Humans have swamped the planet with fences, but there is a gaping hole in knowledge about their ecological impact – an issue scientists are now drawing attention to.
“Fences are so common that they have become nearly invisible, even to ecologists,” says Alex McInturff from UC Santa Barbara, US, lead author of an international paper published in the journal Bioscience.
“The fact that we’ve wrapped the Earth with enough fencing to reach the Sun makes it especially surprising that we don’t have a clear understanding of the scope, scale and types of ecological effects they have.”
Most research to date has only narrowly focussed one species or issue at a time, without synthesising the broader impacts, says McInturff, partly because fences are hard to detect and define. Even then, it’s difficult to evaluate their integrity or functionality.
To explore this issue, his team conducted an exhaustive review of 446 papers covering seven decades, and the news isn’t great.
“One of the most striking findings is that fences can reorganise ecosystems entirely by creating ‘winners’ and ‘losers’,” he says, and the review “suggests that there are multiple losers for every winner.”
The impact of fences on ecosystems is pervasive, he adds, ranging from spiders to birds and dingos to wildebeest, and spanning riparian corridors to entire continents.
Winners tend to be generalist species while more specialised and sensitive species and systems lose out, and where fences are rapidly proliferating, McInturff says “this partitioning of winners and losers threatens ecosystems with collapse”.
On an ecological level, the researchers discovered fences can create “no man’s lands” – much like those with barbed wire during World War I – that only allow a narrow range of species and ecosystems to flourish.
Even fences designed to protect conservation areas, keep out invasive species or shield wildlife from vehicle collisions can have unexplored ecosystem consequences.
An Australian fence designed to protect a nature reserve from pests, for instance, inadvertently affected native reptile populations around the enclosure – particularly eastern long-necked turtles (Chelodina longicollis). The fence interrupted their movements, isolated them and drove their mortality rates up when they became entangled in it.
Fences could also threaten species survival. One study showed how a planned US-Mexico border “would dangerously restrict gene flow among desert bighorn sheep (Ovis canadensis Mexicana)”.
Another paper documented that conservation fences designed to protect vulnerable wildlife from poachers could prevent access to waterholes and other critical resources if not aligned properly.
Even removing fences to increase wildlife connectivity for the “Big 5” charismatic African species impacted smaller grazers such as sable (Hippotragus niger), roan (Hippotragus equinus), tsessebe (Damaliscus lunatus) and eland (Taurotragus oryx) that had thrived in the smaller reserve.
A “particularly ironic” Australian example showed how fences to restrict invasive species assisted the movements of the dreaded cane toads (Bufo marinus) along fence lines.
McInturff’s interest in fences was initially sparked while doing fieldwork in Kenya, where they are commonly used to protect critically endangered wildlife like rhinos from poachers, yet no work was investigating their impact on animal behaviour.
Back in California, he noticed that black-tailed deer would take long detours around fences when they could easily have jumped over them, and field cameras revealed “predator highways” used by carnivores, suggesting they might even use them to trap prey.
The review team included researchers from China, Kenya and Mongolia who shared similar concerns, and Justin Brashares at Berkeley’s University of California, who was curious to understand further ecosystem impacts of fences on vegetation, soil and hydrological processes.
Despite their findings, half of the studies came from just five countries and many focussed on the movement and behaviour of large mammals. Some prevalent fences, like those for livestock, had received very little attention, nor had their broad ecological impacts.
The review included a model of fence lines in the western US that showed more than a million kilometres of fencing, some high density in areas considered to have a low human footprint.
“We live on a fenced planet, and that has enormous ecological consequences,” says McInturff, “many of which we don’t understand yet.”
To address the large gaps in knowledge, the team sought to help lay the groundwork for a new discipline of fence ecology, addressing the different types of fences and their impacts at every level of ecological analysis and making policy recommendations to address this.
“Subtle design changes to fences can have powerful positive impacts on wildlife species,” says McInturff, “while greater care in the position of fence lines can help mitigate other environmental consequences.”
He notes the importance of a holistic approach to this complex but important subject that factors in human impacts, noting instances where fences reinforce isolation of marginalised groups, stagnation of economies and consolidation of power.
“A robust fence ecology needs a strong sociological arm to understand links between fences, ecosystems and people.”
Natalie Parletta is a freelance science writer based in Adelaide and an adjunct senior research fellow with the University of South Australia.
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