Feral pigs and donkeys may be more salvation than scourge for ecosystems, study finds

An unscientific bias against “feral” or “invasive” animals threatens to undercut one of the great stabilizing trends making ecosystems healthier, a new paper argues.

Introduced species such as feral pigs, horses, donkeys and camels represent a powerful force of “rewilding”  — the reintroduction of wild animals into ecosystems where humans had eradicated them — according to a study published Thursday in Science.

In many such ecosystems, big herbivores spread seeds, increase plant diversity and work as “ecosystem engineers” — and that’s true whether those herbivores are “invasive” or “native,” the authors argue. 

“One way to talk about this is: whether a visitor from outer space, who didn’t know the history, could tell what megafauna are native or introduced based solely on their effects,” said Erick Lundgren, a doctoral student in biology at Arizona State University.

Megafauna refers to animals weighing more than 44 kilograms, or about 100 pounds — a key factor, because much of the data on the malignant nature of “invasives” in general rests on research done on small animals, plants and pathogens.

In the case of big animals, however, if our alien visitor couldn’t tell the difference, Lundgren said, “then nativeness isn’t actually a helpful way to understand how ecosystems work.”

The study argues against widely held beliefs about whether invasive species are harmful — or what Lundgren described as the quasi-religious perception that some species inherently belong in a given landscape and others don’t.

That belief is the driving force behind a wave of expensive and often futile campaigns since the 1990s that eradicate species including feral hogs in Texas, wild horses across the American West and donkeys and camels in Australia.

In those culling campaigns, land managers have killed millions of “feral” megafauna and discussed more drastic interventions yet. In the case of Texas, for example, state officials proposed seeding the landscape with the poison warfarin to kill feral hogs. Ranchers argued the poison could enter the food chain and kill scavengers or, potentially, humans who ate the tainted meat.

The Science study made the case that much of this killing is unnecessary — or even harmful to the ecosystems it is supposed to protect. Introduced species “have partly counteracted” the protracted series of extinctions and general decline among populations of big plant-eating mammals since prehistory, the authors wrote.

While they noted that these animals “are thought to have unusually negative effects on plants compared with native megafauna,” by looking at more than 200 studies of impacts from large, introduced herbivores, they found “no differences between introduced and native megaherbivore impacts.”

Instead, they found that the most important determining factor in a species’ effects on the surrounding ecosystem was its size and dietary preferences, rather than where it had come from.

For example, big grazers like horses and camels tended to reduce grass diversity — but that was true whether they those animals were in their home ranges or in new ecosystems abroad.

Native pigs in the forests of Eurasia do just what their feral cousins in America and Polynesia do: They root up plants, eat crops, defecate on the landscapes and create big muddy wallows in their attempts to cool themselves — all without the slightest regard for a farmer’s desire to run a neat, profitable agricultural operation from the same space.

But from another perspective, these actions can be seen as environmentally beneficial — and when those animals are natives, they often are portrayed that way. In disturbing existing vegetation, for example, the pigs also create space for new plant growth. Their poop can lead to algal blooms in waterways, but that’s because it’s so nutrient-rich — meaning it’s an important source of natural fertilizer, not least for the seeds that pigs spread the same way.

And their wallows are essentially tiny ponds that can help trap and retain water in dry landscapes — something that ecologists see as beneficial when it’s done by, say, bison. (Meanwhile, wallows left by introduced African buffalo in Australia have been linked to lower incidence of destructive wildfire.)  

In doing all this, Lundgren argued, the pigs may perform a similar function to a long-extinct species they somewhat resemble — the giant peccaries that rooted and snuffled across North American forests during the last ice age, more than 10,000 years ago.

And often, the impacts of native animals on native plants — such as the Western bison interrupting the recovery of aspen trees in Yellowstone — gets described as ecosystem engineering.

Take elephants: Native in Africa and Asia (and, once, North America) and in frequent conflict with local farmers because of their very different needs from a landscape — needs that, to the agriculturalist’s eye, make them as destructive as any sounder of feral hogs. Elephants knock things over, strip and kill trees, and eat or trample any fruits or vegetables they please. 

On the one hand, those are arguably beneficial functions in forests — though that hasn’t stopped some botanists from arguing that elephants are bad for native trees and shrubs, and in some African national parks, land managers advocate killing them to preserve those species.

But however destructive, established species like bison or elephants have one serious advantage over newer ones, Lundgren argued: Everyone understands that when biologists argue for their removal, what they are proposing “is clearly a preference.”  

“Whereas invasion biologist argue that [what they’re expressing] aren’t even preferences — that they were somehow mandated by the world. That the world told them that those preferences are real.”

Scientists have long distinguished between native and novel organisms — the term “neophyte” refers to a “new plant” on a given landscape.

But the tenor of that debate changed as the number of introduced animals has multiplied — the “global consequence of an increasingly connected world and the rise in human population size,” invasion biologist Petr Pyšek wrote in a 2020 summary that laid out a litany of harms.

“Invasive alien species break down biogeographic realms, affect native species richness and abundance, increase the risk of native species extinction,” he wrote.

This debate has sometimes gotten ugly.

Opponents of “invasion biology” note the sordid connections between early-20th-century concerns about nonnative species — like the Nazi campaign to replace introduced animals in the Third Reich with properly Teutonic species.

But “most judgments about the aesthetics of introduced species, however, cannot be clearly linked to [racist] motives,” leading ecologist David Simberloff wrote in a 2003 article in Biological Invasions.

Unlike Nazi claims of harm by non-German species, however, modern “harm is readily documented,” Simberloff added.

Ecologist Mark Davis, by contrast, has argued in Nature that it is precisely harms — and not origins – that scientists have to evaluate in judging which species to foster and which to cull.

Characterizations of nonnatives as driving extinctions of “beloved ‘native’ species … helped to create a pervasive bias against alien species that has been embraced by the public, conservationists, land managers and policymakers, as well by as many scientists, throughout the world.”

That’s unfortunate, he argued, because “the practical value of the native-versus-alien species dichotomy in conservation is declining, and even becoming counterproductive. Yet many conservationists still consider the distinction a core guiding principle.”

Or as Brown University ecologist Dov Sax told The New York Times: “I think the dominant paradigm in the field is still a ‘when in doubt, kill them’ sort of attitude.”

Those killings and removals have their own unintended effects. Since the 1930s, land managers in Nevada’s Death Valley have removed and sometimes shot burros (wild donkeys). According to the National Park Conservation Association, the burros overburden the ecosystem because they eat so much vegetation and “hog the water.”

Other evidence suggests that donkeys, in fact, increase the supply of water by digging wells that other creatures can access, and a 2007 study of donkey removal in the American West and Australia found that their eradication had led to the inadvertent destruction of the wetlands it was supposed to protect.

“They eradicated [the donkeys], and then wetlands filled in with cattails and reeds, then dried up and went anaerobic — and all these endangered endemic fish and these wetlands went extinct,” Lundgren said. 

“And so now land managers go and clear the vegetation manually. And despite this, they’re still trying to eradicate these animals from all these areas.”

This vision of a war between natives and invasives, he notes, contrasts sharply with one often adopted by Native peoples in both countries — who have watched their ecosystems transformed over the past several centuries.

For example, anthropologists working among the Anishinaabe of the Upper Midwest recorded that many of their respondents saw the colonization of their lands by new plant “nations” as “a natural form of migration.”

And an ethnographer in the Australian Outback, the site of the killing of perhaps half a million donkeys, found that the Aboriginal people he interviewed believed “the worth of an animal lies in its ability to live and flourish in the environment, not in its claim to being an original component of the fauna.”

Among those communities, he added, “it is generally held that [nonnatives] all have a right to live on the country now.”

Lundgren argued these examples suggest how the question of what belongs is complex and contentious. Most Americans want wild horses and burros to remain on public land, and many native Hawaiians are deeply attached to the feral hogs, descendants of domestic swine brought by their ancestors.

“You can even make an argument based on Earth’s history that, that if we introduced elephants to western North America, that would actually be very appropriate for our ecosystems, given that there’s always been animals like that,” he said.

(Or at least up until about 13,000 years ago.)

Decisions about what to do with those animals, Lundgren argued, are political or philosophical, not scientific — which he thinks scientists need to cop to. 

“If we’re going to make decisions to do things, we need to be transparent about what those values are.”