Conservation Northwest

Predators and prey "What's up with predators?" continued from page 7 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." Why big fierce animals are rare Great gray owl—alert predators. © Paul Bannick 8 Spring/Summer 2011 Everybody 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 ThermodySpruce grouse in Okanogan County— namics and natualert prey. © Paul Bannick ral selection have seen to it. 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. In the next issue of the newsletter we will explore the critical roles that predators play in maintaining the ecological health of our planet. www.conservationnw.org

• Cover