Predators: Pivotal Role, yet Increasingly Imperiled

Two distinct, loud “thumps”—that is how, in a September 27, 2014, Facebook post, the president of Lobo Watch, an anti-wolf organization, described the sound when his minivan struck two wolves chasing an elk calf traversing I-90 in remote western Montana. He claimed it was an “accident,” but admits to hitting the accelerator in order to “save that calf.” The outcome was predictable. According to his account, one wolf died on the road shoulder while the other, with a badly broken leg, was observed traversing the ridgeline in obvious distress. The post became fodder for Montana media and the incident was investigated by Montana authorities who, notwithstanding the perpetrator’s own admissions on Facebook and to local media, found no evidence that any wolves were ever struck.

Only days before, on September 24, US District Court Judge Amy Berman Jackson restored Endangered Species Act protections to Wyoming’s wolves when she found that the US Fish and Wildlife Service acted arbitrarily and capriciously in relying on the nonbinding promises of Wyoming officials to maintain a particular number of wolves in the state. Wyoming’s now invalidated wolf management plan permitted a shoot-on-sight policy for wolves in a majority of the state.

Earlier that month, the National Park Service in Alaska—presumably fed up with the state’s anti-predator policies—proposed a ban on the baiting of brown bears, the hunting of wolves and coyotes during the denning and pupping period, and the use of artificial lights to shoot black bear mothers and their cubs at den sites within national preserves in Alaska. Though these unethical practices had been illegal under state law, the Alaska Board of Game recently approved them as part of its decades-long efforts to reduce predators to increase numbers of more popular game animals for hunters.

Humans have conflicting attitudes toward predators. Some view them as vile creatures that kill merely for fun, compete with hunters for game, stalk their children, and kill livestock and pets. Others celebrate the value of predators to healthy ecosystems, their aesthetic beauty, and cultural importance, and routinely dole out funds for their conservation and for the opportunity to see them in the wild. These disparate mindsets create a perfect storm of clashing values.

Meanwhile, predator populations continue to suffer. Grizzly bears occupy a mere 2–4 percent of their historic range in the contiguous United States. Over the same area, gray wolves number only 5,500, and are routinely hunted, trapped, and poisoned for recreation and management control, with little consideration of the impacts to population dynamics, behaviors, or to the ecology of their habitat.

Globally, such threats include habitat loss, degradation, and fragmentation; hunting for trophies, skins, or parts for traditional medicines; international wildlife trade; land use practices; conflicts with livestock; management control actions; human population expansion; and depletion of prey. Notably, on this last threat, human hunting has decimated prey populations and made it nearly impossible for large carnivores to persist in parts of Latin America, Asia, and Africa.

Tigers are on that brink with less than 3,500 remaining in the wild, compared to at least 100,000 in 1900. Wild African lion populations have declined precipitously, with latest estimates of as few as 16,500 remaining—down from an estimated 75,800 in 1980. Many shark species, including hammerheads, oceanic whitetips, and porbeagles have been devastated by the trade in fins and/or meat, with some populations reduced by 99 percent.

In the United States, government decision-makers who dictate predator management rules largely ignore the overwhelming scientific evidence of the ecological importance of predators. State wildlife commissioners and many hunters perceive predators as competition for ungulates—animals preferred by hunters and ones that generate greater revenues for state agencies.  Consequently, anti-predator sentiment predominates among those who dictate wildlife policies.

The folly of this long-held attitude has been in evidence for some time. In the early 1900s, on the Kaibab Plateau in northern Arizona, a coalition of federal and state agents agreed to implement a US Forest Service policy to exterminate predators in an effort to protect more desired species like elk, deer, and bighorn sheep. By 1920, the armed agents had eliminated most mountain lions, bobcats, wolves and coyotes from the Plateau. Without predators to constrain their numbers, however, mule deer populations grew exponentially, consumed all of the available food, and then tens of thousands died from starvation.

It was around this time that scientists began to reconsider the value of predators. In A Sand County Almanac, Aldo Leopold, considered the father of modern wildlife management, tells of his own epiphany about predators when he watched a wolf, struck by his own bullet, die:

We reached the old wolf in time to watch a fierce green fire dying in her eyes. I realized then, and have known ever since, that there was something new to me in those eyes—something known only to her and to the mountain. I was young then, and full of trigger-itch; I thought that because fewer wolves meant more deer, that no wolves would mean hunters’ paradise. But after seeing the green fire die, I sensed that neither the wolf nor the mountain agreed with such a view.

The Kaibab Plateau should have been a wake-up call for wildlife managers. While it did compel a reconsideration of federal predator extermination policies, it did not end the ongoing war against predators.

What predators do for ecosystems

Ecosystem structure and population dynamics are affected by both “bottom up” and “top down” influences. Sea otters, sea urchins, and kelp provide a classic example of “top down” influences. The otters control urchins, allowing kelp to proliferate and improve coastal ecosystem health and function. “Bottom up” control comes from primary production of lower trophic level species (i.e., phytoplankton or plants lower down the food chain). Natural or anthropogenic impacts to either system can have significant effects on ecosystem structure, function, and species-specific population dynamics. When apex predators are removed, the consequences can cascade through the food chain, potentially disrupting ecosystem function and health and increasing conflicts between other, smaller predators and humans.

Apex predators (i.e., predators that have a disproportionate effect on their environment compared to their abundance), in particular, are critical for ecosystem function and health. Whether aquatic or terrestrial, the ecological services of apex predators include controlling mid-sized predators, maintaining the abundance and richness of lower trophic level species, providing food for scavengers, influencing disease dynamics, increasing carbon storage, affecting stream morphology, increasing crop production, and even mitigating climate change impacts.

In a seminal 2014 paper entitled “Status and Ecological Effects of the World’s Largest Carnivore,” Dr. William Ripple of Oregon State University and others provide a comprehensive review of much of the published literature on the ecological value of, and threats to, large predators. They also provide a cautionary note of how humans must adopt a model of tolerance and coexistence to “determine the fate of Earth’s largest carnivores and all that depends upon them, including humans.”

African lions and leopards, for example, exert considerable control over mesopredators (middle trophic-level predators). When the big cats are removed, the natural controls on the mesopredators are released. In West Africa, olive baboons increased in numbers as such apex predators declined. As baboon numbers increased, so did their impacts on small ungulates, other primates, livestock, and agricultural crops which, in turn, impacted humans through competition for food.

The dingo is the sole remaining apex predator in Australia. The species is extensively controlled to reduce impact on livestock. Indeed, a 5,500 kilometer dingo-proof fence has been constructed to keep dingoes out of Australia’s most prominent sheep-producing regions. As reported by Ripple et al., where dingoes exist, they effectively suppress introduced herbivores and red fox (an exotic mesopredator), benefitting plant communities and smaller native prey. Conversely, where dingoes are suppressed, evidence suggests that this has contributed to the endangerment and extinction of small marsupials and native rodents over much of the continent.

The presence of cougars (a.k.a. mountain lions or pumas) in the western United States can limit mule deer densities, thereby reducing deer impacts on woody plants. This can, in turn, benefit other terrestrial and aquatic species, including wildflowers, amphibians, lizards, butterflies, and aquatic plants, while also stabilizing stream banks. Conversely, the loss of apex predators from the eastern United States has contributed to increases in white-tailed deer, with concomitant impacts to ecosystem functions, including plant recruitment and survival, forest structure, and nutrient dynamics. In a series of cougar studies1 in Washington, Dr. Robert Wielgus and others from Washington State University demonstrated that heavy hunting of cougars (1) increased human-cougar conflicts, including livestock depredation incidents; (2) failed to reduce cougar numbers or densities; and (3) led to greater cougar predation of less abundant prey, which has contributed to a seismic shift in cougar management in Washington.

There is compelling evidence that the reintroduction of wolves to Yellowstone National Park in the mid-1990s has resulted in significant ecological changes to the park’s vegetation. Not only are the wolves directly predating elk, moose, deer, and the occasional bison, but they instilled a sense of fear among the park’s herbivores, contributing to their routine movements to evade wolf predation, thereby reducing impacts on park vegetation. Indeed, since wolves have been restored, the park’s floral ecology has improved, with the restoration of aspen groves and other plants. These beneficial impacts have had a cascade of effects—increasing wildlife and plant diversity, restoring locally extirpated species such as beavers, and stabilizing stream banks, further benefitting the park. More broadly, as Ripple et al. report, within North America and Eurasia, cervid (deer family) densities were nearly six times higher on average in areas without wolves than in areas with wolves.

In their study on shark removal and coral loss on Australian reef fish, Dr. Jonathan Ruppert of the University of Toronto found that “fishing [for sharks and other species] was significantly associated with declines in shark numbers and was also associated with high abundances of smaller, mesopredators.” This, in turn, adversely impacted the abundance of herbivorous fish and, ultimately, the health of the coral reefs. Dr. Ransom Myers of Dalhousie University in Halifax, Canada, concluded in a 2007 study that intense shark fishing in the northwest Atlantic over the previous 35 years resulted in a trophic cascade whereby, with the loss of large predatory sharks, the number of mesopredators (e.g., rays, skates, and smaller shark species) exploded and, in turn, wreaked havoc on shellfish populations along the Atlantic coast. Similarly, in a long-term study of predatory fish in the Ligurian Sea, Dr. Gregory Britten, also from Dalhousie University, found “significantly decreasing [ecosystem] stability concurrent with declining predator abundance” and “weaker top-down control, leading to predator-release processes in lower trophic levels and increased susceptibility to perturbation.”

Dr. James Estes, in his 2011 paper entitled “Trophic Downgrading of Planet Earth,” concluded that eliminating large carnivores is one of the most significant anthropogenic impacts on nature. Considering this impact, Dr. Ripple and his coauthors have proposed the establishment of a Global Large Carnivore Initiative (GLCI) to advance public knowledge and recognition of the important ecological role of large carnivores and of their inherent value, and to develop and implement strategies that promote human-large carnivore coexistence. This concept has merit but it should be undertaken with the understanding that an urgent, global campaign is required to save large predators. We must jettison the anti-predator attitude and embrace the scientific evidence revealing that predators are ecological engineers, not evil executioners, and that they must be restored in order to improve ecosystem function and health, and ultimately, to save ourselves.

References

Britten, G.L., Dowd, M., Minto, C., Ferretti, .F, Boero, F., & Lotze, H.K. (2014). Predator decline leads to decreased stability in a coastal fish community. Ecology Letters (in press). doi:10.1111/ele.12354 

Estes, J.A.,  Terborgh, J.,  Brashares, J.S., Power, M.E., Berger, J., Bond, W.J., … Wardle, D.A. (2011). Trophic downgrading of Planet Earth.  Science, 333, 301–306. doi:10.1126/science.1205106

Myers, R.A., Baum, J.K., Shepherd, T.D., Powers, S.P., & Peterson, C.H. (2007). Cascading effects of the loss of apex predatory sharks from a coastal ocean. Science, 315, 1846-1850. doi:10.1126/science.1138657

Ripple, W.J., Estes, J.A., Beschta, R.L., Wilmers, C.C., Ritchie, E.G., Hebblewhite, M., … Wirsing, A.J. (2014). Status and ecological effects of the world's largest carnivores. Science, 343, 1241484. doi:10.1126/science.1241484.

Ruppert, J.L.W., Travers, M.J., Smith, L.L., Fortin, M-J., Meekan, M.G. (2013). Caught in the middle: combined impacts of shark removal and coral loss on the fish communities of coral reefs. PLoS ONE 8(9): e74648. doi:10.1371/journal.pone.0074648

1For more information about the cougar studies by Dr. Rob Wielgus of Washington State University and others, see:
http://environment.wsu.edu/people/faculty/robwielgus.html