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Introduced Megafauna Are Rewilding Ecosystems

Researchers have called for a reassessment of conservation values and attitudes surrounding introduced species following a study suggesting that large herbivores introduced to new regions are rewilding modern ecosystems.

The global study, published in Ecography, shows that introductions have significantly increased megafauna richness, numerically replacing many of the Pleistocene megafauna that were lost around 50,000 years ago. Of the world’s 76 existing megafauna species – defined as herbivorous mammals weighing at least 100 kg – 22 have introduced populations of which almost 50% are either threatened or extinct in their historic native ranges.

“This study challenges fundamental ideas surrounding ‘invasive species’ and shows that the redistribution of species is ‘rewilding’ the world,” says chief investigator Dr Arian Wallach of the University of Technology, Sydney’s Centre for Compassionate Conservation.

“The global decline of megafauna is being driven by habitat loss, changes in land use and overhunting. Despite this, some megafauna have found refuge in new habitats through introductions,” Wallach says.

“Conservation typically ignores these populations by defining them as alien or invasive. However, these populations are likely critical buffers against extinction, and there is growing evidence that they are making positive contributions in their new homes.”

Australia is home to eight introduced megafauna species that may fill vacant niches in the continent’s ecosystems. These include species like endangered water buffalo (Bubalis bubalis), vulnerable sambar deer (Rusa unicolor), one of the largest populations of endangered wild horses (Equus ferus caballus) and the world’s only population of wild dromedary camel (Camelus dromedarius), which has been extinct in its native range for 3–5000 years.

“Australia was once ruled by hippopotamus-sized wombats (Diprotodon) and 3-metre-tall kangaroos (Procoptodon golian), but all Australian megafauna species were lost thousands of years ago, until these recent introductions,” says lead author Erick Lundgren of Arizona State University.

Lundgren found that introduced megafauna contribute unique ecological functions, some of which may have been lost since the late Pleistocene. “As large herbivores, these introduced species can consume plant matter indigestible to smaller herbivores, which may reduce fire frequency, accelerate nutrient cycling, and shape plant communities,” Lundgren says.

In North America’s Sonoran Desert, wild donkeys dig groundwater wells over 1 metre in depth, providing not only new water sources for over 30 mammal and bird species, but also acting as germination nurseries for river vegetation. “The wild donkey is critically endangered in its native range, but has found refuge in its introduced ranges,” Lundgren says.