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The Nature and Importance of Trophic Cascades in Global Ecosystems

scenePI: James Estes, University of California, Santa Cruz; John Terborgh, Duke University*

*External grantees

There is growing evidence that top ocean predators like tuna, sharks and marine mammals exert a strong influence on the populations of their prey, and that ecosystems can become seriously imbalanced when these predators are removed in excessive quantities. One possible effect of top predator removal -- called the “trophic cascade” -- is characterized by explosive growth of the top predator’s prey, followed by a reduction in their prey and so on down the food web. These cascading effects have the potential to cause havoc to fisheries and to change the very structure of marine ecosystems and habitats. To date there has never been a comprehensive synthesis of the evidence for trophic cascades in marine ecosystems, and it remains unknown how common and widespread they are likely to be. As a result, avoiding trophic cascades has not generally been a central objective of ocean management policy.

In response to this deficiency, the Institute for Ocean Conservation Science has provided support for leading scientists to assemble a comprehensive synthesis of trophic cascades in marine ecosystems that will be convincing to the scientific community and compelling to policy-makers and the public. Dr. James Estes, Research Scientist with the U.S. Geological Survey and Adjunct Professor at the University of California, Santa Cruz, and Dr. John Terborgh, Director of the Duke University Center for Tropical Conservation, convened a group of 21 world-class scientists who have studied the ecological roles of large predators. At a workshop at the White Oak Plantation near Jacksonville, Florida, held in February 2008, these experts vigorously discussed the evidence for trophic cascades across major marine ecosystem-types around the world, and explored how to apply knowledge of trophic cascades to biodiversity conservation. The group has now produced an authoritative volume on the subject entitled “Trophic Cascades,” to be published in 2010 by Island Press and co-edited by Drs. Estes and Terborgh. This book is the first comprehensive compendium on trophic cascades and how they operate in the world’s major ecosystems. It includes a chapter from each workshop participant and a foreword by Institute for Ocean Conservation Science Executive Director Dr. Ellen K. Pikitch.

book coverWhile the ecological foundation for trophic cascades appeared in the scientific literature more than half a century ago and has been steadily built upon since then, the tremendous scientific progress made in this field had largely remained scattered, and the extent and importance of trophic cascades in nature has been hotly debated. Dr. Pikitch writes: “This book puts in one place the accumulated wisdom of an impressive array of ecological detectives who through long-term careful investigation have uncovered the complex dynamics of a wide range of ecotypes and regions.”

The resounding conclusion of this volume is that the loss of a predator has indirect yet severe consequences for marine ecosystems. The case of the sea otter is a classic example. As predators, sea otters eat sea urchins, keeping urchin populations in check. When otter populations plummet, urchins proliferate, and they devour their favorite food – kelp. Urchins essentially “mow” the kelp forests down, and leave an undersea desert in their wake. The myriad of species that rely on kelp for habitat or food are then lost from the area. Evidence accumulated in this volume indicates that trophic cascades operate in nearly all ecosystems around the world, both at sea and on land.

The book provides the evidence for what is an emerging paradigm shift in ecology and conservation biology: that ecosystem-wide impacts related to predation and trophic cascades are strong influences on biodiversity, and that apex (top-level) predators are essential in maintaining the structure, function, and biodiversity of most natural ecosystems. Recognition that large marine vertebrates are important predators might change the standards of fisheries management from “maximum sustainable yield,” to the maintenance of functionally intact ecosystems. This perspective would also impact the conservation and management of large predatory fishes and marine mammals, and could dramatically change the spatial strategies and species priorities for effective conservation.

More information

James Estes, PhD
Dr. James Estes is an international expert on sea otters and a specialist in the critical role of apex (top level) predators in the marine environment. He has been a research biologist at the Western Ecological Research Center of the U.S. Geological Survey for more than 20 years. Estes also holds academic posts as research associate and adjunct professor with the Center for Marine Studies at the University of California, Santa Cruz.

His interest in predation as an ecosystem-level process began in the early 1970s, after he began working with sea otters. Using the otters' fragmented distribution across the Aleutian archipelago (which resulted from a history of near-extinction and recovery), he and a colleague discovered the species' keystone role in kelp forests by comparing islands where it was abundant or rare. This work provided a spectacular example of how apex predators influence ecosystem functions. Estes continued to explore the dimensions of sea otter-kelp forest interactions over the next 30 years, including the unanticipated collapse of sea otters and kelp forests in western Alaska.

He has now published nearly 70 scientific articles and reports on wildlife ecology, predation and conservation, and was lead editor of the 2007 book, "Whales, Whaling, and Ocean Ecosystems." Estes is a recipient of the prestigious Pew Fellowship in Marine Conservation (1999).

Dr. Estes' Pew Fellows in Marine Conservation webpage
More information about Whales, Whaling, and Ocean Ecosystems

John Terborgh, PhD
John W. Terborgh is a James B. Duke Professor of Environmental Science and is Co-Director of the Center for Tropical Conservation at Duke University. He is a member of the National Academy of Science, and for the past 35 years, has been actively involved in tropical ecology and conservation issues. An authority on avian and mammalian ecology in neotropical forests, Dr. Terborgh has published numerous articles and books on conservation themes. Since 1973 he has operated a field station in Peru's Manu National Park where he has overseen the research of more than 100 investigators. Dr. Terborgh earlier served on the faculties of the University of Maryland and Princeton University. In June 1992 he was awarded a John D. and Catherine T. MacArthur Fellowship in recognition of his distinguished work in tropical ecology, and in April 1996 he was awarded the National Academy of Science Daniel Giraud Elliot medal for his research, and for his book, "Diversity and the Tropical Rainforest." He has served on several boards and advisory committees related to conservation, including the Wildlands Project, Cultural Survival, The Nature Conservancy, The World Wildlife Fund and both the Primate and Ecology Specialist Groups of the International Union for the Conservation of Nature.
Dr. Terborgh's Faculty Page at Duke University

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