Apex Predators are Taking It to the Top

Large animals at the top of food webs may have as much influence shaping ecosystems as those at the bottom

  • Roger Di Silvestro
  • Animals
  • Mar 11, 2013
IN THE 1990s, biologist James Estes noticed that something was going wrong with the sea otters he had been studying for the past 20 years along Alaska’s Aleutian archipelago. After decades of recovery from near extinction brought on by the fur trade, the otters were dwindling again under what proved to be a more natural force. “By 1997, it became clear to me that orcas were driving the otters down,” Estes says. “They really hammered them.” In some areas, 98 percent of the otters disappeared. Numbers in the Aleutian area fell from more than 100,000 before 1990 to perhaps 3,000 to 5,000 today.

After another 15 years of study Estes, an evolutionary biologist at the University of California–Santa Cruz, believes the sea otter decline is only one link in a series of ecological changes in the archipelago. The starting point was industrial whaling, which in the early 1900s decimated the large whales on which orcas fed in the waters off Alaska’s coast. The killer whales turned to preying on Steller sea lions, which can grow to 2,500 pounds, and caused the sea lion population, perhaps already stressed by climate change and by an intensive commercial fishing industry, to collapse. The orcas then turned to feeding on sea otters, which also may have been debilitated by pollution.

As the sea otters declined, other ecological effects arose: The sea urchins on which the otters fed expanded in number and began eating coastal kelp, destroying habitat that supports fish and other animals. Bald eagles, which fed on fish and sea otter pups, turned to preying on seabirds. Thus the loss of a large species at the top of the food web—whales—disrupted an entire ecosystem. “It’s probably more complicated than that, but that’s probably what’s at the base of it all,” Estes says.

A New View of Ecosystem Dynamics

His work may be among the harbingers of a shift in how biologists view ecosystems and how conservationists protect habitat. “In the past, the default paradigm among biologists was that of bottom-up regulation of ecosystems, where the base of vegetation affected everything above,” says Sterling Miller, a bear biologist and NWF senior wildlife biologist. “But within recent decades scientists have increasingly recognized how important top-down effects are in keeping grazers and browsers from damaging the vegetative base of the pyramid and the cascade of smaller species dependent on this base.” Research by Estes and others underscores the need for strong protection of large consumers atop the ecological pyramid to ensure ecosystem health.

Many apex consumers are dwindling, part of a process that began more than 10,000 years ago with the extinction of such megafauna as the mastodon, giant ground sloth and saber-toothed cats and continued through the extirpation or drastic reduction of such species as Steller sea cows, elephant birds, dodos, brown bears, bison, gray wolves, tigers, rhinos and many more. A recent paper published in the journal Science cites this wave of destruction, caused largely by human activity, as “arguably humankind’s most pervasive influence on the natural world. This is true in part because it has occurred globally and in part because extinctions are by their very nature perpetual, whereas most other environmental impacts are potentially reversible on decadal to millennial timescales.”

The Science paper, “Trophic Downgrading of Planet Earth,” was the brainchild of both Estes and John Terborgh of Duke University, who has spent decades studying tropical rainforest ecology. Their research, which suggested that profound changes in ecosystems could come from the top down, led to the formation of a two-and-a-half-day conference about four years ago that produced the paper, a review of ecological research on large apex consumers written by Estes, Terborgh and 21 other scientists. “What’s important to me is the impact of the paper and its ideas,” Estes says. “Can we use it to influence people’s conceptions about ecosystems and what we do about them?”

The Wolf Connection

Like Estes’ sea otter studies, wolf research in Yellowstone National Park underscores the importance of apex consumers to ecological integrity. After control efforts wiped out park wolves in the 1920s, elk numbers rose, and the elk browsed areas they would have avoided if threatened by wolves. The result: reduced new growth of aspens on some mountainsides and of willows along streams.

When wolves were reintroduced to Yellowstone in 1995 and 1996, a reduction in elk numbers began to show positive effects on tree and shrub growth, says William Ripple, director of Oregon State University’s Trophic Cascades Program and a coauthor of the Science report who has studied wolf, elk and plant dynamics in Yellowstone since the 1990s. In some areas, aspen and willow are growing taller and filling in the plant community, though not at all sites Ripple has studied. “Restoration takes time,” he says. “We’re only 17 years after wolf reintroduction, and we were 70 years without wolves. We need more time to see how Yellowstone unfolds.” Regrowth of willows along streams will shade and cool the water, reduce bank erosion and benefit many species that use such habitat, including songbirds and even beavers, which function as ecological engineers by reconfiguring streams.

The role of wolves in Yellowstone is not unique. In a paper published in 2011 in Geomorphology, Ripple and his Oregon State University colleague Robert Beschta point out that loss of mountain lions in Utah’s Zion National Park led to an overabundance of deer and that destruction of wolves in Washington’s Olympic National Park allowed elk to explode, in both cases affecting the growth of trees along streams and increasing factors such as erosion.

Apex Consumer Ripple Effects

Loss of apex consumers can even affect the occurrence of some diseases. The spread of Lyme disease, which can be deadly to humans, requires the presence of ticks, deer and deer mice, but, according to research published last year by Taal Levi of the Cary Institute of Ecosystem Studies and his colleagues, the ailment also is strongly linked with the extirpation of wolves in the East, which allowed coyotes to move in. Unlike wolves, coyotes kill mouse-eating foxes, which helps deer mouse populations to increase and the disease to spread.

Predators are not the only apex consumers driving changes in ecosystems. In one of the stranger tales from this field of research, a population of deer drove a population of black bears to extinction on 3,000-square-mile Anticosti Island at the mouth of the St. Lawrence River. At one time the forested island, punctuated with peat bogs, probably was home to only half a dozen species of land mammal, including the river otter, pine marten and two bat species. Only two, the deer mouse and black bear, ate plants.

In the 1890s a landowner introduced about 200 white-tailed deer to Anticosti as part of a plan to turn the island into a paradise for hunters. In the absence of efficient predators, the deer rocketed to 50,000 individuals within a few decades, noshing away shrubs that produced a key bear food: berries. Because the island’s forests lacked nuts and seeds and its waters offered few salmon, local bears had to pack away about a third of their body weight daily in berries to make it through winter. As the deer wiped out the shrubs, the berries disappeared, and so did the bears. Researcher Steeve Côté of the Department of Biology at Quebec’s Laval University reported the extinction in the journal Conservation Biology in 2005, crediting his paper as “the only documentation of a large herbivore extirpating a successful and abundant carnivore from a large ecosystem.”

In the Science paper, Estes and his colleagues proposed that “many of the ecological surprises that have confronted society over past centuries—pandemics, population collapses of species we value and eruptions of those we do not, major shifts in ecosystem states, and losses of diverse ecosystem services—were caused or facilitated by [changes in apex consumers].” Examples:

• Destruction of sharks in recent years along the U.S. Atlantic coast allowed cow-nosed rays to stage a population explosion; heavy feeding by the rays later caused a collapse in bay scallop fisheries.

• In East Africa, the introduction in the 1800s of rinderpest, a viral disease that infects a variety of hoofed animals, decimated populations of such browsers as wildebeest and buffalo, resulting in more woody vegetation, which in turn increased the extent and frequency of wildfires. In the 1960s, game and livestock managers eradicated rinderpest. As large ungulates recovered in the early 1980s, woody vegetation declined, grasslands returned and the frequency of wildfire fell off across the region.

• Industrial whaling in the 20th century resulted in the loss of large numbers of plankton-consuming great whales, which are now known to sequester carbon in the deep sea when they die and sink, rotting at the ocean bottom. The decline in whales resulted in the transfer of approximately 105 million tons of carbon into the atmosphere, contributing to climate change.

• The reduction of lions and leopards in parts of Africa has led to population outbreaks of olive baboons, which are drawn into increased contact with people by the monkeys’ attraction to sources of human food. The interaction is causing higher rates of intestinal parasites in both people and baboons.

The authors of the Science article reasoned that the critical role of apex consumers in ecosystem integrity must be included in conservation plans “if there is to be any real hope of understanding and managing the workings of nature.” This conclusion bears profound implications for conservation. “The science is showing that the large predators at the top of the food web are important to the function of ecosystems,” Ripple says. “In short, conserve the large predators so the natural interactions can take place.”

Deposing the King of Beasts

A study released last December reported that lions are rapidly declining across Africa because human population growth and land development have reduced the cats’ habitat by 75 percent. Published in Biodiversity and Conservation, the paper indicated that only 67 regions remain in Africa where significant lion populations can survive, but only about 15 of the sites are home to at least 500 of the big cats. Lions may number fewer than 32,000 animals continent wide.

How this decline will affect African ecology is hinted at in the effect that lion and leopard reductions have had on olive baboons. But, says Luke Hunter, president of Panthera, an organization devoted to wild feline protection, evidence suggests more far-reaching effects. In South Africa, for example, where wildlife is restricted mostly to parks and other protected areas, herds of large ungulates can destroy grassland habitat if lions are absent. The big predators keep zebras, wildebeest and other prey on the move, helping to foster more biologically diverse grassland-woodland mosaics. “Without lions and other top carnivores preventing ungulate populations from localizing, you end up with a golf course that easily collapses ecologically,” Hunter says.

Removing lions from native habitat sets in motion a cascade of ecological changes. “The interactions are so complex, but once you pull the string, it starts unraveling,” Hunter says. For example, he speculates that vultures, already dwindling across Africa from eating the carcasses of livestock treated with an antibiotic toxic to the scavengers, are likely to face even worse inroads as the loss of lions cuts into their primary source of food, the remains of large prey killed by the cats.

But, Hunter says, the relationship of a large predator to other species “is so complicated that the lion decline isn’t universally detrimental.” Lions and spotted hyenas can suppress cheetah populations that share their habitat. In some African nations lions survive only within protected areas like parks, so cheetahs are increasing outside parks. As lions decline, cheetahs have an edge in rebounding. 

NWF in Action: Protecting Apex Consumers

NWF has worked for decades to protect large species such as the grizzly bear, bighorn sheep and Florida panther. The Federation played a lead role in returning wolves to Yellowstone National Park in the 1990s, and last year, working with the Assiniboine and Sioux tribes, helped obtain more than 60 purebred Yellowstone bison for release on the Fort Peck Reservation in eastern Montana. During the past decade, NWF has used a market-based approach to retire more than 500,000 acres of livestock grazing rights on federally administered lands in the Greater Yellowstone Ecosystem to safeguard habitat for native wildlife. See nwf.org/restoringbison.

Roger Di Silvestro is a senior editor for National Wildlife.

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