Trophic Cascades

Washington | Covelo | London

Trophic Cascades:Predators, Prey, and the

Changing Dynamics of Nature

Edited by

John Terborgh and James A. Estes

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Library of Congress Cataloging-in-Publication Data

Trophic cascades: predators, prey, and the changing dynamics of nature / edited by JohnTerborgh andJames A. Estes.

p. cm.Includes bibliographical references and index.ISBN-13: 978-1-59726-486-0 (cloth : alk. paper)ISBN-10: 1-59726-486-5 (cloth : alk. paper)ISBN-13: 978-1-59726-487-7 (pbk. : alk. paper)ISBN-10: 1-59726-487-3 (pbk. : alk. paper)1. Predation (Biology) 2. Predatory animals—Ecology. 3. Keystone species. I.Terborgh, John, 1936–II. Estes, J.A. (James A.), 1945–QL758.T76 2010577′.16—dc22

2009042702

Printed using Bembo

Text design by KarenWenkTypesetting by KarenWenkText font: Bembo

Printed on recycled, acid-free paper

Manufactured in the United States of America10 9 8 7 6 5 4 3 2 1

Contents

vii

Foreword / ixEllen K. Pikitch

Preface / xiiiJohnTerborgh and James A. Estes

Chapter 1. Trophic Cascades:WhatThey Are,HowTheyWork, andWhyThey Matter / 1JohnTerborgh, Robert D.Holt, and James A. Estes

PART I AQUATIC ECOSYSTEMS / 19

Chapter 2. Food Chain Dynamics andTrophic Cascades in Intertidal Habitats / 21RobertT. Paine

Chapter 3. Some Effects of Apex Predators in Higher-Latitude Coastal Oceans / 37James A. Estes, Charles H. Peterson, and Robert S. Steneck

Chapter 4. Trophic Cascades in Lakes: Lessons and Prospects / 55Stephen R.Carpenter, Jonathan J. Cole, James F. Kitchell, and Michael L. Pace

Chapter 5. Prey Release,Trophic Cascades, and Phase Shifts inTropicalNearshore Ecosystems / 71Stuart A. Sandin, Sheila M.Walsh, and Jeremy B.C. Jackson

Chapter 6. Trophic Cascades in Open Ocean Ecosystems / 91Tim Essington

PART II TERRESTRIAL ECOSYSTEMS / 107

Chapter 7. The Role of Herbivores inTerrestrialTrophic Cascades / 109Robert J.Marquis

Chapter 8. Propagation ofTrophic Cascades via Multiple Pathways inTropical Forests / 125JohnTerborgh and Kenneth Feeley

viii Contents

Chapter 9. Large Predators, Deer, andTrophic Cascades in Boreal andTemperate Ecosystems / 141William J. Ripple,Thomas P. Rooney, and Robert L. Beschta

Chapter 10. Islands asTests of the GreenWorld Hypothesis / 163Lauri Oksanen,Tarja Oksanen, Jonas Dahlgren, Peter Hambäck, Per Ekerholm,Åsa Lindgren,and Johan Olofsson

Chapter 11. Trophic Cascades on Islands / 179ThomasW. Schoener and David A. Spiller

Chapter 12. Trophic Cascades,Aboveground–Belowground Linkages, andEcosystem Functioning / 203David A.Wardle

PART III PREDATION AND ECOSYSTEM PROCESSES / 219

Chapter 13. Ecological and Conservation Implications ofMesopredator Release / 221Justin S. Brashares, Laura R. Prugh,Chantal J. Stoner, and ClintonW.Epps

Chapter 14. Fear-Mediated FoodWebs / 241Joel Berger

Chapter 15. Trophic Cascades in African Savanna: Serengeti as a Case Study / 255A.R. E. Sinclair, Kristine Metzger, Justin S. Brashares,Ally Nkwabi,Gregor Sharam,and John M. Fryxell

Chapter 16. Consumer Control by Megafauna and Fire / 275William Bond

Chapter 17. Alternative States in Ecosystems / 287Marten Scheffer

PART IV SYNTHESIS / 299

Chapter 18. Theoretical Perspectives onTrophic Cascades: CurrentTrends andFuture Directions / 301Robert D.Holt, Ricardo M.Holdo, and F. J. Frank vanVeen

Chapter 19. ComparingTrophic Cascades across Ecosystems / 319Jonathan B. Shurin, RussellW.Markel, and Blake Matthews

Chapter 20. Conservation Relevance of Ecological Cascades / 337Michael E. Soulé

Chapter 21. Conclusion:OurTrophically Degraded Planet / 353JohnTerborgh and James A. Estes

References / 369Contributors / 445Index / 453

ix

Foreword

Predators often shape nature in ways that at first glance appear counterintuitive.More than 20 years ago, a colleague at the University of Washington showedme a photo of a sea otter floating on its back, holding a sea urchin with its twofront paws, poised for a meal. I remember my surprise when he told me thatwithout sea otters there wouldn’t be kelp forests.“But wait,” I said,“isn’t that seaotter about to eat an urchin and not a piece of kelp?” He agreed that was thecase.“So why should it matter to the kelp whether otters are around?” I asked.He then explained that it’s because sea otters (as predators) keep urchin popu-lations in check, and without otters, kelp-devouring urchins proliferate, essen-tially mow the kelp forests down, and leave an undersea desert in their wake.

I found this story fascinating and compelling: fascinating because it uncov-ered the hidden and intricate ways nature works, compelling because it demon-strated the broad and devastating effects that can follow the disappearance of asingle species.The transformation of a majestic underwater kelp forest into anundersea ghost town remains a vivid and profoundly sad image.

It turns out that my colleague was making the final preparation for an arti-cle on the sea otter, kelp, and sea urchin saga that appeared in the journalScience. One important factor in the discovery of this tale is that sea otters,through reintroductions and natural population growth, were making a come-back years after they were hunted to local extinction.This provided scientistswith the important and rare opportunity to observe the broader ecologicalconsequences of a world with and without sea otters. This unusual series ofevents afforded an excellent proving ground for uncovering the indirect conse-quences of the loss of a predator on its food web, a phenomenon that has come

to be known as a trophic cascade.This particular trophic cascade has become aclassic example of the phenomenon, and further descriptions of it and manyother trophic cascades appear in this volume.

When I was approached by the co-editors of this book to support this ef-fort, I recognized the important and varied roles a volume on trophic cascadescould play. First, although the ecological foundation for trophic cascades ap-peared in the scientific literature more than half a century ago, until now thetremendous scientific progress made in this field has remained scattered. In ad-dition, over the years the extent and importance of trophic cascades in naturehave been hotly debated.

This first comprehensive work on trophic cascades is excellent, thorough,and timely. It puts in one place the accumulated wisdom of an impressive arrayof ecological detectives who through careful long-term investigation have un-covered the complex dynamics of a wide range of ecotypes and regions.Thereader will be treated to accounts of trophic cascades akin to adventure stories,full of intricate interactions, intrigue, unexpected consequences, and the dis-covery of pattern and order against a backdrop of noise and environmentaldegradation. The studies span the furthest reaches of the planet and includework in some of its most remote and pristine ecosystems. Some accounts gobeyond consideration of trophic effects to examine, for example, behavioral re-sponses of prey to predators, often called the ecology of fear.Together, theseyarns form a rich tapestry. For those who are fascinated by the complex andmysterious ways nature works, the accounts in this book will not disappoint.

Human transformation of the natural world has been broad and durableand has accelerated in recent decades.With few exceptions the trajectory hasbeen to a simplified and degraded natural world. Few if any pristine ecosystemsremain, and the chance to understand them is passing by. Many of the studiesrecounted in this volume could not be duplicated today. Among the manytransformations of nature wrought by humankind is a large and growing pred-ator void.The diminution of top predators now extends to the ocean, the lastgreat natural frontier on this planet, and to the furthest reaches of the deep sea.Forecasting the follow-on effects in ocean ecosystems is of critical importance.Is a great unraveling of ocean life taking place? Are once lush underwater para-dises being transformed into wastelands?This volume may well contain impor-tant clues for answering these questions.

Most importantly, I believe this work will be of vital importance in address-ing the urgent task of stemming the destruction of natural systems and workingto restore them.Understanding the ways ecosystems work is critical to under-standing and devising ways to repair them, and this comprehensive global por-

x Foreword

trait of trophic cascades will be invaluable in that effort. If the wisdom gatheredinto this book is harnessed for the restoration of degraded oceans, seas, rivers,and lands, in my view it will have served its highest and best purpose.

My hope is that this book attracts a broad audience far beyond the scientificcommunity.To some the loss of predators may seem unimportant, irrelevant,perhaps even good. But what this book will teach all of us is that the loss ofpredators is just a starting point, with far-reaching ripple effects.The chancesare good that somewhere in the vast chain of consequences of the great preda-tor void there is something every one of us would sorely miss in a tangible, per-sonal way.My hope is that this volume sparks concern and serves as a call to ac-tion for every reader.

In the end we will protect only what we care about.There is something inthis volume that everyone can care about.

Ellen K. Pikitch, Executive Director,Institute for Ocean Conservation Science

Foreword xi

xiii

Since its origins in the early twentieth century, the science of ecology has em-phasized physical processes to account for the structure and organization ofecosystems. Photosynthesis is the basic energy transduction process of life, andthe primary productivity derived from it underlies all other biotic fluxes. Pho-tosynthesis is supported by light, warmth, and minerals and nourished by car-bon dioxide. It is therefore understandable that the early generations of ecolo-gists found compelling links between these drivers of primary productivity andthe structure of ecosystems (Lindeman 1942; Odum 1957;Walter 1964, 1968;Holdridge 1967; Rosenzweig 1973;Whittaker 1975). Biogeography has fol-lowed in a parallel tradition of seeking understanding through the analysis ofphysical variables and processes, such as area, degree of isolation, elevation, andvicariance (Wallace 1860; Croizat 1958;Walter 1964, 1968; MacArthur andWilson 1967). Productivity in the sea has seemed even more tightly coupled tophysical processes than vegetation on land, and oceanographers have accord-ingly devoted great attention to such processes as nutrient fluxes, temperatureshifts, currents, upwelling, vertical mixing, and convergences (Parsons et al.1983; Lalli and Parsons 1997).

Physical parameters of the environment are easy to measure and obviouslycorrelate with major features of the biological world, so it is natural that theywere emphasized first. A century of studying the relationships between thephysical and biological worlds has yielded a powerful science, able to predictsuch fundamental attributes of ecosystems as the productivity of aquatic andterrestrial ecosystems, the geographic extent of major biomes, responses to

Preface

xiv Preface

gradients in rainfall and elevation, latitudinal diversity gradients, and muchmore (Whittaker 1975;Rosenzweig 1996; but see Chapter 16, this volume).

Coexisting with the bottom-up flow of productivity-driven phenomena isa top-down countercurrent. Products of photosynthesis are harvested by con-sumers, and these in turn are eaten by predators. Predators reign at the top ofthe food chain and prey on consumers such as grazers, browsers, frugivores, and,in the sea, the myriad life forms that feed on phytoplankton. But do predatorseat enough consumers to make a difference at the producer level? Ecologistshave been debating this question for decades.The question seems so simple, yetthe answer has been exasperatingly elusive. But in the answer lies the key tohow nature works, so the answer matters enormously, both as a matter of scien-tific understanding and as a point of departure for how to ensure the survival ofbiodiversity on Earth.

If predators limit consumers, then the plants that constitute the producerlevel at the bottom of the food chain can flourish.This is the famous “greenworld” hypothesis of Hairston, Smith, and Slobodkin (HSS 1960). But if pred-ators are able only to catch the sick, the weak, and the old in prey populations,the prey (here referring to consumers) must be regulated from the bottom upby their principal resources. But the world is obviously green, so how can thisbe true?The retort of bottom-up advocates is that much of that which is greenin the world around us is either toxic or so lacking in nutritional value that itis inedible to consumers. In short, the world is green because so much of it isinedible (Ehrlich and Raven 1964;Murdoch 1966).

A parallel line of debate extends to the ocean margins,where kelps and sea-grasses are often the dominating features of shallow seascapes. Farther out tosea, across the vast surface waters of open oceans where photosynthesis occurssolely through phytoplankton, and in the dark abyssal realm where organismsobtain their energy through detrital fallout and chemosynthesis, alternatives tobottom-up control seem barely to have been considered. Despite the fact thatmost phytoplankton species are edible to a wide range of zooplankton,bottom-up regulation remains an article of faith for most biological oceanographers(Lalli and Parsons 1997;Chapters 6 and 19, this volume).

Those who hold the view that much vegetation is inedible or that rates ofconsumption are intrinsically exceeded by rates of production are skepticalabout the capacity of top-down forces to structure ecosystems (Strong 1992;Polis et al. 2000).According to this point of view, strong top-down regulation isa curiosity or aberration, present in some circumstances but not in others, andoverall, in the big picture, unimportant.

Predation and other types of top-down forces (e.g., herbivory, parasitism)have been relegated to secondary status for a simple reason: Top-down pro-cesses are difficult to study.The laws of thermodynamics require that predatorsbe rare relative to their prey and that they range over much larger areas, render-ing exclusion experiments difficult and expensive to implement.Consequently,most such experiments are conducted at the scale of a few square meters or less(Schmitz et al. 2000).Are such experiments going to convince anyone that or-cas, great white sharks, wolves, tigers, and jaguars are important? At best, suchinference requires a leap of faith (Carpenter and Kitchell 1988).

Acceptance by scientists of the simple logic of HSS has therefore lagged. Inpart, scientists have been held back by their own cultural baggage: institutional-ized skepticism, the existence of alternative hypotheses that have not been de-finitively ruled out, and standards of evidence that exceed achievable limits.Thenative skepticism of science is most readily dispelled via rigorously controlledexperiments.Yet experiments on the scale necessary to study the predator–preydynamics of large vertebrates have been beyond the capacity and the means ofthe scientific establishment in the United States or anywhere else.

These scaling challenges have been hindrance enough on land, but in thesea they have been insurmountable. Large marine predators such as greatwhite sharks, tuna, and toothed cetaceans routinely track prey over hundredsor thousands of kilometers of open ocean. No experiments are possible here.But the fact that appropriate experiments are beyond the reach of investiga-tors does not mean that the large predators of the sea are unimportant or thatthey should be ignored by ecologists.They present a challenge to the ingenu-ity of the investigator, who must search the world for places, events, or specialcircumstances that create “natural experiments” that could not be done in aplanned or replicated fashion. Perhaps the evidence to be derived from such“experiments” is not as airtight or repeatable as it would be from rigorouslycontrolled and replicated treatments, but it is all we have or are likely to havein the foreseeable future as the basis for inferring how the world’s largest eco-system operates.

Biologists have recognized the central importance of predation since themid-nineteenth century (Bates 1862;Müller 1879). But, in retrospect, it is nowobvious that the green world hypothesis of HSS and an even earlier articulationof it by Charles Elton (1927) were ahead of their time. Many ecologists wereconvinced by the logic of the green world argument and conceded that HSSmight be right, but there was no obvious way to put the idea to rigorous tests.Hairston, Smith, and Slobodkin earned a tip of the hat in textbooks, but in an

Preface xv

era of increasing reductionism their thesis was put aside as unsubstantiatedspeculation.

Inspired by the insights of such distinguished forerunners as Charles Elton,Aldo Leopold, and HSS, a handful of ecologists sought to overcome themethodological handicaps inherent in the study of predator–prey systemsthrough intensive investigations of systems that offered special tractability. Painewas the pioneer with his Pisaster removal experiment in 1966.The experimentwas free of confounding variables, and the results were dramatic.The experi-ment was immediately hailed as a landmark and enshrined in textbooks. Butskepticism remained. Perhaps the results were attributable to a particularlypowerful keystone predator, perhaps targeting by the predator of the dominantcompetitor in the prey community was a lucky factor that would not pertain toother systems, perhaps the space-limited system of the rocky intertidal zonecreated special conditions that did not apply to open systems, perhaps the re-sults depended in some unknown way on the multistage life cycles of most ofthe organisms involved, and so on.Such lingering doubts caused many scientiststo regard Paine’s results as an isolated example without far-reaching relevanceto how biological systems work in general.

A GROWING CONSENSUS

Despite methodological handicaps, evidence corroborating key features ofPaine’s findings began to accumulate from carefully constructed case studies.Certainly the most dramatic and quotable of the early studies was the startlingdiscovery by Estes and Palmisano (1974) of how Pacific kelp forests owe theirexistence to sea otters. Exuberant accounts of this signal breakthrough rever-berated through the scientific and popular press, creating public awareness thatbiological interactions are important.Appearing in parallel with these early em-pirical investigations were seminal ideas that began to build a theoretical foun-dation under the concept of top-down regulation (Rosenzweig 1973; Fretwell1977; Paine 1980; Okasanen et al. 1981). On the empirical side, Paine (1980)showed that food web dynamics were largely attributable to strong interactors,often called keystone species; the starfish Pisaster was the prime example.Thenext year, Oksanen and his colleagues confirmed the existence of predictedabrupt state shifts driven by herbivory and predation on arctic productivity gra-dients (Oksanen et al. 1981). Soon after that, Schoener and Toft (1983) andPacala and Roughgarden (1984) demonstrated that Anolis lizards could exert

xvi Preface

strong top-down control on spiders and herbivorous insects, respectively, onsmall West Indian islands. Close on the heels of these meso-scale terrestrialstudies came the remarkable whole ecosystem experiments of Carpenter andKitchell (1988, 1993). Predicted dramatic consequences of the experimentalremoval or addition of the top trophic level from small lakes again stirred thepopular (and scientific) imagination.

Similar narratives began to come in a rush from all types of ecosystems,from unbounded marine situations to freshwater streams and lakes, and fromterrestrial systems from the Arctic to the tropics. Here are some of the better-known examples.

• In the absence of top carnivores, white-tailed deer irruptions occurredover large portions of the eastern United States, with consequent sup-pression of hemlock recruitment in northern forests and oak recruitmentin mid-latitude forests and selective depletion of favored herbaceousplants (Alverson et al. 1988;McShea et al. 1997;Rooney et al. 2004).

• Cage experiments in tropical and temperate streams confirmed the struc-turing effects of top-down forcing and the importance of food chainlength in determining whether autotrophs are enhanced or reduced byapex predators (Power 1990; Flecker 1992).

• Severe degradation of the vegetation of an oceanic island followed thereintroduction of a long-absent native herbivore (Campbell et al. 1991).

• The growth of balsam fir, as indicated by growth rings, is indirectly regu-lated by wolf predation on moose on Isle Royale (McLaren and Peterson1994).

• Overharvest of fishes and invertebrates led to algal overgrowth of Ja-maican coral reefs (Hughes 1994).

• Partial predator exclosures at a 1-square-kilometer scale inYukon, Can-ada resulted in a surge in snowshoe hare densities (Krebs et al. 1995).

• “Mesopredator” release in coyote-free canyons of San Diego County,California had strongly negative consequences for bird populations(Crooks and Soulé 1999).

• Aspen stands and riparian thickets inYellowstone National Park declinedafter wolf extirpation (Ripple and Larsen 2000; Beschta 2005) and havebeen recovering since wolf restoration (Ripple and Beschta 2007b).

• Woody plant recruitment was suppressed by hyperabundant herbivoreson predator-free islets in tropical Lago Guri,Venezuela (Terborgh et al.2001).

Preface xvii

• Decimation of the cod fishery on Newfoundland’s Grand Bank was suc-ceeded by an outbreak of sea urchins, dogfish, skates, and lobsters (Wormand Myers 2003; Steneck et al. 2004; Frank et al. 2005).

• Vegetation was released on a remote oceanic island after local extirpationof a dominant herbivore (O’Dowd et al. 2003).

• Introduction of arctic foxes to islands in theAleutian archipelago resultedin an ecosystem phase shift from grasslands to tundra (Croll et al. 2005).

• Decimation of great sharks in U.S. mid-Atlantic coastal waters precededan outbreak of mollusk-eating cow-nosed rays and the resulting collapseof estuarine shellfisheries (Myers et al. 2007).

• Overfishing of cod in the Baltic Sea triggered a series of algal blooms viaa multilevel trophic cascade (Casini et al. 2008).

As the number and variety of these case studies suggests, top-down forcingand its effects on lower trophic levels have not been overlooked by the scientificcommunity, but they have been overshadowed by a dominant current of opin-ion holding that more was to be gained by investigating bottom-up forcing.Aclear example of the secondary status afforded to top-down processes by fund-ing agencies was the International Biological Program of 1964–1974, a largeU.S.National Science Foundation–sponsored program of comparative researchon ecosystems that emphasized measurements of climate, productivity, nutrientcycling, and other bottom-up processes.The current incarnations of big scienceprograms in terrestrial ecology in the United States are the Long-Term Ecolog-ical Research program and National Ecological Observatory Network, de-signed primarily to monitor the progress and effects of climate change (Kelleret al. 2008). In marine science, the counterpart of these programs is GlobalOcean Ecosystem Dynamics (GLOBEC).“The aim of GLOBEC is to advanceour understanding of the structure and functioning of the global ocean ecosys-tem, its major subsystems, and its response to physical forcing, as the empiricalfoundation to be used in forecasting responses of the marine ecosystem toglobal change” (see www.globec.org). In all these heavily funded programs, in-vestigations of top-down processes have been relegated to a distant secondplace.

Single-minded focus on the role of bottom-up drivers has directed the flowof funding away from investigations of top-down processes, precluding even themost obvious experiments. For example, to date the U.S. National ScienceFoundation has not sponsored any large-scale experiments designed to investi-gate the effect of predator removal on terrestrial ecosystems.The only U.S. ef-fort of which we are aware that controlled a mammalian predator on an eco-

xviii Preface

logically significant scale was a coyote removal experiment, funded by a combi-nation of state and private agencies, that used a 5-square-kilometer block de-sign with replicates (Henke and Bryant 1999).The results demonstrated a pow-erful “Paine effect,” as a six-member rodent community collapsed to onedominant species,Dipodomys ordii. Deer irruptions in the eastern United Statesmotivated the U.S. Forest Service to install a series of 13- or 26-hectare exclo-sures in Pennsylvania (Tilghman 1989). Neither of these experiments is wellknown to ecologists because both efforts were conducted by wildlife managersand published in the literature of that field.We know of no other experimentson a scale greater than 1 hectare in the United States, although a partial preda-tor exclusion experiment conducted on a 1-square-kilometer block design hasbeen implemented in Yukon, Canada (Krebs et al. 1995).These facts provideunequivocal testimony that top-down forcing has been systematically ne-glected by the agencies that fund basic science in the United States.

We believe that the lack of recognition and acceptance of the top-downcountercurrent as a ubiquitous and fundamental structuring force in nature isretarding progress in ecological science at a crucial time when the stability ofecosystems all over the world is being threatened by multiple human interven-tions, not the least of which is the systematic elimination of top predators. Ifdisrupting the trophic cascade were to lead to the dire consequences predictedby theory and anticipated by empirical studies (including those cited earlier),then many of the world’s ecosystems would already be in serious trouble. Ourdeep anxiety at this prospect inspired us to organize a conference that was heldat theWhite Oak Plantation inYulee, Florida (USA) on February 7–10, 2008.For making our stay at the plantation a memorable and pleasurable experience,we are most grateful to Tom Galligher,Troy Miller, and the rest of the unfail-ingly friendly and helpfulWhite Oak staff. For sponsoring our stay at the plan-tation, we express our deep gratitude to the HowardT.Gilman Foundation.Wewarmly thank Patricia Alvarez and Stacey Reese for innumerable forms of as-sistance with production of the book.Defenders ofWildlife and the Pew Char-itable Trusts provided critical financial support, the latter through a grant fromthe Institute for Ocean Conservation Science.

Our purpose in organizing this book was to elevate biotic forcing in ecol-ogy to what we believe is its proper place as coequal with physical forcing.Thetime for a synthesis is right. Evidence of top-down trophic cascades has beenmounting, as indicated earlier, and has lately begun to snowball.The accumu-lated weight of this evidence has become overwhelming, as we shall demon-strate in the ensuing chapters.We hope that this book will be read by doubtersand skeptics as well as the convinced, because we are confident that the theory,

Preface xix

evidence, arguments, and interpretations presented herein will open a new erain ecology.

We shall demonstrate that top-down forces interact with bottom-up forcesthrough a dynamic balance and that this balance confers structure on ecosys-tems and ultimately regulates their species composition and diversity. Becauseof the huge historical disparity in funding and, consequently, scientific attentiongiven to investigating bottom-up and top-down processes, understanding ofhow top-down forces operate via all their myriad and intricate pathways lagsfar behind.Temperature, moisture, production potential, and the serendipitousconstraints of history set broad limits on the distribution and abundance of spe-cies.Yet beyond these rather obvious macro-scale agents of physiological toler-ance, it is the operation of top-down forces that regulates and sustains biodiver-sity on our planet. For this reason alone, the scientific community should focusmuch greater attention on top-down processes and their stabilizing effects onnatural ecosystems.

JohnTerborgh andJames A. Estes

xx Preface