The decline of large predators and other "apex consumers" at the top of
the food chain has disrupted ecosystems all over the planet, according
to a review of recent findings conducted by an international team of
scientists and published in the July 15 issue of Science
The study, which included Jeremy Jackson and Stuart Sandin of Scripps
Institution of Oceanography at UC San Diego, looked at research on a
wide range of terrestrial, freshwater and marine ecosystems and
concluded that "the loss of apex consumers is arguably humankind's most
pervasive influence on the natural world."
According to first author James Estes, a professor of ecology and
evolutionary biology at UC Santa Cruz, large animals were once
ubiquitous across the globe, and they shaped the structure and dynamics
of ecosystems. Their decline, largely caused by humans through hunting
and habitat fragmentation, has had far-reaching and often surprising
consequences, including changes in vegetation, wildfire frequency,
infectious diseases, invasive species, water quality and nutrient
The decline of apex consumers has been most pronounced among the big
predators, such as wolves and lions on land, whales and sharks in the
oceans and large fish in freshwater ecosystems. But there have also been
dramatic declines in populations of many large herbivores, such as
elephants and bison. The loss of apex consumers from an ecosystem
triggers an ecological phenomenon known as a "trophic cascade," a chain
of effects moving down through lower levels of the food web.
Sandin's contribution to the study focused on the importance of sharks
in coral reef ecosystems. For the past several years, Sandin and his
colleagues have explored a chain of islands in the Central Pacific Ocean
that feature virtually pristine, undisturbed reefs but also reefs
adjacent to populated islands and subject to the effects of pollution
and fishing. As documented in the Science
Jarvis Island features a healthy, thriving reef ecosystem with a robust
shark population. Neighboring Kiritimati, or Christmas Island, however,
with a population of 5,000 people and an active fishery, is absent of
sharks and now features an ecosystem dominated by small fishes and
overrun by algae.
"Practically and ethically, we cannot conduct large experiments to
investigate the effects of predator removal," said Sandin. "But in the
Line Islands we can rely on a natural experiment to follow what happens
when sharks are removed. The abundance of prey changes, the way the
energy flows through the ecosystem changes and even the way nutrients
are cycled is altered. The importance of this paper is its recognition
that predator removal is a global phenomenon, affecting reefs and almost
every other ecosystem."
In a separate study recently published in the Public Library of Science (PLoS) ONE
journal, Sandin and his colleagues began probing the details and
mechanisms of how coral reef ecosystems change without the presence of
sharks. Comparing unfished Palmyra to heavily fished Christmas Island,
Sandin and his coauthors found that the ecology of prey fish changed
dramatically in the relative absence of predators. On the reefs of
Christmas, the removal of sharks and other large predators leads to an
increase in the longevity of prey fish, an alteration in their patterns
of growth and ultimately the creation of a wholly new structure for the
"predator-lite" coral reef food web, the study showed.
"Predators have a huge structuring influence," said Sandin. "When you
remove them you change the biology, which is typically profound and
complex. And in many cases it's not necessarily predictable."
"The top-down effects of apex consumers in an ecosystem are
fundamentally important, but it is a complicated phenomenon," Estes
said. "They have diverse and powerful effects on the ways ecosystems
work, and the loss of these large animals has widespread implications."
Estes and his coauthors cite a wide range of examples in their review, including the following:
• The decimation of wolves in Yellowstone National Park led to
over-browsing of aspen and willows by elk, and restoration of wolves has
allowed the vegetation to recover.
• The reduction of lions and leopards in parts of Africa has led to
population outbreaks and changes in behavior of olive baboons,
increasing their contact with people and causing higher rates of
intestinal parasites in both people and baboons.
• A rinderpest epidemic decimated the populations of wildebeest and
other ungulates in the Serengeti, resulting in more woody vegetation and
increased extent and frequency of wildfires prior to rinderpest
eradication in the 1960s.
• Dramatic changes in coastal ecosystems have followed the collapse and
recovery of sea otter populations; sea otters maintain coastal kelp
forests by controlling populations of kelp-grazing sea urchins.
• The decimation of sharks in an estuarine ecosystem caused an outbreak
of cow-nosed rays and the collapse of shellfish populations.
Despite these and other well-known examples, the extent to which
ecosystems are shaped by such interactions has not been widely
appreciated. "There's been a tendency to see it as idiosyncratic and
specific to particular species and ecosystems," Estes said.
The absence of sharks can alter the growth patterns of red snapper and other reef fish.
One reason for this is that the top-down effects of apex predators are
difficult to observe and study. "These interactions are invisible unless
there is some perturbation that reveals them," Estes said. "With these
large animals, it's impossible to do the kinds of experiments that would
be needed to show their effects, so the evidence has been acquired as a
result of natural changes and long-term records."
Estes has been studying coastal ecosystems in the North Pacific for
several decades, doing pioneering work on the ecological roles of sea
otters and killer whales. In 2008, he and coauthor John Terborgh of Duke
University organized a conference on trophic cascades, which brought
together scientists studying a wide range of ecosystems. The recognition
that similar top-down effects have been observed in many different
systems was a catalyst for the new paper.
study's findings have profound implications for
conservation. "To the extent that conservation aims toward restoring
functional ecosystems, the reestablishment of large animals and their
ecological effects is fundamental," Estes said. "This has huge
implications for the scale at which conservation can be done. You can't
restore large apex consumers on an acre of land. These animals roam over
large areas, so it's going to require large-scale approaches."