On the Fence

By Douglas Fox

A three-meter fence, anchored by sections of railroad track driven like stakes into the ground, cuts across South Africa’s Eastern Cape Province.

On one side sits Addo Park, a thriving slice of wilderness containing the largest elephant population in the region. These beasts, with their rare tuskless females, represent a gem of biodiversity unique among Africa’s elephants. Just meters away on the other side of the fence stands row upon row of orange trees sagging heavily with fruit.

This fence might as well separate an oil refinery from a raging brush fire—such is the notorious appetite of elephants for citrus that they would long since have raided those orchards. Only the fence, packing 8,000 volts of gentle persuasion, has protected those trees.
By keeping citrus trees alive, this fence has also kept the elephants alive. It was conflict with farmers and trains which nearly extinguished Addo’s elephants 80 years ago. The construction of the fence between 1931 and 1954 saved these pachyderms from farmers’ guns and from themselves. Since 1931, the population has grown from 11 animals to over 400. “Good fences make good neighbors,” goes the Robert Frost poem—and this African parable seems to confirm it.
But within this quiet gated community called Addo Elephant Park lurk the beginnings of a crisis. Confinement created an ecological echo chamber which transformed the mix of species, the topography of the land, and even the fundamental nature of elephant society. Elephant-on-elephant homicide has soared in recent years, and the fat pads on these beasts’ rumps have deflated. The slow-moving crisis threatens to turn these keystone herbivores into malnourished paupers. It’s just one example of how well-intended fences can exert unintended effects.

The world over, such fences exist on a massive scale. At 5,320 kilometers, Australia’s dingo fence represents one of the longest engineered structures in human history—five times as long as the U.S.-Mexican border fence and roughly equal to the Great Wall of China. These fences even show up from space. Satellites orbiting 700 kilometers above the earth’s surface can spot the edges of Addo Park, where thicket meets orchard, and Australia’s rabbit fence cuts a ruler-straight line for 300 kilometers between green-checkered farmland and brown savannah.

Expansion of human populations is even spawning fences in regions that were traditionally free of fences. In Kenya, desperate, drought-plagued farmers have moved into parks and felled trees—forcing the government to consider building protective fences around Mount
Kenya, the Mau forest, the Aberdare range, and other reserves that were long connected by tracts of open land which linked their individual ups and downs, their population booms and crashes, into a collective equilibrium.

These fences stand as the ultimate response to irreconcilable conflicts. They transform the physiology of landscapes. They block not only the movement of cattle, diseases, and invasive species, but also the dispersal of plants, the flow of genes, and the millennia-old migrations of animals.

We humans have changed our world in many ways, but among the most-profound of those changes stand our fences.

In Addo, the artificial effects of fencing were long mistaken for the facts of nature. Park brochures extolled the gem of genetic diversity preserved inside Addo’s boundaries—136 out of 140 adult female elephants here carried no tusks, distinguishing this rare herd from any other on Earth. But as scientists reconstructed the history of Addo’s elephants, that image of diversity began to unravel.

Anna Whitehouse, a graduate student of Graham Kerley at the Nelson Mandela Metropolitan University in Port Elizabeth, analyzed over 8,500 historical photographs of Addo’s elephants dating from 1931. She identified each elephant based on its pattern of eye wrinkles and ear tears, noted which calves hung beside which mothers, and constructed a universal family tree including every elephant that had lived in Addo for the past 70 years.

Her exercise revealed a classic case of inbreeding. As recently as the early 1930s, half of all females carried tusks. But since then, the frequency of tusked females has plummeted exponentially, to 3 percent in 2000. Without a single male immigrant in 70 years, every elephant born in Addo since 1954 descended from a single male, and this real-world Papa Smurf scenario almost certainly enriched recessive genes which stalled tusk development. A supposed hallmark of diversity was unmasked as a symptom of this herd’s inbreeding and genetic isolation—its confinement, for better or worse, within a fence.

Whitehouse’s 70-year pedigree also revealed another surprising fact. Overall elephant mortality dropped after the fence’s construction—as expected—but males began dying at a much-higher rate than females. Kerley and Whitehouse noticed that males were living 10 to 20 years less than females.

Closer investigation of park records revealed what no one had seen before: 70 to 90 percent of male deaths over the past seven decades resulted from fights—healthy bulls found gored through the neck by the tusks of another elephant. Nowhere else in Africa were elephants known to fight to the death so often.

Kerley and Whitehouse now view those fights-to-the-death as a consequence of Addo’s confinement. Male elephants normally disperse from family groups once they mature. This dilutes face-to-face competition for mating females. But not in Addo. Infrasonic mating calls can travel 20 kilometers—and fences prevent males from wandering beyond that range. A single female making a mating call can attract every male in the park.

Ironically, even as the fence made life riskier for adult males, it made life safer for young calves—and this fueled another problem: runaway population growth. In natural settings, calves often succumb during thirsty marches from one water hole to another in hot years. But by making it impossible for animals to stray more than 20 kilometers from water, the fence reduced this sort of mortality—and neutered a key mechanism of population control.

Biologists in Addo have documented the effects of overpopulation. Elephants have grazed some plants, such as succulents, into oblivion. The diversity of plants has lessened, and numbers of thicket-grazing bushbuck and grysbok have declined. Plant diversity has declined even in parts of Addo where elephants remain fenced out—because some plants depend on elephant dung to disperse seeds and on elephant grazing to provide openings for germination and growth.

Overgrazing has impacted the ability of this landscape to retain nutrients. Topographic surveys now show a decrease in so-called “run-on” areas, which gather leaf litter and other nutrients transported by wind and water. Elephant gorging, in other words, has triggered a slow bleed of biome-sustaining nutrients from the landscape.

Kerley naturally wondered whether these ecological repercussions have reverberated back to the elephants themselves. His student, Christelle de Klerk, decided to find out.

De Klerk pried apart cantaloupe-sized nuggets of elephant dung to assess their fiber and protein content. She found that the beasts in Addo now consume woodier, lower-quality vegetation than do elephants in other parks. This fibrous, colon-blowing diet has taken a toll on the elephants’ bodies. When de Klerk measured the protrusion of hip and shoulder bones beneath the hides of Addo’s elephants, she found them skinnier than elephants in five other parks.
That physical wasting will eventually squelch the production of healthy calves—but not before these Humvees of the animal kingdom have mowed the vegetation down to a putting green. “Fencing was very successful in conserving elephants,” concludes Kerley. “But there are deeper issues which most people don’t pay attention to.”

Biologists managing Addo are working to address those issues. In 2003 they imported four male elephants—a move which injected fresh genetic capital, leading to the birth of several females with tusks. Exporting bulls from Addo could reduce the problem of love polygons and pachydermicide, although it raises the question: Where to move these males? And as biologists prepare to open additional parts of Addo Park to elephants, they’re already debating whether to provide permanent water in those areas. Limiting water could spare areas far from water holes the full brunt of elephant herbivory—providing a refuge for succulent plants. It could also provide a gentle brake on elephant numbers, as thirst drops a few hapless calves from the herd.

One thing is certain: In the case of Addo, a confined population is no longer a natural one. Far from walling off the problem or ending human intervention, the construction of fences guarantees it into perpetuity.

Addo’s small size provided a perfect ecological Petri dish for untangling the subtle effects of fencing. But similar problems probably occur around the world—even if local conditions don’t allow biologists to see them, says Simon Thirgood, a mammologist at the Macaulay Land Use Research Institute in Aber-deen, Scotland. “These effects of fencing,” he says, “must be quite widespread.”

A recent survey of 23 large mammal migrations, from pronghorn and bison in North America to zebra and wildebeest in Africa to reindeer and Mongolian gazelle in Eurasia, found that six of these migrations have fizzled out—while most others declined in number. “Migratory ungulates are in a pretty poor state, and fencing is part of the problem,” says Thirgood, who helped in that analysis.

In Botswana, a web of fences erected to halt transmission of foot-and-mouth disease from wildlife to cattle has decimated migrating wildebeest. One 250-kilometer barrier, the Kuke fence, prevented wildebeest from venturing north toward the waters and grasses of the Okavango Delta. One famous incident, 20 years after the fence went up, left 50,000 sun-baked carcasses scattered across the Kalahari—hapless animals which the Kuke fence prevented from reaching edible forage during a drought. Wildebeest numbers—once well over half a million—have fallen 90 percent since the 1960s, and much of that implosion resulted from blocked migration routes.

The Kuke story reflects a fundamental mismatch between fences—built as immobile infrastructure at thousands of dollars per kilometer—and the evolving face of natural landscapes. Long-distance migrations fluctuate over years, decades, and centuries, pulled in unpredictable directions by changes in rainfall, fire, and forage. “Mobility is the key for a lot of species,” says Keith Lindsay, a U.K.-based biologist who surveyed the impacts of the Kuke fence in the 1980s. “The way they survive in a dry place is to be able to move around.” Even rivers can shift routes in these arid landscapes; biologists who work in southern Africa speak of rivers migrating 15 kilometers in a decade.

The dingo fence in southeastern Australia has seen similar die-offs in dry years—of kangaroos. But it also produced more subtle effects. Populations of small native marsupials have plummeted on the dingo-free side of the fence but remained more stable on northern, dingo-populated side. Biologists now see the decline in native species as a knock-on effect of dingo suppression. By excluding dingoes, the fence allowed smaller invasive predators such as cats and foxes to increase—permitting them to devour small marsupials into oblivion. The fence also led to increased kangaroo numbers—another blow to small marsupials, since kangaroos devour tall grasses which provide small marsupials with cover.

One even finds complications hidden within success stories. Wildlife managers have re-established wild dogs in a dozen or so fenced reserves across South Africa. But the biologists who carried out this project must take an active role in genetic management of the species by periodically transferring dogs from one reserve to another.

Fencing remains a “last resort” for saving species, concludes biologist Matt Hayward, who manages native species reintroductions for the Australian Wildlife Conservancy in Wentworth. “Working out ways to reduce the impact of those fences is going to be a key element of biodiversity conservation for the next couple of decades.”

Perhaps Frost was right when he wrote in his famous poem:

Before I built a wall I’d ask to know
What I was walling in or walling out,
And to whom I was like to give offense.

It’s time that we rethink our fences—how they’re built, what they’ll keep in or out, and how to turn them into appropriately selective barriers. A fence that keeps in cattle needn’t necessarily block the flow of pronghorn, bears, wild horses, or their genes.

Inexpensive radio-frequency ID (RFID) tags implanted in cattle might one day electrify fence wires as soon as a marked bovine steps near. Pronghorn, deer, and moose could duck under or step over the same fence, undeterred.

Even without evoking smart fences or RFID technology, we can already render barriers species-selective. Simply adjusting the spacing of wires on a fence can suffice. Placing the top wire no higher than 42 inches above the ground and the bottom wire no lower than 16 inches (with barbed wire reserved for only the middle wire) allows pronghorn to pass over or under the fence while keeping less-nimble cattle contained. From Wyoming to Colorado to Arizona, conservation groups are already working to restore the permeability of the landscape and reanimate migration corridors which gushed with seasonal pronghorn traffic for thousands of years before the arrival of suburbs and ranches.

Among the rust-red dunes of South Australia, biologists who fenced in populations of native bettongs, bilbies, and bandicoots to protect them from feral cats and foxes are now experimenting with a logical next step. Biologists with a company called Arid Recovery are building into these fences species-specific portals based on the animals’ different burrowing habits and crawling or hopping gaits. These allow small marsupials (but not predators) to exit—thereby enabling native animals to disperse as populations within protected areas reach holding capacity—without humans having to do the job themselves.

Biological approaches to species-specific barriers have also shown promise. Efforts to establish a pack of African wild dogs in Botswana’s Northern Tuli Game Reserve are hindered by the dogs’ tendency to wander into adjacent areas, where preying on cattle is likely to get them shot. But researchers with the Botswana Predator Conservation Trust have used scent markers—derived from the urine of other dog packs—to lay down invisible barriers, which the dogs won’t cross. The team is working to identify specific chemicals in the dogs’ urine that could be mass-produced in order to scale up these bio barriers. If the approach is found to apply to other large predators, one could easily imagine a reserve in which various species are bio-fenced into overlapping but distinct areas of land—each according to its ecological needs.

Biologists have also discovered that the mere sound of bee hives repels elephants. Hive sounds broadcast over loudspeakers—or real hives positioned at gaps in fences—could deter elephants while allowing other species to pass through. In Tanzania and Kenya, cash for maintaining fences is scarce—but the availability of land provides another solution: Thirgood has found that soft-edged buffer zones around parks work just as well as fences—keeping elephants from crops and poachers from elephants.

Making fences more malleable could render them amenable to landscapes where rainfall and riverbeds shift across years and decades. Fences in which individual segments can be lowered and raised already exist; a network of such adjustable fences could provide the first step in that direction. Wildlife managers could also shift scent-based or bee-based bio-fences from year to year.

“Something there is that doesn’t love a wall, that wants it down,” wrote Frost 95 years ago. The reality of suburbs in North America and citrus orchards in southern Africa won’t allow us to raze all our fences, but at the very least we can reinvent them. ❧

Douglas Fox is a freelance writer based in San Francisco. He has written for New Scientist, Natural History, and Discover.