The Unopen Range: How fences hurt wildlife

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Editor’s Note: This story was originally published through The Revealer, an initiative of the Center for Biological Diversity.

With the onset of spring each year, the American Antelope this winter in Wyoming’s Upper Green River Valley begins a journey of over 100 miles to their summer habitat near Grand Teton National Park.

It is one of the longest surviving large mammal migrations in North America. But their trek – and the similar one done by the mule deer – is made more difficult by human developments along the way, especially the fences.

The total length of fences in the world can now exceed that of roads by an order of magnitude and continues to grow due to a global trend towards partition and privatization of land, ”the researchers wrote. UC Berkeley study published in the Journal of Applied Ecology.

Wyoming is no exception. There, the researchers found nearly 3,800 miles of fencing in their study area alone – nearly twice the length of the US-Mexico border. Their search followed the GPS-collared American antelope (Antilocapra americana) and mule deer (Odocoileus hemionus) during two years of their migrations to better understand how fences affect animal movements and what types of fencing can be the most difficult.

Fences aren’t always bad for wildlife (some can prevent animals from moving on roads, for example), but they can also pose a threat.

For animals like the American antelope and mule deer, fences can stop or alter migration routes. Animals that attempt to climb over or under are also at risk of becoming entangled and perishing. Minors are particularly at risk. A 2005 A Utah State University study of ungulate migration through Colorado and Utah found that young people died in fences eight times more often than adults. Many more died of starvation or predation when they couldn’t get past the fences and were separated from their mothers.

Most fences encountered by animals run along the edges of cattle pastures, private property lines or roads, and are made of four or five strands of barbed wire. Some have wire woven into the bottom, the most common type of fence for surrounding sheep, but also the most deadly for wildlife.

A better understanding of wildlife responses to fences is… essential for conservation, ”the UC Berkeley study researchers wrote.

But here’s what we know: The study found that the American antelope and mule deer were largely affected by the fences.

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Every year, an average mule deer encounters fences 119 times and antelope 248 times. In regards to 40 percent of those encounters, the fence changed the animal’s behavior. And this behavior, they found, was more complex than just going over or not going over the fence.

Often animals rebounded ”, or quickly moved away from the fence when they could not cross quickly. Such fence avoidance can keep animals away from high quality resources and reduce the efficiency of habitat use, ”they wrote.

Other times the animals came and went along the fence, behavior that could deplete energy resources. And sometimes they got trapped in areas with a high concentration of fencing, such as pasture for cattle.

This can create other problems.

Restricting animal movements for extended periods of time in limited areas can trigger human-wildlife conflict, ”the researchers found. Pronghorn, for example, was seen in new developments in Colorado Springs, where they were hit by cars and showed up at the airport.

Mule deer and American antelope also behave differently when encountering fences. Mule deer are more likely to jump off a fence and antelopes are more likely to crawl under.

Reluctance to jump means that antelope movements can be completely blocked by woven wire sheep or barbed wire fences with inferior wires – the two most common types of fencing on their home ranges in America. of the North, ”the researchers discovered.

Since the American West may have more than 620,000 miles of roadside and pasture fences, modifications to fencing for conservation may be more urgent than what is currently recognized, ”they wrote.

Efforts are underway to encourage or demand more wildlife-friendly fences ”which are highly visible and allow wildlife to easily jump over or crawl under wires or rails, ”according to Colorado Parks and Wildlife recommendations.

Other advice from the Sustainable Development Code, an organization that works on sustainability issues with local governments, recommends using barbed wire rather than smooth; limit the height of fences 42 inches; allowing 16 inches of clearance at bottom; and including a wide spacing between the threads.

There are other forms of wildlife-friendly fencing, including fixed or temporary fences that allow wildlife to cross during critical migratory seasons, ”the group reports.

Making fences more visible is also useful for other animals, including birds. Low-flying birds, such as the grouse, also die in western range fences.

This is why wildlife managers are starting to push to remove or modify fences, but the effort can be costly. To address this concern, researchers have developed a software package, available to wildlife managers and other researchers, that highlights fences posing the greatest threats to animal movements. They hope this will help make the most of limited conservation funds and help protect critical flyways.

We demonstrate that when summed up and mapped, these behaviors can help identify problematic fence segments, ”they wrote. And it could help save a lot of American antelopes, mule deer and other animals.


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