This story was originally published by Yale E360 and is reproduced here as part of the climate desk Cooperation.
The most famous Fence in the United States is the expanded border wall pushed by former President Donald Trump. The barrier is designed to prevent illegal immigration and also prevents wildlife from moving between the United States and Mexico.
“The primary threat to the boundary wall is not local habitat loss and habitat degradation,” said Aaron Flesch, a wildlife biologist at the University of Arizona who has studied the wall’s impact. “It’s the impact at the landscape level when wildlife movement is restricted or prevented entirely, and large-scale landscape connections are eliminated.”
Bighorn sheep or jaguars, for example, are cut off from conspecifics on the opposite side of the border. This means that the genetic interaction required to keep small populations of jaguars or ocelots healthy can be compromised. It also means that bighorn sheep in Mexico may not be able to migrate north to escape a hotter, drier climate.
These are the types of impacts caused by the many millions of miles of barriers around the globe that slice and dice the natural world. It’s a fast-growing problem as fencing projects expand worldwide. In Europe, nations are building new fences to stop migrants crossing borders illegally in remote areas. In East Africa, cattle fences disrupt the migrations of the region’s legendary wildlife. A new fence between Mongolia and China has blocked the movement of gazelles. And the list goes on.
Until recently, the study of fences and their role in conservation biology has been scattered. Half of the studies were conducted in just five countries, with many focusing on effects on medium-sized animals. And fences are still not part of the Human Footprint Index, a database of human changes on Earth used by researchers to measure the cumulative impact of human development.
But that is changing. A few years ago, biologists in the Northern Rockies published a paper entitled “A Fence Runs Through It: A Call for Greater Attention to the Impact of Fences on Wildlife and Ecosystems.” In 2020, a meta-analysis appeared in life sciences has looked at all studies on the effects of fences and found that their profound effects are often ignored or grossly underestimated.
Recent research shows that these impacts go well beyond blocking animal migratory routes, promoting disease transmission through animal concentration, altering predatory hunting practices, and impeding access to important water and foraging areas. Fences can also prevent “genetic rescue” when an isolated population is decimated by disease or a natural disaster.
Wenjing Xu – a graduate student at the University of California, Berkeley and co-author of life sciences Paper – calls fences “the wires that shape the world”. She and others liken the “aha moment” of growing awareness of the importance of fences to the development of the sub-discipline of road ecology. The effects of roads on ecosystems – particularly how they open up previously undisturbed areas for exploitation – have also been previously ignored. But when the impacts of roads on nature were identified and named, it changed the way people perceived them and helped draw attention to how important roads are in conservation planning.
The reason conservation has been largely fence-blind is partly a matter of perception, scientists say. “Fences have been everywhere for as long as people can remember,” Xu said. “When you look at a landscape you don’t see fences themselves, you see roaming cattle and wildlife. You don’t see the fence because you’re so used to it. People take them for granted.”
Fences are also difficult to see from above with satellite and other types of aerial mapping, essential tools in modern conservation. “You can’t see the fence, but you can see shadows from fence posts and a clear difference in the type of land cover, and you have to assume there would be a fence there,” said Andrew F. Jakes, a wildlife biologist at the Smithsonian Conservation Biology Institute and lead author of the 2018 article “Fence Runs Through It.”
“Things are at the very beginning of how we’re going to map fences… Considering fences creates a better understanding of the cumulative impact of development on the landscape. Fence ecology not only affects wildlife but also ecosystem processes and has a wonderful interface with people and communities. There is much more to learn about the effects of fencing.”
The 2020 analysis found that fences affect ecosystems at every scale, from reducing insect abundance by giving spiders ample space to build their webs, to impeding the long-distance migration of wildebeest to mule deer. By concentrating animals more tightly than in the wild, fences could increase disease transmission, a problem that hasn’t received much attention so far. This could be especially important now that Covid and chronic wasting disease are spreading rapidly among wildlife populations.
Wildlife faces two categories of threats from fences, one direct and one indirect. For example, one of the main direct threats to the Great Sage Grouse – a species in severe decline in the western United States – is that they fly low and are often killed by crashing into strands of barbed wire. Among other things, antelopes in North America and guanacos in South America become entangled in fences and starve or injure themselves.
Indirect threats include blocking migratory routes, habitat destruction, and changes in predator hunting. For example, fences are sometimes used by wolves to imprison their prey. And a rapid increase in the number of fences in game-rich East Africa has blocked the great wildebeest migration in Kenya’s Greater Mara region, where the animals must follow the rain to find food and water. Some die from lack of water.
“People are mapping fences there, and you can clearly see how they have exploded over the past 20 years,” Xu said. “There is evidence that wildlife numbers have really declined as fences have increased. It’s a tragedy.”
Fences are built in regions like the Mara because the people who build them need to be able to manage their livestock, often their most valuable asset, and keep animals away from their crops. Therefore, fence ecology is intertwined with social and cultural issues.
On the China-Mongolia border, a Mongolian gazelle was tracked along a fence for 20 days as it traveled 30 miles trying to find a spot to cross.
“Fences can create ecological ‘no man’s lands’ in which only a limited number of species and ecosystems can thrive,” says Alex McInturff, wildlife ecologist at the University of Washington and lead author of the 2020 paper Ecosystems likely to collapse.”
The longest fence in the world and a textbook example of how fencing transforms nature, the Wild Dog Barrier Fence stretches 3,488 miles across the corner of South East Australia. Researchers say the massive fence created two “ecological universes” on either side of the wire. On the inside of the fence, where farmers capture, shoot and poison the dingoes that manage to get through, it has caused a trophic cascade. The lack of dingoes this side of the fence has meant many more kangaroos, leading to overgrazing, soil erosion, loss of soil nutrients and even altered geomorphology of the sand dunes and river course. This has reduced cover for the dusky bouncy mouse, an endangered species, making it far more vulnerable to predators.
Fence construction is growing rapidly worldwide. An additional 460-mile extension of the Dingo Fence is underway. Sturdy, often impermeable fences are being erected at national borders in Eastern Europe to deter migrants, and Europe now has more kilometers of border fences than it had during the Cold War. In China, the main threat to the Przewalski’s gazelle is habitat fragmented by new barriers. The fences in the American West alone are estimated to be more than 620,000 miles long – nearly three times the distance from Earth to the Moon.
A better understanding of the role fences play in ecosystems can often lead to simple solutions by figuring out which ones can be removed or modified, and then doing things like making them jump over them.
In the western US, simple reflectors are added to fences to allow sage grouse, peregrine falcons, and other birds to see the wires and avoid collisions. Some ranchers use portable fences and move them from paddock to paddock as needed, rather than leaving the fences open all the time. And virtual fencing, in which cattle wear collars that vibrate when they approach a wire in the ground — like an invisible fence for dogs — is widespread.
Many miles of fence have been removed by tribes, land trusts, ranchers and government agencies as a better understanding of its role in the natural world emerges. One such effort is Wyoming’s Absaroka Fence Initiative, which has taken up miles of unused “ghost fences” in Wyoming.
Fences are not always a problem – in the right place, they can be an indispensable tool for conservation. New fencing, for example, has become an important tool in controlling predators — particularly grizzly bears near more populated areas. Sturdy electric fences are essential for enclosing chicken coops, beehives, and other grizzly bear attractants.
The Confederate Salish and Kootenai Reservations in western Montana have dozens of underpasses and flyovers with high fences that guide wildlife so grizzly bears, wolves, deer and other species can safely cross Interstate Highway 93.
One of the pioneers in the field of fence ecology, Arthur Middleton, assistant professor of wildlife management and policy at UC Berkeley, has studied the role of fences in antelope migration in Wyoming. He hopes the new focus on these barriers “will open people’s eyes to the magnitude of the fencing effects.”
“Our next steps,” he said, “are to better understand the true biological costs that all of these fence-driven behavioral changes are having on wildlife populations and to find ways to mitigate those impacts at really large scales.”