The decline of large predators such as big cats, wolves, sharks and giant whales may be “humankind’s most pervasive influence on the natural world,” causing prey animals to swell in population and throw food chains out of balance, a new report says.
Humans have touched off the world’s latest mass extinction, according to the report, published Thursday in the journal Science, and the consequences are being felt on land and in water systems as large predators vanish.
“Recent research suggests that the disappearance of these animals reverberates further than previously anticipated,” says the report, “Trophic Downgrading of Planet Earth.” In addition to creating an overabundance of prey, the dwindling number of predators contributes to the spread of disease, wildfires and invasive species.
The decline of wolves in Yellowstone Park is cited as an example of what can happen. Elk and deer in the park once flourished on willow trees and saplings, threatening a crucial part of the forest on which other creatures rely.
The report also mentions the slaughter of lions and leopards by hunters and herders in parts of sub-Saharan Africa. As a result of the killings, disease-carrying olive baboons have thrived without their top predators and inched closer to food crops and people.
The decimation of sharks along the U.S. Atlantic Coast has allowed their main prey, the cow-nosed ray, to proliferate and dine heavily on the threatened Chesapeake Bay oyster.
A reduction of big herbivores such as buffalo and wildebeest in East Africa through hunting is also a problem, the report says. Their demise has led to increases in plants that fuel giant wildfires in the dry season.
Americans don’t have to visit federal parks or sub-Saharan Africa or plunge into seas to see the consequences, said Ellen K. Pikitch, a co-author of the report and a professor at Stony Brook University in New York. Many experience the problem every day in their own back yards.
“People who live in North America know it’s hard to grow a garden because deer will eat it,” said Pikitch, a marine biologist. “The lack of wolf populations throughout North America has led to an expansion of the deer population.
“You may hate wolves. You might think they’re dangerous. But without them, the land changes,” Pikitch said. “Deer carry ticks. We humans become more susceptible to diseases such as Lyme disease.”
Wildlife advocates say efforts to protect one species of predator in the United States were set back when the Obama administration signed a bill in April that removed 1,300 wolves from the endangered species list in northern Rocky Mountain states. It was the first time Congress had taken a species off the endangered list. The law allows limited hunting of the animals to begin this summer.
Other studies have examined the collateral damage caused by the near-extinction of large predators and herbivores. But the report in Science is the first to tie together the impact on land animals as well as salt and freshwater marine life, Pikitch said. It was conducted by an international team of 24 scientists and funded primarily by the Institute for Ocean Conservation Science at Stony Brook.
Much of the science in this area of study has focused on the threat to life at the bottom of the food chain, theorizing that small animals and plants are important because so many creatures rely on their survival.
Although “bottom-up” research is fundamental and important, the report says, “top-down” research deserves wider consideration “if there is to be any real hope for understanding and managing the workings of nature.”
The report acknowledges that top-down research of the food chain is difficult to conduct, noting that it can take decades to measure the effects of the disappearance of large predators.
“The irony . . . is that we often cannot unequivocally see the effect of large apex consumers until after they have been lost” and the ability to restore the species has also been lost, the report says.
Large predators, or apex species, include animals that people adore, such as otters, and others not so popular, such as vultures.
On the Pacific Coast, from Alaska to the southern tip of California, sea otters were hunted in the 1900s to near-extinction for their pelts. Their absence started a chain of events that nearly eliminated the kelp forests that nurture all manner of marine life on the coast.
Sea otters feed on sea urchins, which dine on kelp. Without otters, the sea urchin population exploded. The kelp forest started to disappear. When sea otter populations elsewhere were re-introduced to a few areas along the coast, the kelp started to rebound.
A telling consequences of the absence of large predators can be found on the Scottish island of Rum, where wolves have been gone for more than 250 years and red deer thrive, the report says. The once forested island is now treeless.