Predators and their prey – why we need them both

by Joe Scott, international conservation director

Originally published in the spring/summer 2011 edition of Conservation Northwest Quarterly

Like CSI detectives investigating a crime scene, lynx and hare researchers in north central Washington recently responded to a “mortality” signal from a snowshoe hare that they had fitted with a satellite tracking collar to monitor hare movements. When they arrived at the scene the biologists were able to reconstruct the events around the hare’s demise.

Predators & prey: Why don’t predators eat all their prey?

A great horned owl had killed the hare, but predator became prey as a lynx killed the owl and pirated the hare for itself.

Everything eats snowshoe hares. In boreal forests, hares are the cheeseburgers for the fries, the fish for the chips, the meatballs for the spaghetti, and the corned beef for the cabbage.

Lynx are the most famous hare junkies, but the fleet-footed rabbits are also favored by wolves, coyotes, foxes, martens, eagles, goshawks, owls, and other raptors. In the ultimate insult, even red and ground squirrels eat them. People eat them.

Speed, stealth, aerial ambush and traps are all used on hares. Cute has no currency in the wilds, except as lunch. Scott Fisher, biologist with the Washington Department of Natural Resources, describes it this way: “When you’re a hare, everybody else on the block is a bully.”

But despite being every animal’s comfort food, snowshoe hares not only persist but do so in often ridiculous numbers.  How does an animal in such demand avoid being eaten out of existence?

It’s tempting to think that hares’ prodigious breeding ability is the evolutionary response to hyper predation.

But we have to dig a little deeper. All animals will have as many babies as they can successfully rear, whether their eggs are small and many or large and few, because that’s the best way of ensuring your genes survive in competition with those of your neighbors. And whether a species has many, many small eggs like a salmon, or a couple large eggs like a grizzly bear, it’s all about getting more of your genes out there, which is in turn rooted in the concept of ecological niches.

Predators & prey: What is a niche? Jobs in the woods


Paul Colinvaux describes a species’ niche as its place in the “grand scheme of things,” its “profession,” that is, “everything it does to get its food and raise its babies.” A plant or animal’s habitat is its address.

Spaces in each ecological niche, like welding jobs in a shipyard, are limited. Consequently the number of species that can fill a particular niche is more or less set by limitations on habitat imposed by climate, food, den sites, cover, etc—all the things that a species needs to survive and reproduce.

The niches, or jobs, of each species are crafted over millennia by natural selection. In other words, as Colinvaux concludes, “the common stay common and the rare stay rare,” unless something drastic happens to change the environment, like, for instance, clear cutting an old-growth forest or deregulating the financial industry. In each case you have a proliferation of weedy species, a reduction in diversity, and fewer real jobs.

Snowshoe hares have evolved to exploit a niche that has few competitors, since the boreal forests home to hares and lynx are very tough neighborhoods, especially in winter. In other words there are lots of hare “jobs” out there as long as the boreal remains the boreal and something doesn’t happen to radically alter it—like climate change.

So hares have lots of babies, very often, to supply the demand for hare jobs, not because so many animals like to eat them. High hare predation rates are a consequence of hare fecundity not a cause. Hare deaths are the grim cost of a reproductive strategy that floods the market with baby rabbits. And, lots of hares provide hare eating niches, or jobs, for many different predators.

So ultimately the numbers of any wildlife species are not determined by breeding strategies. They are set by opportunities for a particular plant or animal to live according to its needs. The number of welding jobs in the shipyard, not graduates of welding schools, determines the jobs for welders.

Predators & prey: Natural selection: Nature’s golden rule

Cooperation and conflict drive plant and animal adaptation. Species and their habitats thrive as interactive, dynamic systems that are constantly reshaping each other.

Natural selection is the ultimate arbiter—the universal law by which Mother Nature governs the biosphere. Quite simply, organisms are driven to survive, so prey animals respond over time with physical and behavioral adaptations to all the natural forces and conditions that conspire to kill them. Predators respond in kind with adaptations that allow them to exploit particular prey species. Otherwise neither would survive.

Predators have helped make snowshoe hares, well, snowshoe hares. Natural selection has equipped them with outsized hind legs and feet to run at lightening speed over snow. Their huge ears magnify the slightest sounds. Most ingeniously from a genetic standpoint, hares have developed a natural camouflage and change color with the seasons—white for winter, brown for summer. With so many things trying to eat them, they need lots of defensive weapons.

The hares that live longest and have the most babies pass more of those successful genes to the next generation, refining the traits over time, like camouflaged fur, that allow hares to escape lynx and owls long enough to reproduce, thus ensuring the survival of the species and its competitive “fitness” to adapt to dynamic landscapes and dozens of hungry predators.

Lynx are the ultimate hare specialists and as such their fortunes rise and fall with those of their big-footed prey. And they’ve kept pace in the evolutionary race for survival: their huge furry feet and long hind legs allow them to run and cut on top of snow like an NFL running back on juice. Many animals hunt hares but none with the efficiency of lynx. Yet they still can’t kill all the hares.

Predators & prey: Wild fluctuations

bighorn-does-david-moskowitz.jpgIn the north of their range, hare numbers have historically “crashed” spectacularly before rising again dramatically. In Alaska and western Canada, hare numbers rise and crash in roughly 10-year cycles, sometimes going from 10,000 to fewer than three animals per square mile in a single year. Female hares can produce two to five litters per year with three to four young per litter.

The phenomenon has been a source of scientific scrutiny for decades and was first described by fur trappers in the mid-nineteenth century. On large scales, scientists think that cyclical sun spot activity affects weather patterns and fire frequency in boreal forests, which in turn affect hare survival and food availability.

On smaller scales, over-browsing by the hyper-reproductive hares at their population zenith leads to food shortages that cause starvation and reduced reproduction, which in turn start the population declines. Just as the health of individual hares diminishes and they become more vulnerable to disease, predation from a larger number of hungry hare eaters kicks in and bingo, hares are scarcer than hens’ teeth—for a little while.

In essence, such wild fluctuations “reset” complex multiple predator prey systems until the next crash, rather like the way our economic and regulatory systems work—or not. Since they’re so tightly linked to snowshoe hares for food, lynx populations in the north rise and fall with them.

Northern lemmings undergo similar population booms and busts. Snowy owls are so tuned in to lemmings that they actually lay fewer eggs when lemming populations are down.

But such boom and bust predator prey economies are the exceptions, not the norms.

Predators & prey: From Arctic to Africa

Bees swarm, birds flock, fish school, and ungulates such as elk form herds because they’re less likely to become a predator’s next lunch special if they do so. Why some predators form groups, on the other hand, like lions in prides or wolves in packs, could have less to do with food sharing and more with defense of territories and rearing of young.

Research on the most well-known predator/prey dance partners on earth has shown that Africa’s Serengeti lions and wildebeest coexist in relative stability without the dramatic fluctuations in numbers that typify some arctic predators and prey like lynx and hares and lemmings and snowy owls.

A research team lead by John Fryxell of University of Guelph in Canada and Craig Packer from the University of Minnesota wanted to know why. Their modeling of four decades of data showed that wildebeest drastically reduced lion predation when they kept to groups and large herds.

Interestingly, the greater the tendency to form groups, the higher the stability of numbers of both species over time. According to Fryxell and colleagues: “When both the lions and wildebeest formed groups, predation was reduced even more. Compared with no-group ecosystems (all animals strewn across the Serengeti), grouping caused a 90-percent reduction in kill rates for lions.”

The complex social “cliques” seemed to work as ecosystem stabilizers, not dissimilar to human communities, with “both lion and wildebeest populations remaining relatively level over time.”

Social cliques among wild animals in the Serengeti are actually the glue that holds the ecosystem together and keeps population numbers stable. Wildebeest thrive in great numbers alongside zebra, Thompson’s gazelles, and several other ungulate species, all prey for lions, leopards, hyenas, crocodiles, hunting dogs, and cheetahs.

Predators & prey: Wolves and white-tails

There are 3,000 wolves in Minnesota. They eat on average about 50,000 of the estimated 450,000 white-tailed deer a year. This represents about 11.5 % of the deer population, with minimal supplements of snowshoe hares, beavers, and moose.

In a recent comprehensive 15-year study of white-tailed deer and wolves, the Minnesota Department of Natural Resources (MDNR) monitored the movements, survival, and mortality causes of 450 radio-collared does in four study areas. Simultaneously, department biologists monitored 55 radio-collared wolves from eight packs whose territories overlapped the deer study areas.

The research showed that doe mortality from wolves ranged from 4% to 22% per year but most typically was between 5% to 10% per year with the highest rate observed in the very severe winter of 1995-96.

Despite the fact that deer outnumber wolves in Minnesota by 150 to 1, wolves are not particularly effective hunters of white-tails.

According to the MDNR, “Wolves end up surviving primarily on the most vulnerable individuals in the deer population, such as very young, old, sick, injured, or nutritionally compromised deer, because those are the ones they can catch. The result being, that under certain conditions…many of the deer that wolves kill likely would have died from other causes, such as starvation or disease.”

Biologists refer to such predation impacts as “compensatory” as opposed to the “additive” effects of human hunters, who kill most prey in the prime of their reproductive lives.

Predators & prey: Where have all the mule deer gone?

doe-mule-deer-looking-out-david-moskowitz.jpgResearchers from Washington State University wanted to understand the reasons for long-term mule deer declines in the intermountain West. Hunters had long been blaming cougars. They were right…sort of. Cougars do kill mule deer. So do wolves, coyotes, bobcats, black bear, and grizzly bears.

But as with all natural systems, nothing’s that simple.

It turns out that the open, mixed forest habitat preferred by mule deer has been so dramatically altered in the West through irrigated agriculture that it’s provided wonderful white-tailed deer habitat. White-tails, historically rare in Washington, now outnumber mule deer in eastern Washington.

And as white-tailed deer numbers grow, mule deer decline. It appears as though landscape level habitat changes have created the white-tailed equivalent of tenements for cockroaches. It also appears that cougars have responded in kind.

But while there may be a slight uptick in cougar numbers as a result of increased ungulate numbers, cougar numbers have not exploded as some people seem to think.

“It’s particularly striking how little difference there is in resident cougar densities across cougar range in western North America,” says Gary Koehler, carnivore biologist, Washington Department of Fish and Wildlife. According to Dr. Koehler and his colleagues, “North American cougars exist in densities of about 1 to 2 adult animals per 100 sq. km everywhere they live—almost without fail. Female cougars are limited by prey availability, but males are limited by the availability of females in their territories, which they defend vigorously.”

However, the WSU researchers have found that cougar predation is having a greater impact on mule deer than on white-tails and occurs in the summer when white-tails move into higher elevation mule deer habitat. Mule deer are the “secondary” prey, but as they’re already in decline, predation is having a greater effect on them.

A similar dynamic has happened with mountain caribou in British Columbia’s inland rainforest. As the caribou’s historically extensive old-growth forest habitat has been increasingly fragmented, it’s opened more niches for deer, elk, and moose. Cougars and wolves follow and opportunistically prey on caribou which cannot withstand the “new normal.” For centuries the mountain caribou old forest and high elevation niche was at the heart of their predator avoidance strategy. Predators simply weren’t able to get to them enough to make a difference in caribou numbers.

Like steelworker jobs in Pittsburgh, jobs for mountain caribou have diminished. Now the wolves are literally at the door and it’s forced some tough choices for managers and conservationists alike until the habitat and historic prey species numbers are restored.

The Mule Deer Working Group of the Western Association of Fish and Wildlife Agencies has been studying mule deer dynamics, particularly mortalities and predation.

According to their findings, many factors confound the question about mule deer declines. Most deer mortality occurs in young animals soon after birth or in winter of their first year. Some biologists believe that the question of whether mortality is compensatory or additive is density dependent—it has to do with how many of the mule deer jobs are filled—also referred to as “carrying capacity” or the ability of the habitat to support the herd.

Carrying capacity can be measured by the overall condition of the animals and their range. When the herd numbers are consistent with what the habitat can support—at carrying capacity—deaths that happen are compensatory. They will occur one way or another naturally keeping animals at levels where the land to support them. As numbers of deer fall below range capacity, additional deer deaths become additive—and unsustainable, contributing to herd declines.

Climatic conditions such as long-term drought or severe winters can reduce the quality of the range and thus the overall physical condition of mule deer making them more vulnerable to predation. Significant habitat changes that result in different movement patterns could make deer even more susceptible to predation.

According to the Working Group, “…most of the environments where mule deer exist today have been altered by fire suppression, development, habitat fragmentation” etc. In these habitats (most of the West), biologists believe predation does not cause declines in deer populations. The effect predators have on prey populations in these environments is more complex and related to how humans affect predators, prey and habitat, and the types and densities of predators that exist.

“In years when mule deer populations are lean, some predators such as mountain lions and wolves may consume several wildlife species including elk and small mammals, causing the predators to maintain artificially high numbers. While this has the potential to slow the growth of mule deer populations, scientific studies show that reducing predators does not increase the number of fawns that survive to adulthood. And it’s the number of fawns that survive to adulthood that determines the growth rate of a mule deer population.”

Predators & prey: Why big fierce animals are rare

lynx-hunting-photo-c-patrick-reeves.jpgEverybody knows that, in nature, small things are common and large things are rare. To understand why, we need to dust off our high school physics textbooks and reacquaint ourselves with the second Law of Thermodynamics, which dictates that the harvesting of solar energy cannot be 100% efficient. This is the real reason that big fierce animals are rare. And rarity is one reason that predators can’t eat all their prey or compromise their numbers to the point that those prey animals are themselves threatened as species.

Ecosystems have structure, like the rows of stone in a pyramid. This structure is organized into what ecologists call “trophic levels,” which are quite simply the different plant and animal communities that inhabit a given area. In a typical simple system there are three trophic levels: plant communities, herbivores, and carnivores. Plants form the large pyramid base, herbivores in the middle level, carnivores at the small, pointy top.

Plants are less than 10% efficient converting light energy to produce plant tissue. Ninety percent is lost as heat to the atmosphere. In the transfer of energy to the second trophic level, the herbivores follows suit, so essentially energy is degraded by 90% at each level from plants through herbivores to carnivores. Of the 1000 calories of solar energy captured by a plant, 100 calories are available to a deer, and 1 calorie is available to a wolf, to grow, reproduce, and have enough strength and energy to hunt again. For this simple reason alone, predators generally can never number more than 10% of their prey.

The upshot is that predators have to work really hard to make a living. It’s definitely blue collar: complete with long hours, physical exertion, shorter life span, high risk of injury or death, and being frequently ostracized by neighbors. For example, wolves are considered efficient hunters for only about two years of their lives and rarely live beyond seven years in the wild.

Predators are limited by available calories, particularly in winter, territorial behavior, the rigors and risks of hunting, rapid decline in their athletic abilities. It’s no life of Reilly. The Second Law of Thermodynamics and natural selection have seen to it.

Predators & prey: Balance in all things

The bottom line is that ecosystems are complex. And, like it or not, predators are a necessary and beneficial part of natural systems. If we remove them from the picture, there are consequences.

Predators provide ecological stability by regulating the impacts of grazing and browsing animals, thus ensuring the overall productivity of the habitat. They cull weak, sick, and old prey, thus ensuring the maximum fitness of elk, deer, antelope, and hares. They foster biological diversity by “enforcing” ecological boundaries or preventing what ecologists refer to as “competitive exclusion,” the tendency of one prey animal to outcompete another. So-called “apex predators,” the wolves, lions, and tigers are the Godfathers, as they also control the numbers of “meso predators,” the coyotes, raccoons, possums, foxes—even domestic cats—which when left unchecked can do enormous damage to birds and native rodents.



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