Large animals at the top of food webs may have as much influence shaping ecosystems as those at the bottom
03-11-2013
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Roger Di Silvestro
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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