The Case of the Missing Predator, or Please Pass the Shark Fin Soup
THE PREDATOR/PREY RELATIONSHIP. WHY IS IT SO IMPORTANT?
From time to time in this website, we examine important scientific papers in light of what they show us about subjects of interest to ourselves and our readers.
The following paper (Crooks and Soule, Nature, 1999) has become a classic because it vividly shows how the presence or absence of a predator can have unexpected and important effects on an ecosystem.
(I regret that I cannot reprint the entire paper, and can show you only the abstract. Copywrite policies prevent me from doing so, even if I were to purchase it. This is detrimental to the free flow of scientific information, but I have no control over the situation.)
Mesopredator release and avifaunal extinctions in a fragmented system
Kevin R. Crooks1 & Michael E. Soulé2
- Department of Biology, University of California, Santa Cruz, California 95064, USA
- The Wildlands Project, PO Box 1302 2010, Hotchkiss, Colorado 81419, USA
Correspondence to: Kevin R. Crooks1 Correspondence and requests for materials should be addressed to K.R.C. (e-mail: Email: [email protected].).
Mammalian carnivores are particularly vulnerable to extinction in fragmented landscapes1, and their disappearance may lead to increased numbers of smaller carnivores that are principle predators of birds and other small vertebrates. Such 'mesopredator release'2 has been implicated in the decline and extinction of prey species2, 3, 4, 5, 6. Because experimental manipulation of carnivores is logistically, financially and ethically problematic6,7, however, few studies have evaluated how trophic cascades generated by the decline of dominant predators combine with other fragmentation effects to influence species diversity in terrestrial systems. Although the mesopredator release hypothesis has received only limited critical evaluation8 and remains controversial9, it has become the basis for conservation programmes justifying the protection of carnivores6. Here we describe a study that exploits spatial and temporal variation in the distribution and abundance of an apex predator, the coyote, in a landscape fragmented by development. It appears that the decline and disappearance of the coyote, in conjunction with the effects of habitat fragmentation, affect the distribution and abundance of smaller carnivores and the persistence of their avian prey.
Letters to Nature
Nature 400, 563-566 (5 August 1999) | doi:10.1038/23028; Received 22 February 1999; Accepted 5 July 1999
The Soule paper was a sort of scientific detective story.
Michael Soule, who is considered to be the dean of the field of Conservation Biology, if not its creator, examined a puzzling situation in his home city of San Diego, California. The city, although it is highly urban, is transected by many deep and mostly wild ravines that extend out to the ocean front. The ravines contain animal populations and are basically isolated from each other by houses, streets, etc. One can think of them therefore as islands of life, the denizens of which can enter and leave only with great difficulty.
When Soule surveyed the animal populations in these ravines, he noted a strange situation. In some of the ravines there were dense populations of song birds, whereas in others, there were hardly any at all.
The mystery was clarified when Soule examined the other animal populations in these “urban islands.” He found cats in some ravines, and coyotes in others, but never cats and coyotes together. He also noted that in those ravines where there were coyotes but no cats, there were plentiful populations of song birds, but in others where there were no coyotes, but many cats, song birds were missing.
This is partially a big fish, little fish story, with intriguing consequences and implications. The picture that Soule put together was the following: Coyotes and cats are both predators. We call coyotes mesopredators because they themselves are the prey of others, such as mountain lions and wolves. Coyotes, in turn prey on cats and cats eat birds as well as their eggs and young. Did you ever think of your kitty as a predator when it brought a dead creature home? Well, domestic as well as feral cats are responsible for the deaths of millions of song birds every year!
In those ravines where the only predators were cats, the cats decimated the bird populations. However in those ravines where there were urban coyotes, they preyed on the cats, keeping their numbers way down, thus allowing the bird populations to flourish. Mystery solved.
So, what has this to do with our interests? Just this. Wolves are keystone predators, meaning that they sit on top of the food chain (if humans are ignored), and due to this situation, their behavior and eating habits result in a cascade of effects to the rest of the ecosystem.
Yellowstone National Park, like those San Diego canyons, is a sort of island of wildlife, surrounded by ranches, highways, houses and other accoutrements of human habitation. Animals do not easily get in or out. When a few wolves were reintroduced there in 1995, some of the effects were anticipated, but others were not.
The effects of these predators have been widespread and profound. First, they reduced the number of coyotes in the park considerably as the wolves took their place as the top predators in Yellowstone. Then they reduced the elk population from roughly 15-17 thousand to around 4,500 – 5,000 at present. Lest you think that this drop in elk population is the nightmare hunters fear it is in other locales, it all depends on whose ox is gored.
In fact, the elks, themselves an introduced species in the 1930s, had increased greatly in numbers, due mainly to the lack of predators. They overran and overgrazed the park, considerably changing the ecosystem. How profound these effects were became apparent when the wolves reduced elk numbers and changed their behavior, by chasing them from the river bottoms into the hills.
Aspens, willows and other water–loving plants began to grow in riperian areas, resulting in the stabilization of crumbling creek banks and increasing the amount of shade. Cold water fish, like trout, returned to shaded, cooler waters. Other animal species also benefitted. Animals that depended on the availability of carrion left by the wolves, such as vultures, crows, and foxes made a strong comeback.
These changes have been examined in many studies of the Yellowstone ecosystem by Ripple as well as Creel, W.D. Smith, and Hebblewhite. The documentary film, Lords of Nature, dramatically shows the effects of these changes, both in Yellowstone and in Zion National Park, where extirpation of mountain lions in the main valley has resulted in its being highly degraded by the ever-increasing deer population. In side canyons, where mountain lions still live, the riperian plant and animal life remains lush.
In Yellowstone, the elk and wolves have alternated in population bursts. When wolf numbers increased, the elk numbers diminished. Then the wolf population, stressed by lack of prey, dropped in turn, enabling the elk to make a comeback. These cycles have been observed a number of times during the wolve's 16 year sojurne in Yellowstone, as the wolves and elk repeated their predator/prey dance.
In recent years, the roles of predators in keeping an equilibrium in other species have been intensively studied in many ecosystems. Their value more appreciated by biologists than by the general public. Who knew for example what profound effects the loss of sharks, who are top marine predators, would have on the world’s fisheries. Shark fin soup anybody?