PerspectiveConservation

When the hunter becomes the hunted

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Science  19 Jun 2015:
Vol. 348, Issue 6241, pp. 1312-1314
DOI: 10.1126/science.aaa8465

Although gray wolf (Canis lupus) populations are recovering in parts of Europe and North America, wolves continue to be killed in attempts to protect livestock and threatened species such as caribou.

PHOTO: DENNIS DONOHUE/THINKSTOCK

Historically, wild predators were overwhelmingly viewed as threats to livestock, wild “game,” and public health. Over time, public perceptions have broadened to include recognition of predators' intrinsic value and their role in structuring ecosystems. Nowhere are these changing perceptions better illustrated than in Yellowstone National Park, where the U.S. government deliberately eliminated wolves in the 1920s, only to actively restore them in the 1990s. Large carnivores are now recovering across much of North America and Europe but declining elsewhere (1, 2). Predator control, once widely accepted by the public, has become a source of intense social conflict (3, 4). Robust scientific evidence and broad stakeholder involvement are crucial for effective management of predator populations.

Randomized controlled experiments have repeatedly shown that predator control can bolster populations of prey species, including many of conservation concern (5, 6). However, the complexity of ecological systems means that predator control does not invariably benefit wild prey. In some ecosystems, factors such as habitat loss or weather conditions influence prey numbers more strongly than does predation. Suppressing the populations of one predator species may cause other predators to increase in number, leaving prey to face unchanged or even heightened predation rates. For example, Ellis-Felege et al. have shown in an experimental study that predator control efforts reduce mammalian predation on bobwhite quail nests in the southeastern United States, but these benefits are offset by increased predation from snakes (7). Similarly, pronghorn (which are preyed on by coyotes but seldom by wolves) appear to have declined in response to the extirpation of wolves, which caused a dramatic expansion of coyotes across North America (8).

Predator control is thus not invariably beneficial, and decisions about its use need to be informed by case-specific evidence. The best evidence comes from controlled experimental studies, which have been widely used to understand the effects of predator control on wild prey. However, decisions about controlling predators to protect livestock or manage disease have often been based on much weaker evidence. For example, despite the enormous effort invested in controlling populations of red foxes, coyotes, and wolves, we are not aware of any randomized experiments that investigate whether such control reduces livestock predation as intended. This is problematic because it is often difficult to disentangle cause and effect in observational studies.

The ability of experimentation to improve evidence in such cases is illustrated by the management of bovine tuberculosis in Britain. The role of European badgers in transmitting tuberculosis to cattle was recognized in the 1970s, but decades of badger culling failed to prevent the infection from spreading across Britain. A randomized controlled trial revealed that culling consistently increased infection rates in badgers, spreading the disease in space and, under some circumstances, elevating disease risks for cattle (9). This failure of predator control to effect disease control is linked to the social behavior of these territorial animals. When badgers are killed in one area, other badgers migrate into the newly available territory. The resulting social instability is thought to increase opportunities for disease transmission (9). A similar behavioral response was seen among red fox populations culled across Europe in the 1970s to control rabies (10). Such fox control efforts were gradually abandoned as practical experience showed fox vaccination to be far more effective (10).

Predator control conducted for a specific purpose often has broader consequences, which may be as unwelcome as they are unintended. Control or elimination of large carnivores, mainly to reduce livestock depredation, has been linked to increased deer numbers across Europe and North America. These larger deer populations damage forests and crops, reduce road safety, and have cascading effects on ecosystems (11). Extirpating wolves across much of North America stopped wolves from killing sheep, but introduced a new sheep predator as coyotes spread across the continent. Likewise, culling badgers for disease control purposes in the United Kingdom doubled the numbers of another farm pest, the red fox (12). The potential for predator control to cause unwanted side-effects should be considered seriously in environmental impact assessments and other policy decisions.

In deciding how and whether to manage predation, it is also important to bear in mind that the need for predator control is itself often a consequence of human action. For example, in Patagonia, guanacos are kept in low numbers by their natural predator, the puma. Normally predator numbers would fall as their prey decline, but pumas are sustained by sheep, deer, and hares introduced by people, allowing them to maintain high predation pressure on the few remaining guanacos (13). Similarly, Canadian oil developments have opened up the boreal forest, allowing deer numbers to increase and improving access to the forest for wolves. As a consequence, endangered woodland caribou face unsustainable predation, prompting the Canadian state governments to shoot, trap, and poison wolves in an attempt to protect the caribou (14). More generally, many studies have found predation on birds' nests is higher where human activities have fragmented natural habitat. Predator control efforts aimed at conserving threatened prey may need to be maintained in perpetuity unless conservation seeks to restore ecosystem functioning.

Decisions about how to manage predation in such human-altered systems are driven as much by the priorities of decision-makers as by rigorous scientific evidence. For example, decision-makers responding to woodland caribou declines might choose to prioritize biodiversity conservation. They would then prefer habitat restoration over wolf control, because this approach preserves predation as an ecosystem process. Alternatively, decision-makers might prioritize economic gains. They might then consider the need for wolf control (or indeed the loss of woodland caribou) a small price to pay for the prosperity achieved through oil exploitation. The caribou inhabiting Canada's boreal forest have come to resemble the partridges struggling to survive in England's wheat fields; both may remain reliant on predator control unless or until society is willing to restore their natural habitat.

European badgers (Meles meles) transmit tuberculosis to cattle, but efforts to control tuberculosis by culling badgers have not been effective because they do not take their social behavior into account.

PHOTO: SETH JACKSON/ZSL INSTITUTE OF ZOOLOGY

Robust scientific evidence alone is not sufficient to manage predators effectively; social acceptability is equally important. Pragmatic conservationists have long recognized that allowing some predator control—whether or not it achieves its stated aims—can help to build tolerance among land managers who might otherwise block conservation efforts (3). Unfortunately, such compromise is not always effective. For example, small-scale culling of badgers on and around farms experiencing tuberculosis would be more socially acceptable than widespread badger control. However, such localized culling consistently increases cattle tuberculosis (9). Policy-makers are thus faced with a stark choice between near-elimination of a native carnivore (unpopular with the public and potentially unlawful) or no badger control at all (unpopular with farmers). The British government's decision to pursue large-scale badger culling became an issue in the recent national election, and culls have been repeatedly delayed by legal challenges.

Controversy about predator management can lead to intense social discord, which may undermine management decisions. For example, in Britain conservation efforts for hen harriers have been hampered by persistent illegal killing, while attempts to control cattle tuberculosis by killing badgers have been disrupted by dedicated protestors. Where social conflict is intense, scientific evidence is often used selectively, contested, or dismissed. In such situations, involving stakeholders in the design, implementation, and interpretation of experimental studies may help to build trust and improve social learning. For example, controversy over grizzly bear management in Banff National Park, Canada, was successfully resolved by engaging stakeholders in a problem-solving group, which shared responsibility for interpreting scientific evidence and making management decisions (15).

Similar approaches might benefit the management of ecologically complex and socially divisive issues such as tuberculosis in badgers, wolf predation on caribou, and hen harrier predation on grouse (4). The challenge, especially in more intense social conflicts over predators, is that polarized views may prevent parties from engaging with the process at all. If policy-makers, scientists, and stakeholders from all sides can show leadership in overcoming this challenge, predator management might become more evidence-based, as well as more responsive to changing social perspectives.

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