Loss of Large Predators Upsets Ecosystem Balance

August 31, 2011

The decline in recent years of large predators could severely impact Earth's ecosystems, according to a recent study from researchers at Oregon State University in conjunction with 21 other scientific and educational institutions in six countries.

The decline in recent years of large predators could severely impact Earth's ecosystems, according to a recent study from researchers at Oregon State University in conjunction with 21 other scientific and educational institutions in six countries. 

"In the past, when studying ecology, scientists would typically think of energy flowing from the bottom-up, starting with plants. In more recent times, we understand that the top-down is also very important in structuring ecosystems," says Dr. William Ripple, ecologist and professor at Oregon State University and study co-author.

The study found that large predators have a much greater impact on many ecological processes than previously understood. Researchers also point out that our planet in the midst of a sixth mass extinction, unique in fact that this extinction period is characterized by the significant loss of these larger animals.

Over the years, the livestock industry has quite successfully eliminated large predators from farmlands and rangelands. In fact, the federal government got involved as well starting in 1915 with the idea of killing off large predators because they compete with hunters for game. Additionally, these predators sometimes attack sheep, goats, and cattle. In recent decades, however, Ripple says we are seeing more emphasis on endangered species, which is somewhat of a reversal, and there is more interest in restoring large predators to forests and rangelands.

Ripple says that the loss of wolves, lion, sharks, sea otters, and other large predators may represent the most powerful impact we have ever had on Earth’s ecosystems. This decline, in which the loss of top-down predation disrupts other animal and plants species – something scientists call "trophic cascades" – affects everything from habitat loss to pollution, deforestation, carbon sequestration, climate, the spread of disease, and more.

When large animals, also know as "apex consumers," start to disappear, the effects are far-reaching and diverse. For example, the loss of apex consumers reduces food chain length, which eventually alters the abundance of plants. This increase in plants can then fuel fires during wildfire season.

One telling example of this process revolves around the loss of wolves in Yellowstone National Park, says Ripple. When wolves were removed, elk populations soared and they no longer avoided feeding on young aspen trees. As a result, growth of these trees practically stopped, and the beaver population decreased. When the wolves were returned to Yellowstone, the balance shifted with aspen, willow, cottonwood, and beaver recovering in some places.

"Interestingly, in Northern Minnesota, where wolves were never extripated, they live in quite good harmony with farmers and ranchers," says Ripple. "Meanwhile, in other areas, like the Western landscape, wolves that have been gone from these regions for 70 years have been reintroduced to some regions."

The study cites additional examples of these trophic cascades in both land and marine populations. One case details how industrial whaling may have shifted the diet of killer whales, leading to the dramatic decline of sea lions, seals and sea otters. Another details how the loss of lions in Africa has led to a population explosion in olive baboons, which bring intestinal parasites to humans living nearby.

"If we acknowledge that predators are important for healthy ecosystems, then indirectly humans benefit. If we have degraded ecosystems, humans will lose out in the end. We need functioning ecoystems to provide services to humans. When ecosystems are healthy, the production and conservation of plants, animals, and water are much better than when we have disrupted ecosystems," says Ripple.

A major disconnect in past research has been in excluding top-down food chain members from examinations of ecosystem shifts. Larger animals are often viewed as sitting atop the food chain and having little effect on other animals and plants. While bottom-up forces are extremely important, they are just one part of a very complex picture. Larger animals must be included in future conceptual overviews, says Ripple, in order to grasp a true understanding of the workings of nature.

"What we hope to do is make people more aware of how strongly top predators affect these various ecosystems. We also want to provide a larger context for future researchers when looking at specific study systems. The absence, or presence, of large predators is an important factor that should be considered," he adds.