Since the emergence of Homo sapiens as a species, many wild herbivore populations have been driven to extinction, and alongside them we also lost their ecological functions. While non-native species are usually bad news for conservationists, new evidence1 suggests that introduced wildlife may actually help restore the ecological balances that existed before the megafauna extinctions of the Late Pleistocene (130,000- 11,700 years ago).
Throughout the last 100,000 years, humans have caused the extinction of a multitude of large-bodied mammals, including many wild herbivore species. These extinctions have led to cascading events in natural ecosystems and their repercussions still affect us today, such as through increased wildfire frequency and reduced ecosystem productivity.
While rewilding efforts are increasingly implemented across the world, hoping to restore healthy ecosystems, the idea that non-native species could help bring ecosystems back to their natural balances has been given less consideration. In fact, still today, introduced species are often associated with negative conservation outcomes.
The argument that non-native species can help restore healthy ecosystem received support by a new paper1 published in Proceedings of the National Academy of Sciences, coauthored by a group of international researchers led by Erick J. Lundgren of the University of Technology Sydney, Australia. “Although introduced species have long been a source of contention, our findings indicate that they may, in part, restore ecological functions reflective of the past several million years before widespread human-driven extinctions”, the authors report.
Even though different species may have evolved in separate geographical regions, they may still share similar physiological, behavioural and ecological aspects that would make them good replacements for each other. Following this logic, non-native species could be introduced in novel landscapes to fill the ecological niches of animal populations that are not longer around today.
The study involved comparisons of specific traits of native, non-native and extinct species, looking for matches. Such analysis excluded domestic herbivores, as their populations are not necessarily ecologically viable without human interventions and their interactions with the environment are heavily modified by people.
The findings reveal that over half (64%) of currently introduced species are most similar to extinct herbivores, and 42% of them may actually be replacing the ecological functions of extinct herbivore populations.
“The similarity of introduced herbivores to extinct ones indicates that introductions have restored lost trait combinations and, thus, functions. To better understand which functions have been restored, we focused on key ‘metabolic’ functions herbivores contribute in ecosystems by consuming plant biomass and by cycling and redistributing nutrients”, the authors explain.
Some introduced herbivores and their extinct nearest neighbors in those continents most impacted by extinctions and introductions. The color of the top bar indicates the number of extant species (per body mass bin and climate zone) that are more similar to the nearest neighbour than the introduced species is, while the lower bar color indicates dietary guild. Figure credits: 1
The study opens up the idea of future rewilding initiatives that go beyond the reintroductions of only native wildlife. Nevertheless, the authors stress that today’s ecosystems and landscapes are very different than those of many thousands of years ago, and even though the introduced species could in theory play the ecological role of extinct taxa, in practice there could be several issues hindering such process.
“Many apex predators continue to face declines. Likewise, ongoing landscape fragmentation restricts herbivore movements and can lead to concentrated herbivory. Therefore, while introductions make herbivore assemblages more functionally similar to [those of the past], they do not necessarily restore ecosystems to [past] conditions due to ongoing anthropogenic pressures”, the authors conclude.
Introducing non-native species to novel ecosystems may still not resonate with the ethos of many conservationists, who may see this approach too risky and controversial to be implemented in practice. The research of the next decades will be crucial to bring new evidence and insights into the rewilding debate, and accordingly inform management.
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- 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. and Wallach, A.D., 2020. Introduced herbivores restore Late Pleistocene ecological functions. Proceedings of the National Academy of Sciences. https://doi.org/10.1073/pnas.1915769117
Cover image credits: https://bit.ly/2JDXEnP