The Relative Importance of Predators and People in Structuring and Conserving Ecosystems


Trophic Cascades. Predator, Prey, and the Changing Dynamics of Nature . Terborgh, J., and J. A.Estes , editors. 2010. Island Press, Washington, D.C. 488 pp. $45 (paperback). ISBN 978-1-59726-487-7.
The Wolf's Tooth. Keystone Predators, Trophic Cascades, and Biodiversity. Eisenberg, C. 2010. Island Press, Washington, D.C. 248 pp. $35 (hardcover). ISBN 978-159726-397-9.
Rewilding the World. Dispatches from the Conservation Revolution. Fraser, C. 2009. Metropolitan Books, New York, NY. 416 pp. $28.50 (hardcover). ISBN 978-0-8050-7826-8.

There are few among us who will not feel a rush of awe, wonder, and maybe a touch of fear when glimpsing a large mammalian predator, such as a wolf, leopard, or bear, in the wild. Even a fresh track is enough to make the average mortal's heart beat faster, and I challenge readers not to glance over their shoulders when finding a steaming grizzly scat on a trail. Maybe all animals are created equal, but these large predators are definitely more equal than others—at least when it comes to how we humans feel about them. But as conservation professionals, we are forced to move beyond this emotional response and ask serious questions. What roles do these animals serve in their ecosystems? Are their roles more important than those of other species? And, how do these roles affect the conservation of biological diversity in general? Scientific questions about the role of predators are as old as ecology itself, and debates have raged for years between the champions of top-down and bottom-up effects. How the results of scientific studies can be applied to conservation of top predators is also hotly contested. Some argue that pristine wilderness must be conserved to conserve top predators, whereas others argue that conservation of predators can be achieved via coexistence and the integration of humans and wild animals. These are the issues Trophic Cascades. Predator, Prey, and the Changing Dynamics of Nature; The Wolf's Tooth. Keystone Predators, Trophic Cascades, and Biodiversity; and Rewilding the World. Dispatches from the Conservation Revolution explore.

Trophic Cascades is a serious scientific tome, weighing in at 464 pages. In 21 chapters, a star-studded cast of ecologists attempts to answer the elusive question of the extent to which top-down processes structure natural communities. The book presents a good overview of the state of knowledge on this topic, covering a wide range of species, communities, and ecosystems (terrestrial, freshwater, coastal, and marine) in boreal to tropical latitudes. Through an inspiring array of often ingenious and opportunistic experimental and observational approaches, the researchers describe the complexity and diversity of interactions within biological communities. Their comparative approach emphasizes the many differences in ecological processes in the various ecosystems. The 17 chapters that present case studies and reviews of key aspects of trophic cascades offer convincing arguments that trophic cascades exist and are widespread. One is left with little doubt that top-down processes complement bottom-up processes in structuring communities. Nevertheless, the authors stress there is much subtlety and context dependence in these effects, such that the editors’ conclusion that “trophic cascades are a universal property of ecosystem functioning” may seem a premature generalization. The most important objections to this universality, repeated by multiple authors, are the role of disturbance agents such as fire, weather (drought, wind, floods), and disease and the fact that many ecosystems remain poorly studied. Furthermore there is a disturbing frequency with which the same handful of classic studies (e.g., Yellowstone National Park wolf reintroduction, sea otters and kelp forests, Lago Guri islands, and Baro Colorado Island) are cited.

Surprisingly, there is relatively little focus on population dynamics or predator–prey theory, which is the route through which predation and herbivory influence the abundance of a given prey or forage species and therefore trigger a cascade. Apart from this, the book presents a solid and coherent overview of trophic cascades. Nevertheless, the editors and some authors go beyond exploring the ecology of the phenomenon and attempt to place trophic cascades into the context of conservation, and it is here that the book is at its weakest. Quite correctly, many authors point out how large predators and megaherbivores (which are singled out for particular attention) interact strongly and that most of them have had dramatic reductions in range and abundance. The authors accurately note that most of our planet's ecosystems are no more than depauperate “anthropogenic artifacts.” Their prescription is the restoration of large predators and megaherbivores and the maintenance or restoration of connectivity of their habitats, themes familiar from past publications by the editors and some of the authors. Again, this is a dramatic oversimplification and overgeneralization. Human effects on ecosystems go far beyond the removal of a few large, strongly interactive species. These impacts include pollution (toxins, greenhouse gases, nitrogen), destruction of species’ habitats, harvesting (trapping, hunting, fishing, grazing of livestock, cutting of timber) of all trophic levels, introduction of diseases and non-native species, and behavioral disturbance. Reversing the effects of human activity will require far more than the restoration of a few charismatic and strongly interactive species.

The Wolf's Tooth is a well-written popular-science book that covers much of the same ground as Trophic Cascades. Cristina Eisenberg uses personal anecdotes to illustrate the importance of predators in structuring ecosystems. She takes the reader on a fascinating journey across the continental United States and Canada as she conducts her own field work, visits sites of famous studies that have influenced the science of ecology, and attends workshops and meetings with key players in conservation biology. On this journey readers come face to face with both nature in all its beauty and the researchers who are trying to unlock its secrets. She brings one close to the men and women behind the science, helping one to see their personalities. Eisenberg's writing style makes the book highly readable, and without effort one learns a lot about the evidence supporting “trophic cascade science,” gaining insights into how the underlying data were collected, how theories were developed, and what this scientific discipline means for conservation. She shows a good understanding of where science stops and philosophy begins and gives fair treatment to the competing ideologies that guide conservation.

But the best of the batch by far is Caroline Fraser's Rewilding the World. Fraser whisks readers off on a whirlwind tour across the planet's surface, stopping off in numerous locations in the Americas, central and eastern Europe, Africa, Nepal, and Australia. The diversity of locations is designed to reflect the diversity of conservation practices. Like Eisenberg, Fraser explains in simple terms what the science says about the importance of predators and their potential for triggering trophic cascades. Nevertheless, the main thrust of her analysis concerns the practicality of achieving conservation on the ground. She sharply cuts through the rhetoric of some of the larger conservation organizations and is generally skeptical about the actual successes achieved by the geographically extensive and overhyped megaconservation initiatives. Instead she focuses on the people who actually get things done, often at a small scale, one step at a time. Fraser's view of rewilding is pragmatic. This pragmatism allows her to celebrate every conservation success no matter how small. The origin of her pragmatism is to recognize the importance of people. The view of rewilding she advocates is one in which people and their needs are part of the equation. Rewilding is motivated by people, driven by people, and must ultimately help satisfy the needs of people, especially the people who live in and around the areas being rewilded. She summarizes her perspective in a simple sentence—“Rewilding is a science, but it is also a social movement on a grand scale.”

Looking beyond the science in these books and focusing on the application of the science to real-world conservation, very different views emerge. The books by Eisenberg and Terborg and Estes advocate for top predators and wilderness. Their ideas may be relevant for those few truly wild areas that remain, and there are very few among us who would not argue for the preservation of these “crown jewels” of biological diversity as truly intact and fully functioning systems. Nevertheless, these places are the exceptions and are of little relevance for conservation of biological diversity across most of the world with its 7 billion human occupants. These 7 billion people are all looking for a better life and the biological diversity we care so much about will have to be conserved in and around the places where these people live and work and play. In such landscapes it is highly unlikely that the “Yellowstone model” will achieve anything apart from generating conflict. The European ideas of coexistence and compromise are far more likely to facilitate sustainable conservation of biological diversity.

Furthermore, conservation of biological diversity in a human-dominated arena will require a philosophy that places humans within nature and encourages humans to share their living space with wild animals, rather than to dream of some remote Eden, where humans and wild animals are neatly segregated. The motivation for conservation ultimately comes from the human heart, and fostering this motivation requires giving people the chance to connect with nature, every day and in every way, not just when they visit a distant wilderness on a day when their permit lets them in. In other words, we must not forget our own humanity in our zeal to save nonhuman life.