Large herbivores’ effects on ecosystems depend more on size and diet than on herbivore origin

The effect of large herbivores on plant abundance and diversity depends more on their size and diet than whether they are native or introduced into their host ecosystems, according to a meta-analysis of more than 200 studies worldwide. The findings counter the widely held notion that the impacts of introduced megafauna are distinct and more harmful than those of native megafauna and suggest that trait-based ecology provides better insight into megaherbivore-plant interactions than concepts of species origin. Large mammal herbivores play a key role in shaping ecosystems and biodiversity by consuming vegetation, dispersing seeds and nutrients, and creating disturbances. However, since the end of the Pleistocene, many native megaherbivore species have gone extinct or have experienced precipitous population declines. More recently, humans have introduced many large herbivores to regions worldwide. Although these introductions have, in some ways, counteracted the loss of native megafauna and their functionality, introduced megafauna are thought to have greater negative effects on the ecosystems that host them. As a result, conservation policies in some areas have prioritized eradicating or culling introduced megafauna populations, even though many of these animals are threatened with extinction in their native ranges.

To better understand whether introduced megaherbivores have stronger and more negative impacts on plant abundance and diversity than native species and the factors that account for these effects, Erick Lundgren and colleagues performed a meta-analysis encompassing the datasets from 221 studies spanning 6 continents. Lundgren et al. found no evidence that herbivore origin shapes the effects of megafauna on plants. Nor did species ‘invasiveness,’ ‘feralness,’ coevolutionary history with local flora, or phylogenetic and functional novelty of large herbivores. Instead, the authors discovered that megafauna functional traits, specifically species’ body size and dietary selectivity, have stronger effects on plant diversity. “We argue that the effects of introduced megafauna should be studied as any other wildlife would be studied, through the lens of functional ecology, with the normative dimensions of their ‘belonging’ considered separately and with transparency,” write Lundgren et al. In a related Perspective, Yvonne Buckley and Andrew Torsney discuss the study in more detail.

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