Both extinctions and introductions of large mammalian herbivores (>45 kg) affect biodiversity and ecosystem function. An important question is whether the origin of herbivorous megafauna predicts impacts on an ecosystem that are specific to introduced species in general, more severe than those of native species, or both. On page 531 of this issue, Lundgren et al. (1) report that functional traits, rather than the origin (introduced or native), of large herbivores are correlated with native plant diversity. Together with previous contradictory findings on the ecological effects of introduced and native herbivores (2), the study raises the question of whether herbivore size and dietary specialism are general determinants of invasion impact, origin effects, or both.

Large mammalian herbivores that range in size from red kangaroos (Osphranter rufus) and goats (Capra hircus) to African bush elephants (Loxodonta africana) have profound effects on ecosystems through their functions as consumers and modifiers of plant biomass (3), dispersers of seeds (4), and, when dead, concentrated resources for scavengers and decomposers (5). The largest adult megaherbivores (>1000 kg; e.g., Rhinoceros spp., Hippopotamus amphibius, and elephants) are relatively resistant to predation pressure, which has important consequences for the flow of energy through ecosystems and landscape heterogeneity (6). However, large animals are also vulnerable to local and global extinctions (7). This is attributed to a mixture of intrinsic traits, such as low reproductive output, and extrinsic exposure to human impacts. Although large mammalian herbivores have been lost in many habitats, some ecosystems have gained them through human-mediated introductions to areas where they did not previously occur.

Feral goats, buffalo, and horses (wild populations of formerly domesticated species) are high-impact invasive species, particularly in regions where humans have caused the decline and extinction of other large herbivores or where large herbivores with traits similar to the introduced species never occurred. Indeed, goat (C. hircus), fallow deer (Dama dama), and wild boar (Sus scrofa) are included in the list of 100 of the world’s worst invasive species (8). Goats that were introduced to islands have proven to be particularly problematic as a driver of local species extinctions (9), and their eradication from islands is a conservation success story (10)...

Large terrestrial mammalian herbivores (≥45 kg; henceforth “megafauna”) have distinct effects on ecosystems by causing disturbance, consuming low-nutrient vegetation, and dispersing seeds and nutrients (1, 2). These effects were ubiquitous for ~55 million years until the extinctions of the Late Pleistocene and Holocene (~130,000 to 7000 years before present) (3). More recently, humans have introduced numerous megafauna, which have partially counteracted these declines numerically (4) and functionally (5, 6), and which contribute some lost ecological functions, such as increasing water availability through well digging and reducing wildfire (7, 8).

However, introduced megafauna can also reduce native plant abundance and diversity and promote introduced plants (9). These effects are generally interpreted as evidence that the impacts of introduced megafauna are distinct from those of native megafauna (10). Accordingly, conservation policy has prioritized the eradication and culling of introduced megafauna, even though 50% of these species are threatened or extinct in their native ranges (11).

The notion that native and introduced species have distinct effects is most often justified by the functional postulate that long-term community-wide coevolutionary history shapes ecological interactions (12–14). Coevolution has been inferred at broad macroevolutionary scales [e.g., the evolution of grasses and grazers throughout the Cenozoic, or the evolution of plant defenses (15, 16)] and plays a role in specialized interactions, as evidenced by the consequences of introduced pathogens (17). However, these observations have been extended to justify a broader biological reality to nativeness in which coevolution also shapes diffuse, generalist interactions with high taxonomic precision, such as between individual plant and megafauna species. Nativeness has thus become central to conservation policy (18); widespread notions of ecological “health” (19); and basic biodiversity data, which only count populations thought to be native (20).

However, critics have argued that coevolution is unlikely to shape generalist interactions in the same way it does specialized ones and that long-term community-wide coevolution is unmeasurable (21, 22). Instead, critics have suggested that ecological factors, such as predation, the environment, and functional traits, may sufficiently explain the effects of both introduced and native organisms (23, 24). If so, and if it were impossible to determine the nativeness of an organism from their actual effects, then nativeness would remain a description of dispersal history but would not be a meaningful way to understand ecological interactions (23).

We employed a meta-analytic dataset of 3995 responses from 221 studies to evaluate whether nativeness and/or ecological factors (Table 1) could explain the effects of wild herbivorous megafauna (≥45 kg) on plant abundance (N = 3221 responses) and plant diversity (N = 774) (25, 26)...

Fig. 4. Dietary selectivity influences megafauna impacts on plant diversity.

(A) There was strong evidence that megafauna communities dominated by bulk-feeding generalists increased local plant diversity. Dietary generalism was estimated with muzzle width of each megafauna community (maximum, weighted by relative biomass per species; see fig. S3 for mean muzzle width). Letters in the plot indicate the taxa highlighted in (C) to (F). [Icons: Gabriela Palomo-Munoz, Jan A. Venter, Herbert H. T. Prins, David A. Balfour, and Rob Slotow (vectorized by T. Michael Keesey)] (B) Effect sizes for select groups of representative taxa from communities where these species constitute >50% of total megafauna biomass. Deer include all Cervidae, and wild pigs include all Suidae (primarily introduced wild boar, Sus scrofa). Equids include all Equidae but primarily feral horses (Equus ferus caballus). Large, broad-muzzled bovids include the genera Bison, Bos, and Syncerus. (C) Native and introduced deer can reduce plant diversity by selectively browsing preferred plants (49, 50). [Photo: Murray Foubister] (D) Pigs are distinct for belowground foraging and are dietary generalists, despite their relatively narrow muzzles (51). Feral pigs often increase plant diversity, at times doubling native plant diversity by suppressing competitive dominants (52). [Photo: Valentin Panzirsch] (E) Feral horses (E. ferus caballus) appear to have mixed effects on local plant diversity. (F) Bulk-grazers, like cape buffalo (Syncerus caffer) and bison (Bison bison), tend to increase plant diversity (53). Our results suggest that this is driven by their inability to selectively feed, forcing them to consume the most abundant (i.e., competitively dominant) plants. [Photo: Stig Nygaard]