Top Predators and Ecosystem Health

The presence of predators is a key indicator of ecosystem health. (Photo credit: Doug Smith/NPS)

Ecologists have long known that the health of an ecosystem depends on productivity at its base. Known as “bottom-up” control, nutrients available at the bottom of a food web dictate the abundance of organisms at each trophic level above it. But now mounting evidence is turning the bottom-focused theory on its head, pointing to the key role played at the very top of the food web.

Although they make up little of an ecosystem’s total biomass, top predators help regulate the entire web. For example, sea otters are a top predator in the kelp forests of the Pacific Ocean. When the sea otter population dropped, sea urchins went unchecked and decimated the kelp forests. As a result, many other species that depended on the kelp forests also declined, including raptors, shorebirds, fish, and invertebrates. Now that sea otters are protected and their numbers are back up, the kelp forests and the species  that depend on them are recovering as well.

If such chain reactions, or trophic cascades, are common to other ecosystems, there could be major implications for conservation. Top predators of terrestrial ecosystems such as wolves, bears, jaguars, and lions have declined throughout the world due to human activities. In North America and Eurasia, wolves in particular have been eradicated from many areas where they were once widespread. If the wolf populations were required to keep their prey populations in check, we would expect to see much higher numbers of deer, caribou, elk, and moose in those areas. We would also see changes in the plants the herbivores consume, as well as impacts to the biodiversity those plant communities harbor.

A recent study at Oregon State University set out to look at that very data. Researchers analyzed published data on populations of wolves, bears, and cervids (members of the deer family) in boreal and temperate forests of North America and Eurasia from 42 separate studies. They used remote sensing imagery to identify productive forests and historical range maps to determine areas where wolves and bears have gone locally extinct.

The investigators used statistical analyses to compare cervid density in forests where top predators remain to areas where they have been removed. What they found was strong evidence for “top-down” control of herbivores by predation. The average cervid density was almost six times greater in areas with wolves than areas without them.

Many authors of the reviewed studies also noted impacts of unrestricted cervid populations on the rest of the ecosystem. They observed declines in specific tree and shrub species, as well as changes to invertebrate communities. Other effects included increased stream bank erosion, which has led to changes in stream shape and fish habitat.

The researchers’ analysis also revealed that, although humans hunt deer, humans do not adequately stand in to fill the role of the top predator. The reason may have to do with how humans hunt compared with how large mammalian carnivores hunt. Apparently, the presence of predators does more than just limit a population’s numbers; it also affects how they behave.

When wolves were reintroduced to Yellowstone National Park in the mid-1990s after almost a 100-year absence, elk populations not only shrunk but also changed their behavior. They moved to safer areas, avoiding the riparian ecosystems which have since made a striking comeback. Vegetation came back to the stream banks along with beavers, beaver dams, and the fish and bird habitats they create.

As a strategy for habitat restoration, then, reintroduction programs or protection programs for top predators may yield far-reaching results. But they must be implemented with careful planning. Wolves have not historically been ranchers’ best friends, and their reintroduction in and around Yellowstone has been rife with controversy. One organization, Defenders of Wildlife, has come up with a creative solution to overcome ranchers’ losses due to wolves by paying them for killed livestock.

However, one thing is becoming clear from the mounting data on the complexities of ecosystem regulation: no one can do it quite like the top predators. It’s a job best left to the wolves.

More to Explore

The Reintroduction of Wolves to Yellowstone 
Top-Down Regulation
Defenders of Wildlife

Society and the Environment: Conserving Top Predators

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