July 15, 2011

Decline Of Large Predators Threatens Global Ecosystem

The worldwide decline of top predators, or "consumers", such as wolves, sharks and lions, is threatening to drive other species to extinction, an international team of 24 scientists reported on Thursday.

The research shows for the first time the critical importance that large animals have within the world's ecosystem.

"Until recently, large apex consumers were ubiquitous across the globe and had been for millions of years. The loss of these animals may be humankind's most pervasive influence on nature," the scientists said.

The decline of such species is much greater than previously understood, and now affects many other ecological processes through what scientists call "trophic cascades," in which the loss of "top down" predation severely disrupts many other plant and animal species.

"Although such losses are widely viewed as an ethical and aesthetic problem, recent research reveals extensive cascading effects of their disappearance in marine, terrestrial, and freshwater ecosystems worldwide."

Indeed, such disruption is sufficiently severe that it now affects everything from habitat loss to pollution, carbon sequestration, wildfire, climate, invasive species and spread of disease, the scientists said.

It is also a driving force in the sixth mass extinction in Earth history, which the researchers said is now under way.

"We now have overwhelming evidence that large predators are hugely important in the function of nature, from the deepest oceans to the highest mountains, the tropics to the Arctic," said William Ripple, a professor of forestry at Oregon State University, co-author of the report and an international leader in this field of study as director of OSU's Trophic Cascades Program.

"In a broad view, the collapse of these ecosystems has reached a point where this doesn't just affect wolves or aspen trees, deforestation or soil or water," he said.

"These predators and processes ultimately protect humans. This isn't just about them, it's about us."

Historically, there has been little consideration of how large predators affected so many other species, the researchers said.  Indeed, such processes were typically studied one plant or animal at a time in a small area, failing to consider the larger disruption under way.

Based on the new understanding that is emerging, the scientists argued that the burden of proof should now be shifted, to assume that top predators have major effects on ecosystems until proven otherwise.

"We propose 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 altered top-down forcing regimes," the scientists wrote in their report.

Previous research has described the effect that the loss of wolves had within Yellowstone National Park, finding that elk populations increased and their behavior changed because they were no longer fearful of wolf attacks.

Furthermore, without the wolves, the growth of young aspen trees and willow nearly ground to a halt, and there were fewer beaver. Plant communities, tree growth and stream ecology were also affected.

With the return of wolves, those areas are now returning to health, and in places, aspen and willow are recovering where they had been declining.

In the current study, the scientists cited many examples of terrestrial and marine changes as a result of the decline of top predators.  For instance, the reduction of cougar in Utah led to an eruption of deer, loss of vegetation, altered stream channels, and a decline in biodiversity."¨

Likewise, industrial whaling in the 20th century likely caused a killer whale diet shift and a dramatic decline of sea lions, seals and sea otters.  Decimation of sharks resulted in an outbreak of cow-nosed rays and the collapse of bay scallop fisheries.  Sea otters enhance kelp abundance by limiting herbivorous sea urchins."¨ The reduction of lions and leopards in Africa led to a population explosion in olive baboons, which bring intestinal parasites to humans who live in close proximity to them.

For too long, large animals have been seen as "riding atop the trophic pyramid," but not really affecting the species and structure below them, the researchers said.

That's a fundamental misunderstanding of ecology, they said.

"Top-down forcing must be included in conceptual overviews if there is to be any real hope for understanding and managing the workings of nature," they concluded.

The study was published July 15, 2011 in the journal Science.

The paper's coauthors include 24 scientists from various institutions in six countries.

The Institute for Ocean Conservation Science, Defenders of Wildlife, White Oak Plantation, NSERC Canada and NordForsk provided other support for the research.


Image 1: Wolves are among the many "apex predators" that have been affected by human activities. Credit: Getty Images

Image 2: Fishing alters predator patterns; when large fish are removed, corals don't fare as well. Credit: S. A. Sandin et al.


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