Why top predators matter

by Jeremy Hance


Wolves chasing an elk in Yellowstone National Park. Photo courtesy of the National Park Service.

Three recent studies reveal just how important top predators are to their ecosystems.

Few species have faced such vitriolic hatred from humans as the world’s top predators. Considered by many as pests—often as dangerous—they have been gunned down, poisoned, speared, ‘finned’, and decimated across their habitats. Even where large areas of habitat are protected, the one thing that is often missing are top predators.

However, new research over the past few decades is showing just how vital these predators are to ecosystems. Biologists have long known that predators control populations of prey animals, but new studies show that they may do much more. From controlling smaller predators to protecting river banks from erosion to providing nutrient hotspots, it appears that top predators are indispensable to a working ecosystem.

Not easy being a top predator

Top predators sit at the apex of an ecosystem’s food chain. Wolves in Alaska, tigers in Siberia, lions in Kenya, white sharks in the Pacific are all examples of top predators. Some top predators have been introduced by humans, such as dingos in Australia, while others have taken over after humans have extirpated the ecosystem’s natural top predators, such as coyotes in the US after wolves and mountain lions vanished. Either way, the expanse and population of top predators has changed drastically as humans have taken over the world.

In the continental United States genetic evidence shows that there were once 200,000 wolves when Europeans arrive; today there are less than 5,000. Despite millions of dollars and years of conservation effort wolves are only present in 5 percent of their historic range in the US. Wolverines, though largely a scavenger, are terrific hunters in their own right (they are even known to harass both wolves and mountain lions). But they have it even worse in the US than wolves. While there are only an estimated 500 wolverines in the continental US, the Bush Administration denied them any coverage under the Endangered Species Act stating that wolverines still thrived in Canada, essentially arguing that this predator was unworthy of protection.

The world’s largest cats—tigers—are endangered throughout all of their range. Despite being one of the world’s most recognizable, and beloved, animals, tigers are on the edge of extinction. The species is classified as Endangered by the IUCN Red List, while two of the six surviving subspecies of tiger are considered Critically Endangered. Few animals have received the amount of conservation attention and funds as tigers, yet every year the great cat moves further from a comeback. Recent reports show tiger populations dropping in both India and Russia, both of which were considered the bright spots in tiger conservation.

Even when top predators bring in millions in tourist revenue—such as is the case of lions in Africa—they still face a barrage of trouble. Habitat loss, poisoning, and killing by gun or spear has crippled African lion populations. Recent reports state that they could vanish altogether from some of their best habitat—i.e. Kenya’s grasslands—in twenty years if nothing is done.

To think such species are somehow immune to extinction is erroneous: three tiger subspecies (the Javan, the Bali, and the Caspian), two wolf subspecies (both from Japan), one lion subspecies (the Barbary), and the thylacine—once apex carnivore in Australia—all vanished during the 20th Century. This past decade has seen the loss of the baiji: top predator and river dolphin of China’s Yangtze River.

Other top predators linger on the edge of extinction: the Amur leopard, the Indo-Chinese tiger, the Arabian leopard, the Javan leopard, and the Asiatic cheetah could all vanish during this century. In some parts of the world populations of large mammalian carnivores have dropped a staggering 95-99 percent.

It’s not just on land where top predators are vanishing. In the oceans, many shark populations have been decimated. Overfishing, by-catch, and ‘finning’ (whereby fishermen cut off a shark’s fin and then dump the animal back in the water, where it soon succumbs) are all taking a toll on some shark species. A study in 2006 found that up to 73 million sharks are killed by fining in a single year—all this to keep up orders of the Asian delicacy: shark fin soup. The first global survey of sharks and rays found that nearly one-in-three species are threatened with extinction, higher even than amphibians, which are said to be in the midst of an extinction crisis. Some shark species populations have plummeted by over 90 percent in just a few decades.

At a time when top predators are vanishing worldwide, three recent research papers show a very new side of top predators. Peeling off the dangerous, fierce veneer, these studies show that top predators are actually protectors of many aspects of the ecosystems they inhabit and show just how many detrimental ecological ripples their losses entail.

‘My enemies’ enemy is my friend’

It has long been known that top predators affect and control populations of prey species (such as wolves and elk, lion and zebra, tigers and deer), but recent studies have shown that top predators also affect carnivorous species just one rung beneath them on the food chain, known as mesopredators. Coyotes in North America, hyenas in Africa, ocelots and jaguarundis in South America, and weasels in Europe are all examples of mesopredators. 

A recent paper in Ecology Letters titled ‘Predator interactions, mesopredator release and biodiversity conservation’ reviews 94 top predator-mesopredator related studies, discovering just how much impact top predators can have on those beneath them.

The paper defines mesopredators as often “versatile generalist hunters, with a capacity to reach high population densities and have large impacts on a wide range of prey species.” However, the situation is sometimes complicated. For example, in parts of North America where mountain lion and wolves have vanished, coyotes move up from mesopredator to the apex of the food chain (top predator) though coyotes hardly practice the same hunting habits or possess the same skills as the continent’s bigger hunters.

Euan Ritchie, lead author of the paper, outlined two ways that top predators impact lesser mesopredators: these “can be referred to as fear and loathing,” he told mongabay.com, “first of all, top predators loathe mesopredators (think dogs and cats), perhaps through perceived competition and therefore they often actively seek them out and kill them, thereby reducing the overall abundance of mesopredators.”

According to the paper this ‘loathing’ leads a top predator to kill a smaller one “for food and to eliminate an ecological competitor.” Some top predators will kill a mesopredator and leave the body without eating it.

Secondly there are few things in the world mesopredators fear more than a run-in with a top predator: studies have shown that fear alone can cause great behavioral shifts in mesopredators.

“Fear may cause mesopredators to reduce or change their times of activity and/or habitats they use,” Ritchie explains. “This can lead to the reduced ability of mesopredators to find food, therefore lowering reproduction and survival, and hence can have large impacts on their populations.”

Through reviewing field studies, the paper found that a reduction in top predators allows mesopredators to increase disproportionately, sometimes as much as fourfold. In other words if a wolf population drops by a hundred that may allow, under certain conditions, the coyote population to explode by as much as four hundred. This ecological occurrence, termed ‘mesopredator release’ by scientists, in turn affects many other species.

As Ritchie explains: “When top predators are removed from an environment (e.g. dingoes), mesopredators (e.g. cats or foxes), which tend to be more generalist and opportunistic species with a high reproductive rate relative to larger predators, can quickly increase in abundance and drive prey species to extinction,” adding that, “this is especially true where the prey species themselves have quite low reproductive rates, such as many of Australia’s native mammals.”

For example, a population of rufous hare-wallaby vanished in Australia following the poisoning of local dingoes. Once the dingoes were gone, fox (an alien species in Australia) invaded the area and the rufous hare-wallabies, who had survived side-by-side with the dingo, were quickly hunted out of existence. Rufous hare-wallabies are listed as Vulnerable by the IUCN Red List.

In cases such as this, top predators actually aid the survival of certain prey species. By keeping a constant check on mesopredators, top predators in turn become protectors of prey species, especially smaller prey. It may not be too much of a stretch to label the world’s top predators: ‘guardians of small prey species’.

“In short,” Ritchie says, “my enemies’ enemy is my friend.” He adds that “even if large predators also occasionally eat the same prey species as mesopredators, their impact is lower relative to mesopredators, due to their larger territories and smaller overall population sizes.”

Although there is a general trend of top predators keeping a check on mesopredators—and thereby aiding a number of prey species—studying the relationship between top predators and mesopredators can prove incredibly complex. According to the paper some underlying factors that need to be considered include resource availability, habitat types, and the relationship between various predator groups.

To illustrate this, Ritchie points again to Australia: “A classic example perhaps is the relationship between dingoes, foxes and cats. Dingoes kill foxes and cats. Foxes kill cats too. The problem is that in some circumstances, by killing foxes, dingoes may be indirectly helping cats. However to date, no study has yet been able to resolve the complexity of this relationship. There’s no doubt the same situation could apply to other groups of predators, such as wolves, coyotes and cats/foxes/raccoons/skunks etc. We’re only now beginning to delve into the true complexity of these relationships.”

Despite the complexity, Ritchie and colleagues have found considerable and varied evidence of the role top predators play in regulating the ecological system.

How predators protect plants

Top predators impact prey populations, the mesopredators below them, and—indirectly—the mesopredators’ prey species, but what about plants?

At first glance it may appear ridiculous that a top predator could drastically affect an ecosystem’s plant life. However, a recent study in Biological Conservation of five National Parks in the United States (Olympic, Yosemite, Yellowstone, Zion, and Wind Cave) shows just how much plants, and thereby healthy ecosystems, rely on big predators. Not only could they be called ‘guardians of small prey species’, but in addition ‘guardians of native flora’.

During America’s short history, top predators—wolves and cougars—were largely wiped out from their habitats due to hunting, trapping, poisoning, and even government campaigns established to eradicate these ‘pests’. The study shows that this decline—and in many places complete expiration—of top predators has had drastic impacts on plants.

“The removal of top predators from landscapes allows, via reduced predation and predation risk, unimpeded foraging by large herbivores such as elk and deer,” explained Dr. Robert Beschta, lead author of the paper, to mongabay.com. “Heavy utilization of plants by these animals, over time, can greatly alter the composition of plant communities and thus impact other animals that are dependent upon these plants as part of their life cycles.”

As an example he says that “in areas where wolves have been extirpated, greatly increased foraging pressure by elk on aspen, cottonwood, and willows can occur. If high levels of foraging continue year-after-year, this can eventually lead to the local extinction of these plants and others.”

Scientists call this process a ‘trophic cascade’, which Beschta says “is used to denote effects of predators upon their prey and, in turn, upon plants.”

Beschta and co-author William J. Ripple found that in the five parks, twenty years after top predators were displaced, tree recruitment (i.e. the number of trees surviving to designated height) declined to 10 percent of the number required to maintain historical tree communities. Within fifty years, the affect was even more acute: recruitment levels dropped to 1 percent. Eventually, the authors write, this trend would lead to many native trees’ local extinction.

The study concludes that these changes in tree survival were due to top predator loss, after carefully eliminating other possible impacts, such as climate, fires, decline in impact by Native Americans, and land use.

“None of the alternative factors explained the observed long-term declines in tree recruitment,” write the researchers.

The decline in surviving trees and the loss of particular species of plants due to predator loss can have varied impacts on the ecosystem, affecting everything from erosion to fire.

“Accelerated erosion of hill-slope soils or of stream banks can occur as the diversity and biomass of plant communities is increasingly affected,” says Beschta. In addition, “fire is an important mechanism for rejuvenating aspen stands but, in the presence of high levels of herbivory, fire accelerates the removal of large trees while sprouts and seedlings are unable to grow above the browse level of elk or deer.”

The loss of top predators—and the uptick of herbivores foraging—can also have massive impacts on aquatic environments, including degrading plant communities to a point where they “may be no longer capable of maintaining stable stream banks during periods of high flow,” says Beschta, “once riparian plant communities are degraded, increased channel widening or channel down-cutting can occur.”

Such impacts can raise summer water temperatures due to shallower streams, increase sediment runoff, and destroy important fish-rearing habitat.

A previous study in Zion National Park shows just far the loss of top predators ripples outward: the study found that abundance measurements for a number of species—including water plants, wildflowers, amphibians, lizards, and butterflies—were lower in areas where mountain lions were scarce and more abundant in areas where mountain lions still roamed frequently.

In the end, the loss of top predators can actually be linked to an overall decrease in ecosystem services, since “a diversity of native plant species, as well as the composition and structure of plant communities, are necessary to provide food-web support, maintain habitat, contribute to soil development, and a variety of other ecosystem services. The key to maintaining ‘ecosystem services’ is a healthy and vibrant plant community,” says Beschta.

But without top predators excessive grazing by big herbivores “can fundamentally alter the capability of native plant communities to function in a normal manner,” say Beschta, adding that, “unimpeded herbivory is a powerful ecological ‘force’ that can have profound consequences to terrestrial and aquatic ecosystems.”

Predators enrich the ecosystem

One of the most surprising recent studies on predators shows that not only do they affect plant species, but through hunting they actually create nutrient hotspots that keep ecosystems rich and varied.

Researchers from Michigan Technological University used a 50-year-record of moose prey kills by wolves on Isle Royale National Park, an island in Lake Superior, to find that moose corpses create hotspots of forest fertility by enriching the soil with biochemicals.

Measuring these chemicals in the soils of kill-sites and control sites, the scientists found that the soils of kill-sites were 100 to 600 percent richer in inorganic nitrogen, phosphorous, and potassium than control sites. In addition, the wolf kill-sites show an average of 38 percent more bacterial and fungal fatty acids; while nitrogen levels in foliage at kill-sites were 25 to 47 percent higher than control sites.

“This study demonstrates an unforeseen link between the hunting behavior of a top predator—the wolf—and biochemical hot spots on the landscape,” said Joseph Bump, an assistant professor in Michigan Tech’s School of Forest Resources and Environmental Science and first author of the research paper. “It’s important because it illuminates another contribution large predators make to the ecosystem they live in and illustrates what can be protected or lost when predators are preserved or exterminated.”

Bump says that he and his colleagues were shocked just how clear the biochemistry of the kill was, especially considering wolves—with the help of scavengers—pick a corpse clean.

“The fact that we observed strong effects even when carcasses are so well utilized was surprising. We suspect that the stomach contents are important in create the fertilization effects because wolves and scavengers do not eat the decomposing plant material and microbial soup in the stomachs of moose,” Bump told mongabay.com.

If it is in fact the stomach contents that serve as the primary source of the rush of nutrients added to the ecosystem, Bump says that human hunters likely provide a similar uptick in nutrients. However, Bump adds that there is a major caveat to this.

“[Hunter-left] guts piles occur in different places and at different times of the year than wolf-killed prey,” Bump explains. “Hunter left gut piles are highly concentrated temporally during the hunting season, and are generally much closer to roads.”  In other words, wolves play an important role in the distribution of nutrient hotspots. According to the paper: “in contrast [to human hunters], wild predators hunt continuously and across a broader range.”

“Wolf-killed moose were found in some areas of the study landscape at 12 times the rate of occurrence for moose that died from other causes,” Bump says. “This means that wolves, in part, are shaping where a moose hits the ground. In some areas in which wolves apparently have greater kill-success more moose carcasses are deposited and the soil changes we observed are highly clustered.”

By clustering their kills, wolves create areas of greater soil fertility, a clustering that isn’t reproduced by human hunting, car collisions, starvation, or other means of moose mortality.

According to the paper it is unlikely that these results are unique to only wolves and moose: “The results we observed in a forest ecosystem are likely to occur elsewhere where large carnivore-ungulate relationships are intact. For example, we have observed similar above- and belowground biogeochemical effects at elk carcass sites in Yellowstone National Park, USA […] In the low resource environment of the Arctic tundra, the impact of a muskox (Ovibos moschatus) carcass on surrounding vegetation was still dramatic after 10 years, which emphasizes that carcass effects may last longer in some systems. Similar dynamics likely occur in South American, African, and Asian systems with intact large carnivore–ungulate prey relationships.”

The writers say this research is vital because it demonstrates an unknown and unexpected ecosystem service provided by top predators, which in scientific terms is described as “creating ecosystem heterogeneity at multiple scales”.

“What is important,” concludes Bump, “is that wolves are not intuitively connected to dirt and how fertile a spot of dirt may be. Identifying and describing such connections tells a more complete story of what we have when we have healthy moose and wolf populations on the landscape. If ecologists continue to tell such stories then we will understand what is lost or gained with wolf expiration or restoration respectively.”

Where do we go from here?

As researchers discover more ways in which top predators contribute to working environments, the question then becomes where do we go from here?

One relatively recent answer is to reintroduce top predators into habitat where they have been lost. To date top predators have been reintroduced into a few select areas, the most famous example being wolves in North America. But the process of reintroducing such species is new and the researchers are hesitant to recommend it without first knowing the full ecosystem picture and predicting possible effects.

“We need to take a whole of ecosystem view, and not a single-species approach,” says Ritchie, co-author of the paper on top predator impacts on mesopredators. “It is inevitable that whenever we tinker with a natural system, there will be some winners and some losers. So before we go ahead and change things, we need to ask why are we doing this, what do we hope to achieve and what are the likely results going to be? If we can’t answer these questions then we shouldn’t proceed.”

Yellowstone National Park has proven an especially intriguing example of the effects a reintroduced top predators can have on ecosystems, since the wolf, the region’s top predator, was absent for nearly 90 years.

Following the demise of wolves in Yellowstone, Beschta’s study found that Aspen populus declined rapidly due to intensified browsing by group elk herds. During this time, elk culling programs were initiated to control over-browsing in Yellowstone and other parks, but none could replicate the affect of a top predator on the ungulate populations.

Eventually in 1995 and 1996 a cautious reintroduction began in Yellowstone National Park: thirty-one wolves were returned to the wild. Despite being controversial, the measure was quickly a success.

“With reintroduction of wolves into Yellowstone, the large carnivore guild is again complete,” says Beschta. “Within a few years following reintroduction, we began to document a decrease in browsing pressure and an increase in height growth of young willows, aspen, and cottonwood in some areas. This result is extremely exciting as it appears that this is the first time in many decades that such plants have been able to grow above the browse level of elk and produce seed for subsequent generations of plants. Observations by others indicate that beaver counts are increasing and small predators and scavengers may be doing better. In contrast, elk and coyote numbers have been decreasing.”

One example of wolf impact on mesopredators—coyotes—comes from a study that shows Yellowstone has seen a fourfold recovery of juvenile pronghorn antelope.

“Overall, the reintroduction of wolves appears to have initiated a ‘reshuffling’ of Yellowstone’s ecosystem, a reshuffling that is continuing,” adds Beschta. “Over time, we hope Yellowstone will provide an improved understanding of the extent to which top predators such as wolves may have influenced other ecosystems across public lands in the American West.”

The wolves of Yellowstone are a good example of how top predator reintroductions can prove an unreserved ecological success.

Despite some remaining questions, Ritchie sees top predator reintroduction as one means to reestablish healthy, working ecosystems.

“In many situations our environments have been so badly degraded through human impacts, there is often the case to be made that we have nothing to lose and everything to gain from bold experiments,” Ritchie told mongabay.com. “As an example, in Australia the Tasmanian devil (a native predator) is in decline in its native range of Tasmania as a result of devil facial tumor disease. This animal also used to be on the mainland of Australia until quite recently. From ecological theory and anecdotal evidence we know that this species may be able to control foxes and cats, and therefore help some of our other most threatened species. So why not introduce devils back to the mainland? They might be able to reverse some of the damage currently being done by foxes and cats, with the added benefit of establishing an insurance population of the devil on the mainland, free from disease.”

There are of course political pressures on both sides—pro-predator and anti—that complicate the issues. Many people—much like mesopredators—still fear and loathe top predators. One only has to look at the recent debate over allowing wolf hunts in the US to see how emotional the issue can become.

While the reintroduction of wolves in Yellowstone was an ecological success, politically it has proved far less smooth. After years of pressure from anti-wolf groups, this year the Obama Administration allowed Wyoming and Montana to begin hunting wolves again. Well-known Yellowstone packs were quickly devastated. No one knows yet how this latest experiment in human-managed reintroductions will impact the remaining wolves and, in turn, the greater ecosystem. Yet, for his part, Ritchie, suggests that by culling top predators, especially pack-leaders, one may worsen rather than alleviate predator-human problems.

“Many large predators (e.g. wolves) have complex social structures and behaviors, and by killing individuals, especially the older, dominant ones, we can have large impacts on how a group of animals behave,” explains Ritchie. “In the case of dingoes, there is some evidence that by killing dingoes, we are breaking down their social structure […] In some cases where dingoes are being killed, dingoes actually appear to be killing more livestock than when they were left alone. This is probably happening because few old dingoes are left, which in normal circumstances train young dogs how to hunt species such as kangaroos. So in effect what you’re left with is a bunch of rowdy, uninformed teenagers who go for the easiest target, which are often things like calves.”

Currently, Australia is mulling reintroducing dingoes to some areas in order to help over-preying on endangered native mammals. Recent research has also suggested that reintroducing wolves into the Scottish highlands (absent since the mid-1700s) could help native foliage return, which is currently over-browsed by deer. Many political difficulties stand in the way of such reintroduction schemes; in the end it’s not the science, but the politics that dictates where we go from here.

As Beschta says, “the underlying conclusion of our research is that loss of large predators has been incredibly important. Where we go next is up to society based on this ‘new’ information.”

Citations: Euan G. Ritchie and Christopher N. Johnson. Predator interactions, mesopredator release and biodiversity conservation. Ecology Letters. Volume 12, Issue 9.

Beschta, R.L. and W.J. Ripple. Large predators and trophic cascades in terrestrial ecosystems on the western United States. Bological Conservation.

Bump, J.K., Peterson, R.O., & Vucetich, J.A. 2009. Wolves modulate soil nutrient heterogeneity and foliar nitrogen by configuring the distribution of ungulate carcasses. Ecology. Vol 90, Issue 11.

This story was posted on mongabay.com on February 02, 2010, with photos and captions that are not included here. You can post a comments at: 
http://comments.mongabay.com/pages/news.mongabay.com/2010/0202-hance_toppredators.html

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