The Devastating Impact of Blocking Wildlife Corridors

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Some types of fencing and barriers can have the same harmful affect on ecosystems as highways do—they block apparent wildlife corridors* and may limit gene pools, separate animal families and negatively impact wildlife food sources. Additionally, homeowners in suburbia and urban edge areas are directly affected when these wildlife corridors are blocked. Unable to access wild food, deer will often graze landscaped lawns and gardens. Because animals follow their food, deer attracts large predators such as mountain lions and coyote. This isn’t just an abstract problem; it is occurring right now and is directly impacting the wildlife in our canyon as well as other wild areas in neighborhoods.

“As humans have developed and destroyed habitats worldwide, it is critically important to keeping remaining isolated patches of habitat connected to each other with corridors of similar habitat, to allow animals (and thus their genes) to move between them.  Conservation biologists refer to this as ‘connectivity'”. Laurence Frank PhD–Living with Lions, Kenya

Wild animals go where there is food and water. It doesn’t matter what service they provide in the ecosystem—predator and prey must eat to survive. Deer are drawn to areas where there is plenty of vegetation—generations follow the same wildlife paths, going back and forth between sites that are proven to be rich in vegetation. They browse and graze on leaves and grasses in one area, then move to others, eventually circling back to the original spot. Predators, including mountain lions and coyote follow them. It is vital that these wildlife routes are kept open and are not made impassible. Additionally, keeping these wildlife corridors accessible discourages inbreeding and may help to deter predators from venturing into urban and suburban areas.

Deer are drawn to areas where there is plenty of vegetation—generations follow the same wildlife paths

This wildlife path has been followed for years by many generations of deer

Wildlife corridors

Barbed wire fences, barriers and tall fences in our neighborhoods often obstruct the natural movement of wildlife. This is becoming more problematic as expanding urbanization divide and limit animals from accessing wild areas. The neighborhood canyon is no exception. Because the 88-acre canyon is too small to fully support a diverse and healthy group of animals, wildlife cross the busy street that borders one side of the canyon to access the open space. They graze in the meadow, browse on the vegetation, hunt prey and drink the water from the creek in the pristine habitat that is managed by the San Francisco Public Utilities Commission (Hetch Hetchy Regional Water System). The canyon side of the corridor is vital for wildlife as well—it is also a riparian zone and provides food, water and shelter for the animals. One of my cameras is aimed at the crossover spot at the fence the animals go through when crossing between the two wild areas. For many years it’s faithfully recorded a variety of species crossing back and forth.

For years and countless generations, animals have consistently passed through the fence at this spot. Youngsters, watching and following their parents, learned that this is the best place to make the crossing. They wait, hesitating at the fence, blending in with the shadows from the trees until the road is quiet and free of traffic and then dash over to the safety of the canyon.  Families of deer as well as coyotes, bobcats and mountain lions have worn paths under the horizontal barbed wires to go back and forth between the two wild areas. That spot at the fence is rich with the familial scents of thousands of animals over many years passing under and through the wires. Until recently, it worked well.

Everything changed a few weeks ago.

The SFPUC blocked the wildlife corridor

The SFPUC, recently weaved strong twisted wire strands between the horizontal wires in the fence, making it impossible for the deer to squeeze through. This wildlife artery, used by many wild species for crossing back and forth between the canyon and the open space is now impassible and dangerous for many animals.

SFPUC Added vertical wire to the fence, blocking the wildlife corridor

SFPUC Added vertical wire to the fence, blocking the wildlife corridor

The SFPUC most likely made the poor decision to add the vertical wires because they don’t want people trespassing into the open space. Based on years of consistently filming the spot, their trespassing concerns are unfounded. In the 3.5 years that the camera has filmed the fence, there have only been four occasions when people have squeezed through the wires into the open space. Twice utility workers picked their way through the barbed wire, another time a house painter climbed through the wires to urinate. In 2019 a couple of trespassers did maneuver their way through the fence, probably to take pictures. While the SFPUC concerns are understandable, it appears that they are not considering the environmental implications of blocking a well-travelled wildlife corridor. It also seems that they are not honoring their own policy “Water Enterprise Environmental Stewardship Policy” concerning native species habitats and enhancing ecosystem function.

I don’t know if the SFPUC is aware of the detrimental impact that adding vertical wires between the horizontal ones has on the wildlife. The wires are now locked in place, making it almost impossible for some of the animals to make those crucial trips between the canyon and the SFPUC land.

Does and fawns have it a little easier in some ways. Being smaller and not equipped with antlers, they can squeeze between the closely spaced wires, but at a painful price. They are frequently scraped and scarred by the sharp barbs.

In addition to the fence being almost impossible to pass through, bucks can get their antlers tangled in the barbed wire.  Tragically, bucks who are trapped by the fence, can become easy targets for coyotes or may die from starvation. They are particularly vulnerable right now because it’s fall, the rutting season, when the males are intent on finding mates. They wander more, increasing the possibility of getting entangled in the fence as well as being stuck on the road. Although a few does have managed to fit through the small openings between the wires, some of the members of their groups can’t—leaving them in dangerous and precarious situations on the road.

Finding safe passage

Of course, there are other places they can use to cross over between the canyon and the open space, but they aren’t as safe as the familiar spot in the fence: animals must walk a distance on the road and risk being hit by cars and trucks.

Mature buck can't get through the fence--he's stuck on the road.

Mature buck can’t get through the fence–he’s stuck on the road

A possible solution to avoid blocking wildlife corridors

The SFPUC needs to stop blocking these important wildlife corridors with injurious fences and barriers. There is a solution that is low-cost as well as relatively easy to implement and it can include local communities. First, the SFPUC needs to identify these important wildlife corridors. These access points and corridors are easy to spot—they are the paths where the vegetation has been flattened and the soil hardened by many generations of hooves and paws. Wildlife cameras should be strategically placed to document the wildlife using the trails to cross between the wild areas. An added plus is that the cameras serve as a deterrent to trespassers. A lot of this work has already been done.  Local conservation/wildlife groups including the Felidae Conservation Fund has identified many wildlife corridors and has been monitoring them for many years. Additionally, the SFPUC can solicit volunteers from the community to monitor cameras and submit and analyze the data. Instead of blocking these spots, cameras can continue to monitor them. When appropriate, signs should be placed on roads, indicating to drivers where the wildlife crossings are located.

What is occurring to the wild animals in the neighborhood canyon is not a unique situation. Tragically, wildlife corridors are being blocked in suburban and urban edge communities throughout the west. Who knows, maybe the local community and the SFPUC can set an example that might eventually be widely adopted, benefiting wildlife and helping people successfully cohabitate in areas that are shared by wild animals.

*The term “wildlife corridors” may not be scientifically accurate. According to the following definition, the term refers to a strip of natural habitat connecting populations of wildlife otherwise separated by cultivated land, roads, etc. 

Thank you Laurence Frank. PhD for reviewing, fact checking and commenting.


Marilyn is a certified cat behavior consultant (The Cat Coach, LLC). Not surprisingly, she’s fascinated by feline behaviors. This started with household cats and then after witnessing a puma being killed a few blocks from her home in the suburbs, expanded to include local mountain lions and bobcats. A few years after the tragedy, she joined the Bay Area Puma Project/Felidae Conservation Fund, maintaining trail cameras, writing and helping wherever she can. She is focusing on how urbanization is affecting apex predators’ behaviors.

She is also an author and educator. Her book Naughty No More! focuses on solving cat behavior issues through clicker training, environmental changes as well as other positive reinforcement techniques. She gives presentations throughout the United States as well as writes columns and articles for a variety of venues and helps clients change their cats’ unwanted behaviors though on site and remote consultations. She is also frequently interviewed for print and on-line publications. Additionally Marilyn is guests on television and radio and has appeared, along with her Bengals and Savannah Cat on Animal Planet, CBS, ABC, KGO and others.

Join Marilyn for lively discussions about all things feline on her Facebook page

 

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