PerspectiveEcology

To Fence or Not to Fence

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Science  04 Apr 2014:
Vol. 344, Issue 6179, pp. 46-48
DOI: 10.1126/science.1246251

Habitat fragmentation undermines the functioning of ecosystems, and so biodiversity conservation often entails maintaining or restoring landscape connections. However, conservationists also destroy connectivity by constructing wildlife fences. A recent debate about the use of fences to protect African lions (13) highlights a more general need to evaluate the role of fencing in conservation.

People and wildlife can be uneasy neighbors. Many wild species damage valuable livestock, crops, or infrastructure; some carry livestock diseases; and a few threaten human lives. At the same time, people kill wild animals for food, trade, or to defend lives or property, and human activities degrade wildlife habitat. Separating people and wildlife by fencing can appear a mutually beneficial way to avoid such detrimental effects.

While some fences may be last-ditch attempts to preserve wildlife areas already isolated by human development, others are constructed within relatively contiguous wildlife habitat. For example, in parts of southern Africa, fencing of individual land parcels secures wild animals as privately owned commodities in a wildlife economy centered on sport hunting. In North America, roads may be fenced to minimize collisions that can kill people and wildlife. Fences have been constructed in Australia to protect native marsupials from invasive species, and in Kenya to separate critically endangered hirola antelope from natural predators. Botswana is traversed by veterinary cordon fences intended to prevent disease transmission from wildlife to livestock, and fencing has also been considered as a way to halt the spread of infectious cancer among Tasmanian devils. In Africa, containing rhinos in small fenced areas makes them easier to protect from poachers.

Although fencing can have conservation benefits, it also has costs. When contiguous habitats are converted into islands, the resulting small and isolated populations are prone to extinction, and the ensuing loss of predators and other larger-bodied species alters interactions between other species in ways that cause further local extinctions, a process that has been termed ecological meltdown (4). Areas isolated by fencing are likely to experience similar consequences unless the wildlife populations they contain are intensively managed.

Fencing can lower not only the number but also the density of organisms that can be supported in a landscape. In highly variable environments, both wild herbivores and pastoral people move widely to track resources such as food and water. Constraining these movements by fencing lowers the carrying capacity of such environments. Just as settlement of formerly nomadic pastoralist people can lead to overgrazing and land degradation (5), so construction of fences has been linked to vegetation changes (6) and marked declines in wild herbivores, even where fences were constructed for conservation purposes (7). Such impacts are likely to be especially severe where climate change increases the frequency and severity of extreme weather events (5, 8).

Pros and cons of fencing.

Fences may allow some species, such as lions, to reach high densities, but they also profoundly alter ecosystems. For example, for species like the African wild dog (A) and many of its ungulate prey, fencing may increase the risk of extinction and reduce their resilience in the face of climate change. Furthermore, in some cases fences may fail to prevent passage of wildlife, such as these elephants in Kenya (B).

(DOGS) ROGER DE LA HARPE/WWW.AFRICAIMAGERY.COM; (ELEPHANTS) LAUREN EVANS/WWW.GEOG.CAM.AC.UK/PEOPLE/L.EVANS AND WWW.SPACEFORGIANTS.ORG

Fencing structures themselves can exacerbate pressures on wildlife. They offer a ready supply of wire, which can be used to fashion snares for poaching. Furthermore, predators such as wolves and African wild dogs (see the figure, panel A) can learn to improve their hunting success by chasing prey into fences (9). These impacts and the resulting fence damage in turn prompt hostility from managers, which can lead to deliberate predator removal. Less tangibly, fencing alters people's relationship with nature. Where policy changes have constrained the movements of formerly nomadic people, fences can be perceived as symbols of the policy, generating local hostility to wildlife conservation efforts (10).

In principle, these harmful effects of fencing on ecosystems might be countered by improved protection from human activities, as well as the societal benefits of protecting people from wildlife. However, the challenges of appropriate fence design, location, construction, and maintenance mean that fences often fail to deliver the anticipated benefits (see the figure, panel B). In Africa, fences are a negligible barrier to determined poachers, who need little more than a pair of pliers to gain access to valuable wildlife. For example, fencing has not prevented substantial illegal killing of rhinos in recent years. Although many fences provide effective barriers to wildlife, failures are commonplace. For example, a study of 37 fences in South India found that 49% failed to prevent passage of elephants, mainly because of poor maintenance and deliberate breaches by local people seeking access to the fenced areas (11).

The balance between beneficial and detrimental effects of fencing wildlife was recently debated after a call to fence African parks to conserve lions, which reach higher densities (relative to estimated carrying capacity) inside fenced reserves than in unfenced areas (1). If fencing is effective in resolving human-wildlife conflict, it should reduce lion mortality from human predation (a top-down effect). However, herbivores may not benefit in the same way, because fencing constrains their ability to escape lions and other natural predators. At the same time, restricting herbivores' ability to exploit ephemeral food sources increases the likelihood of their populations being food-limited (bottom-up effects), and fencing has been repeatedly associated with herbivore declines (7). Changes in herbivory prompt cascading effects on vegetation (6) and are likely to influence many other ecosystem components. Although relatively high lion densities inside fenced areas have been portrayed as successful conservation of a key ecosystem process (3), they might indicate food webs profoundly altered by fencing.

Despite these concerns, fences are a powerful tool for conserving and restoring wildlife in landscapes that are highly modified by human activity. In New Zealand, where invasive species extirpated mainland populations of species such as the hihi, fencing of Maungatatauri and other sites has facilitated the creation of “mainland islands” free of invasive species, allowing restoration of native fauna (12). Likewise, in South Africa more-or-less intact (albeit highly managed) assemblages of large mammals have been restored in fenced areas of former farmland at reserves such as Ithala and Pilanesberg, and these areas are increasingly viewed as nuclei for more extensive restoration efforts entailing progressive removal of fencing.

Where wildlife habitat remains extensive, however, alternative approaches will usually be more appropriate than large-scale fencing. A variety of approaches—including traditional farming practices such as herding, planned grazing, and crop guarding, as well as wildlife-sensitive land-use planning—can help to mitigate conflicts between people and wildlife without the need for fencing (13). Likewise, a combination of testing, vaccination, and meat preparation can prevent transmission of foot-and-mouth disease without the need to separate cattle from wildlife by fencing (14). Rather than enclosing wildlife, fences may be used to enclose small areas of intense conflict such as settlements, wells, or grain stores. Where rhinos need to be contained for their own protection, special fences that allow passage of other species have proven effective. Likewise, in North America, livestock fences have been constructed that allow passage of pronghorn. Even virtual fences have been developed for particular species; for example, scent marks have been used to constrain African wild dogs in an otherwise unfenced reserve (15). The wider applicability of such costly and labor-intensive approaches is uncertain.

Reconciling the needs of people and wildlife is a perpetual challenge, and separating the two may be appealing. Too often, however, fences are constructed without a realistic assessment of the costs and benefits. In the United States, growing populations of large carnivores and megaherbivores are conserved in unfenced reserves, showing that fencing is not a necessary condition for conservation of such species. As climate change increases the importance of wildlife mobility and landscape connectivity, fence removal may become an important form of climate change preparedness, and fencing of wildlife should become an action of last resort.

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