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Barriers like this electric fence restrict the habitat of African elephants, and other key species.David Woodfall/Balance/Photoshoot/via ZUMA Press
This story was originally published in Yale E360 and is reproduced here as part of the Climate Desk collaboration.
The most famousThe Trump-era expanded border wall is the reason for the US fence. It was designed to stop illegal immigration but also prevent wildlife from moving between the United States, Mexico.
“The main threat of the border wall is not the localized area of habitat loss and habitat degradation,” said Aaron Flesch, a wildlife biologist at the University of Arizona, who has studied the wall’s impacts. “It’s the landscape-level impacts of curtailing or completely precluding wildlife movement and eliminating landscape connectivity at large scales.”
For example, bighorn sheep and jaguars are cut off from their counterparts on the other side of the border. This means that genetic interaction necessary to maintain healthy populations of jaguars and ocelots may be reduced. It could also mean that bighorn sheep in Mexico might not be able migrate north to escape a hotter or drier climate.
These are the effects of the many millions upon millions of miles that have been built around the world to keep the natural world from being destroyed. It’s a rapidly growing problem, with fence projects expanding worldwide. Europe is building new fences in order to prevent migrants from crossing borders in remote areas. In East Africa, livestock fencing is interrupting the migrations of the region’s storied wildlife. A new fence separating China and Mongolia has stopped gazelles from moving between the two countries. The list goes on.
The study of fences and their roles in conservation biology was sporadic until recently. Half of the studies were done only in five countries. Many of these studies focused on medium-sized animals. Fencing is still not considered part of the study. Human Footprint IndexA database of human impacts on the earth that researchers use to calculate the cumulative effect of human development.
But this is changing. A few years ago, biologists from the Northern Rockies published a Paper titled “A fence runs through it: A call for greater attention to theInfluence of fences on wildlife and ecosystems.” In 2020, a meta-analysis in BioScience looked at all the studies of the effects of fences and found that their profound impacts are often ignored or greatly underestimated.
Recent research has shown that these effects go beyond blocking animal migration routes. They can also increase disease transmission by concentrating livestock, altering predator hunting practices, and preventing access to water and food sources. Fences may also prevent “genetic rescue” if an isolated population is decimated by disease or a natural disaster.
Wenjing Xu—a PhD student at the University of California, Berkeley and a co-author of the BioScience paper—calls fences “the wires that shape the world.” She and others liken the “aha moment” of the growing realization of the importance of fences to the evolution of the sub-discipline of road ecology. The impacts of roads on ecosystems—most notably how they open up previously undisturbed areas to exploitation—were once similarly ignored. It was only after the effects of roads were discovered and named that people started to see them differently. This helped to bring attention how important roads are in conservation planning.
Scientists believe that perception is partly responsible for conservation being largely blind to fences. “Fences are everywhere, for as long as people can remember,” said Xu. “When you look at a landscape, you don’t see fences themselves, you see livestock roaming and wildlife crossing. You don’t see the fence, because you’re so used to it. People take them for granted.”
Satellite and other types aerial mapping are essential tools for modern conservation. However, it is difficult to see fences from the sky. “You can’t see the fence, but you can see shadows of fence posts and a distinct difference of land cover type, and you have to make an assumption of that’s where a fence would be,” said Andrew F. Jakes, a wildlife biologist with the Smithsonian Conservation Biology Institute and the lead author on the 2018 “fence runs through it” paper. “It’s not easy. And then you don’t know if it’s a four-strand barbed wire fence or a woven wire fence.
“Things are right at the beginning stages of how we are going to map fences … Taking fences into account creates a better understanding of the cumulative effects of development on the landscape. Fence ecology impacts not only wildlife but also ecosystem processes. This is a great intersection between people and communities. There is a lot more to learn about the impacts of fencing.”
2020 analysis showed that fences can have an adverse effect on ecosystems at all scales. They prevent long-distance migration and reduce insect abundance. Fences can increase disease transmission by concentrating animals closer together than they would be in the wild. This is an issue that has not been given much attention. This is especially important considering the rapid spread of Covid and chronic diseases like wasting disease among wildlife populations.
Emus trying cross the Rabbit-Proof Fence, Western Australia.
Western Australia Department of Agriculture & Food
There are two types of threats that fences can pose to wildlife. One is direct and the other is indirect. For example, one of the key direct threats to the greater sage grouse—a species in steep decline in the US West—is that they fly low and are often killed by slamming into strands of barbed wire. Amongst other species, antelope in North America as well as guanaco from South America, get caught up in fences and become starved or injures.
Indirect threats include blocking migration routes, habitat destruction, and predator hunting. Wolves, for example, sometimes use fences to keep their prey in check. The rapid growth of fences in wildlife-rich East Africa has impeded the great migration of wildebeests from the Greater Mara region in Kenya. Wildebeests must follow the rains to find food. Some animals are dying because they lack water.
“People are mapping fences there, and you can clearly see how it has exploded in the last 20 years,” said Xu. “There is evidence that wildlife numbers have really decreased with the increase in fences. It’s a tragedy.”
Fences are being built in Mara because people need to be able manage their livestock, which is often their most valuable asset. They also need to keep animals away form their crops. That’s why fence ecology is intertwined with social and cultural issues.
A Mongolian gazelle, which was trying to cross the border from China and Mongolia, was tracked for 20-days along a fence. It traveled 30 miles to find a safe crossing.
“Fences can create ecological ‘no man’s lands’ where only a narrow range of species and ecosystems can thrive,” says Alex McInturff, a wildlife ecologist at the University of Washington and lead author of the 2020 paper. “As they are built at an accelerating pace around the world, ecosystem collapses are likely to follow.”
The world’s longest fence, and an object lesson in how fences change the natural world, is the Wild Dog Barrier Fence, which stretches 3,488 miles across the corner of southeast Australia. Researchers say the massive fence has created two “ecological universes” on either side of the wire. The fence’s inside has created a trophic cascade, where farmers trap, kill, and poison dingoes that are able to pass. There are now many more dingoes along that fence, which has resulted in overgrazing, soil erosion, loss of nutrients, stream flow alteration, and soil erosion. This has made the dusky hopping mouse less protected and more vulnerable to predators.
Construction of fences is increasing rapidly all over the globe. A further 460 mile extension of the dingo fencing is in progress. To deter migrants from Eastern Europe, strong, often impermeable fences are being built at the borders to deter them. Europe has more miles of border fencing than it did during the Cold War. In China, the chief threat to Przewalski’s gazelle is habitat fragmented by new barriers. The length of fences in the American West alone is estimated at more than 620,000 miles—nearly three times the distance from the Earth to the moon.
Understanding the role fences play in ecosystems is often a key to finding simple solutions. This includes raising the height of the bottom wire to allow animals to pass underneath it or lowering it to allow them to jump over it.
Wyoming wildlife can pass through a fence that volunteers have modified.
Absaroka Fence Initiative
Simple reflectors have been added to fences in the west to allow sage gruse and peregrine falcons to see the wires and prevent collisions. Ranchers are switching to portable fencing. They can move it from one pasture to the next as needed. And virtual fencing, where livestock wear collars that vibrate when they get close to a wire in the ground—like an invisible fence for dogs—is being widely used.
Tribes, landtrusts, ranchers and government agencies have removed many miles of fence to better understand the role of the natural world. One example of such efforts is the Absaroka Fence Initiative in Wyoming, which has taken up miles of unused “ghost fencing” in Wyoming.
Fences aren’t always a problem—in the right place they can be an essential tool for conservation. New fencing, for example, has become an important tool in managing predators—especially grizzly bears near more populated areas. For fencing off chicken coops and other grizzly bear attractors, strong electrical fences are necessary.
The Confederated Salish and Kootenai Reservations in western Montana have dozens (or more) of underpasses and overpasses with high fencing that funnel wildlife into them. This allows grizzly bears and wolves to safely cross Interstate Highway 93.
Arthur Middleton is an assistant professor at UC Berkeley of wildlife management policy and policy. He is one of the pioneers of fence ecology. He has studied the role of fences on antelope migration in Wyoming. He hopes the new focus on these barriers will “open people’s eyes to the scale of fence effects.”
“Our next steps,” he said, “are to better understand the actual biological cost that all these fence-related behavioral changes have on wildlife populations, and find ways to mitigate those effects at a really large scale.”
