DOI] [PubMed] [Google Scholar]
  25. Svenning, J.‐C. , Lemoine, R. T. , Bergman, J. , Buitenwerf, R. , Roux, E. L. , Lundgren, E. , Mungi, N. , & Pedersen, R. Ø. (2024). The late‐Quaternary megafauna extinctions: Patterns, causes, ecological consequences and implications for ecosystem management in the Anthropocene. Cambridge Prisms: Extinction, 2, Article e5. [DOI] [PMC free article] [PubMed] [Google Scholar]
  26. Titcomb, G. , Allan, B. F. , Ainsworth, T. , Henson, L. , Hedlund, T. , Pringle, R. M. , Palmer, T. M. , Njoroge, L. , Campana, M. G. , Fleischer, R. C. , Mantas, J. N. , & Young, H. S. (2017). Interacting effects of wildlife loss and climate on ticks and tick‐borne disease. Proceedings of the Royal Society B: Biological Sciences, 284(1862), Article 20170475. [DOI] [PMC free article] [PubMed] [Google Scholar]
  27. Trepel, J. , Le Roux, E. , Abraham, A. J. , Buitenwerf, R. , Kamp, J. , Kristensen, J. A. , Tietje, M. , Lundgren, E. J. , & Svenning, J.‐C. (2024). Meta‐analysis shows that wild large herbivores shape ecosystem properties and promote spatial heterogeneity. Nature Ecology & Evolution, 8(4), 705–716. [DOI] [PubMed] [Google Scholar]
  28. Vimercati, G. , Probert, A. F. , Volery, L. , Bernardo‐Madrid, R. , Bertolino, S. , Céspedes, V. , Essl, F. , Evans, T. , Gallardo, B. , Gallien, L. , González‐Moreno, P. , Grange, M. C. , Hui, C. , Jeschke, J. M. , Katsanevakis, S. , Kühn, I. , Kumschick, S. , Pergl, J. , Pyšek, P. , … Bacher, S. (2022). The EICAT+ framework enables classification of positive impacts of alien taxa on native biodiversity. PLoS Biology, 20(8), Article e3001729. [DOI] [PMC free article] [PubMed] [Google Scholar]
  29. Waldram, M. S. , Bond, W. J. , & Stock, W. D. (2008). Ecological engineering by a mega‐grazer: White rhino impacts on a South African savanna. Ecosystems, 11(1), 101–112. [Google Scholar]
  30. Wallach, A. D. , Ramp, D. , Benítez‐López, A. , Wooster, E. I. F. , Carroll, S. , Carthey, A. J. R. , Rogers, E. I. E. , Middleton, O. , Zawada, K. J. A. , Svenning, J.‐C. , Avidor, E. , & Lundgren, E. (2023). Savviness of prey to introduced predators. Conservation Biology, 37(2), Article e14012. [DOI] [PubMed] [Google Scholar]
  31. Wallach, A. D. , Ripple, W. J. , & Carroll, S. P. (2015). Novel trophic cascades: Apex predators enable coexistence. Trends in Ecology & Evolution, 30(3), 146–153. [DOI] [PubMed] [Google Scholar]
  32. Wooster, E. I. F. , Middleton, O. S. , Wallach, A. D. , Ramp, D. , Sanisidro, O. , Harris, V. K. , Rowan, J. , Schowanek, S. D. , Gordon, C. E. , Svenning, J.‐C. , Davis, M. , Scharlemann, J. P. W. , Nimmo, D. G. , Lundgren, E. J. , & Sandom, C. J. (2024). Australia's recently established predators restore complexity to food webs simplified by extinction. Current Biology, 34(22), 5164.e2–5172.e2. [DOI] [PubMed] [Google Scholar]

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