Water buffalo grazing in a Welsh marsh Joan Gravel/Alamy

Moose and water buffalo do a lot of munching and trampling, so it might seem a bad thing that these large animals have escaped their native realms and invaded other regions. But there appears to be an upside to their walkabout across the planet.

Beasts like camels, moose, water buffalo and donkeys – known as “megafauna” because of their size – live in large numbers outside their native ranges, thanks mostly to human introductions. Often, ecologists give these aliens the cold shoulder, presuming they do harm. For instance, conservation biologists have called for feral horses in the US – which came over with Europeans within the last 500 years – to be removed from certain areas.

But that’s wrong, says Erick Lundgren of Arizona State University. He says it is increasingly hard to tell where such animals “belong”.


Lundgren and his colleagues have studied the whereabouts of 76 species of large plant-eating mammals. He found that 22 have significant populations outside their native range. Ten of those travellers are extinct or threatened back home. For instance, there are an estimated 5 million wild donkeys round the world, but only a few hundred of their pre-domesticated ancestors, the African wild ass.

Remaking nature

So the team says the newcomers should often be welcomed as nature’s comeback. They describe them as “rewilding the Anthropocene”.

For one thing, introduced populations can be good for biodiversity. Killing them might put other species at risk. They also perform useful ecological services.

Sometimes those benefits were intended. Romanian water buffalo graze the marshes at a nature reserve in Wales, maintaining areas of open water. Similarly, giant tortoises from the island of Aldabra have been installed on Mauritius to replace their extinct counterparts and spread the seeds of native trees.

At other times, it has just happened. In his Master’s thesis, Lundgren describes the behaviour of wild donkeys or “burros” in the Sonoran desert of the American Southwest. They turn out to be brilliant at digging down to find underground water that is now drunk by dozens of native bird and mammal species. When native megafauna disappeared, nothing remained in the area with the ability to do that.

Journal reference: Ecography, in press