- Email: [email protected]
Landscapes of hope, oceans of promise: trophic cascades
Update is a reflection of the paradox of conservation. In order to succeed, conservation needs a whole range of specific conservation solutions to meet different ecological and political situations. Whereas Conservation Biology for all is a good place to find out about the range that is currently out there, conservation biology as a discipline is still dominated by western scientists, if dominance is measured by articles published and chapters written. There are a large number of non-western biologists and
Trends in Ecology and Evolution Vol.25 No.10
practitioners delivering successful conservation projects against a background of competing political and economic concerns. Whereas some members of this group have contributed to the book, there are still many more out there from whom we need to hear in order to redress the balance. 0169-5347/$ – see front matter doi:10.1016/j.tree.2010.07.003 Trends in Ecology and Evolution 25 (2010) 553–554
Book Review
Landscapes of hope, oceans of promise: trophic cascades Trophic Cascades: Predators, Prey, and the Changing Dynamics of Nature by John Terborgh and James A. Estes (editors). Island Press, 2010. US$90.00/£25.65 hbk (488 pages) ISBN 978-1597264877. The Wolf’s Tooth: Keystone Predators, Trophic Cascades, and Biodiversity by Cristina Eisenberg. Island Press, 2010. US$28.00// £19.95 hbk (272 pages) ISBN 978-1597263979.
Joseph K. Bump School of Forest Resources and Environmental Science, Michigan Technological University, 1400 Townsend Drive, Houghton, MI 49931-1295, USA
Predation’s importance as an evolutionary force is unquestioned. As an agent of natural selection, predation drives the spiraling levels of defense and counterdefense seen in evolutionary arms races between more effective predators and more evasive prey. Such widespread evolutionary impact of predation has been easily observed and accepted. The same cannot be said for predation as an ecological force. In contrast, physical, abiotic factors such as light, temperature, moisture, and salinity, have traditionally been used to explain ecological dynamics. In Trophic Cascades, John Terborgh and James Estes reason that predation’s proper place as an ecological force is as a ‘‘coequal with physical forcing.’’ This edited volume offers the ‘‘universal importance’’ of trophic cascades as the mechanism elevating predation to a ‘‘law of nature as essential and fundamental to ecology as natural selection is to evolution.’’ Cristina Eisenberg’s popularly written counterpart, The Wolf’s Tooth, narrates the same thesis with interviews (including ones with Terborgh and Estes) and insight from her gray wolf (Canis lupus) research, concluding that ‘‘wolf presence affects everything.’’ Given predation’s far-reaching effects through trophic cascades, these authors advocate restoration and stewardship of predators as necessary to maintain ecosystem function and resilience, to increase
Corresponding author: Bump, J.K. ([email protected]).
554
biodiversity, to preserve grandeur, to create what Eisenberg calls ‘‘landscapes of hope.’’ A trophic cascade, in which a perturbation at one trophic level propagates through lower levels with fluctuating positive and negative effects, is an iconic metaphor of ecological theory, being historical, contentious, charismatic, and consequential. Early ideas about predatordriven, top–down forcing are found in the writing of Charles Elton and Aldo Leopold and since then the strength and ubiquity of trophic cascades has been repeatedly debated. Standout examples involve well-loved species like sea otters and orcas, wolves, deer, sharks, and unfortunate human introductions such as rats and foxes. According to these books, acknowledgement of the relative importance of top–down forcing via trophic cascades is beyond important because the apex predator species that are most responsible for cascading ecological effects are also frequently most at risk of extirpation. Both books are fundamentally about relationships. What is the relationship between predators and ecosystems? What conditions strengthen or weaken this relationship? What shall we decide is our relationship with predators? According to the evidence presented by Terborgh and Estes, the jury is in on the importance of predator–ecosystem relationships. They explain why a fair and accurate perspective is one that acknowledges top–down forcing via trophic cascades as a determinant of species abundance and distribution within the broader constraints of species ranges and global productivity gradients. The first two sections (‘Aquatic Ecosystems’ and ‘Terrestrial Ecosystems’) in Trophic Cascades detail that across biomes, powerful ecological effects from top–down forcing have been found ‘‘almost wherever people have taken the
Update time to look very carefully.’’ The last two sections (‘Predation & Ecosystem Processes’ and ‘Synthesis’) explore the theoretical perspectives and conservation relevance of trophic cascades, demonstrating that there is both much research still to be done and results to be applied. Mechanisms of top–down forcing have been identified that extend beyond trophic cascades or, in an alternative view, expand the concept of trophic cascades. The terminology in the literature is a bit muddy here (e.g. trophic control, behaviorally-mediated cascades, trait-mediated effects or indirect interactions, disease-mediated trophic cascades), reflecting a swiftly moving area of ecological research. New mechanistic understanding continues to emerge [1,2], and with each discovery a nuanced appreciation for what is lost and gained with predator extirpation and restoration. Chapters in Trophic Cascades on alternative states in ecosystems (by Marten Scheffer) and theoretical perspectives (led by Robert Holt) present major research fronts exploring links between predation and ecological stability [3], spatial heterogeneity, and disease dynamics [4]. Within the book, these chapters balance the dramatic evidence illustrating why we should care about trophic cascades, asking how we can predict the nonlinear effects of trophic cascades and whether such effects can be reversed. Given their research programs, Terborgh and Estes are expert contributors and fine editors for the subject. In 1995, Estes contributed to Linking Species and Ecosystems [5], the only book that can be considered a potential predecessor to Trophic Cascades. In his chapter 15 years ago, Estes posed five questions concerning top-level carnivores and ecosystem effects. Most generally he asked: what are the ecological effects of top predators, how general are they, and under what conditions do they occur? The answers to the first two questions and an exploration of the third are largely provided in Trophic Cascades by an impressive list of contributors with extended expertise on the topic. A contribution from Oswald Schmitz on non-consumptive effects of predators is lacking and given Schmitz’s pioneering experimental work in the field, his perspective is a notable absence [6]. We are fortunate to have two complementary books suited to separate audiences. Eisenberg’s narrative is earnest and personal; great for students and nonscientists. Terborgh and Estes’ volume is comprehensive, synthetic, and urgent; geared toward researchers and resource managers. Eisenberg also interviewed two other contributors to Trophic Cascades, Robert Paine and Michael Soule´. Yet the overlap between these books is enriching and does not create an either/or choice for reading. With insistent commentary, Terborgh and Estes document dynamics of top– down forcing wherever we have looked. Eisenberg offers insight into how one might begin to look for predation’s influence individually. Learning how to look is not a trivial point. Lack of awareness and entrenched resistance to the importance of top–down forcing is still widespread. For example, Soule´’s chapter emphasizes the critical role of connectivity in facilitating the dispersal and migration of apex preda-
Trends in Ecology and Evolution
Vol.25 No.10
tors, a factor which is required for healthy top–down forcing. When inadequate subpopulation connectivity for genetic exchange was raised as an issue in a recent wolf delisting debate in the northern Rocky Mountains, Edward Bangs (wolf recovery coordinator for the U.S. Fish and Wildlife Service) said, ‘‘Connectivity can happen through a ride in the back of a truck [7].’’ Given this response in favor of ultra pragmatic solutions, Bangs’s stance against the mere suggestion to pragmatically use small populations of wolves for ecosystem restoration is inconsistent [8,9]. Such attitudes miss the comprehensive value of connectivity and reflect the challenge of applying top–down forcing restoratively in resource management (predator removal acknowledges top–down forcing, albeit as an undesirable force). As Eisenberg writes, ‘‘When it comes to resource management, it’s still largely a bottom–up world.’’ Resistance to conservation imperatives logically stemming from acceptance of predation’s full ecological importance might reflect extended bottom–up bias and failures of imagination, but it also reveals an important limitation of trophic cascades to influence human behavior and policy. No matter how well we understand and champion the ecological significance of predation, our endeavors to share resources and live successfully with competing predators requires an ethical discussion on coexistence. This much is explicitly sidestepped, unfortunately, by Terborgh and Estes, but can be found implicitly in Eisenberg’s prose. Do we really need to know more about the ecological significance of predators to do a better job of living alongside them? Can the fascinating story of trophic cascades create the ‘‘emotional salience’’ that Soule´ feels is necessary to create real conservation action? Eisenberg is decidedly hopeful; Terborgh and Estes less so, writing ‘‘only time will tell.’’ Acknowledgments I thank Amy J. Schrank for editorial comments and discussion.
References 1 Bump, J.K. et al. (2009) Wolves modulate soil nutrient heterogeneity and foliar nitrogen by configuring the distribution of ungulate carcasses. Ecology 90, 3159–3167 2 Schmitz, O.J. et al. (2010) Predator control of ecosystem nutrient dynamics. Ecol. Lett. DOI: 10.1111/j.1461-0248.2010.01511.x 3 Scheffer, M. (2009) Critical Transitions in Nature and Society, Princeton University Press 4 Holt, R.D. and Roy, M. (2007) Predation can increase the prevalence of infectious disease. Am. Nat. 169, 690–699 5 Jones, C.G. and Lawton, J.H. (1995) Linking Species and Ecosystems, Chapman and Hall 6 Schmitz, O.J. (2010) Resolving Ecosystem Complexity, Princeton University Press 7 Morrell, V. (2008) Wolves at the door of a more dangerous world. Science 319, 890–892 8 Licht, D.S. et al. (2010) Using small populations of wolves for ecosystem restoration and stewardship. BioScience 60,147–153; response commentary by Mech, L.D. et al. DOI:10.1525/bio.2010.60.2.9 9 Mech, L.D. et al. (2010) Restricting Wolves Risks Escapes. Bio Science 60, 485–486. DOI: 10.1525/bio.2010.60.7.19
0169-5347/$ – see front matter doi:10.1016/j.tree.2010.07.004 Trends in Ecology and Evolution 25 (2010) 554–555
555
Trends in Ecology and Evolution Vol.25 No.10
practitioners delivering successful conservation projects against a background of competing political and economic concerns. Whereas some members of this group have contributed to the book, there are still many more out there from whom we need to hear in order to redress the balance. 0169-5347/$ – see front matter doi:10.1016/j.tree.2010.07.003 Trends in Ecology and Evolution 25 (2010) 553–554
Book Review
Landscapes of hope, oceans of promise: trophic cascades Trophic Cascades: Predators, Prey, and the Changing Dynamics of Nature by John Terborgh and James A. Estes (editors). Island Press, 2010. US$90.00/£25.65 hbk (488 pages) ISBN 978-1597264877. The Wolf’s Tooth: Keystone Predators, Trophic Cascades, and Biodiversity by Cristina Eisenberg. Island Press, 2010. US$28.00// £19.95 hbk (272 pages) ISBN 978-1597263979.
Joseph K. Bump School of Forest Resources and Environmental Science, Michigan Technological University, 1400 Townsend Drive, Houghton, MI 49931-1295, USA
Predation’s importance as an evolutionary force is unquestioned. As an agent of natural selection, predation drives the spiraling levels of defense and counterdefense seen in evolutionary arms races between more effective predators and more evasive prey. Such widespread evolutionary impact of predation has been easily observed and accepted. The same cannot be said for predation as an ecological force. In contrast, physical, abiotic factors such as light, temperature, moisture, and salinity, have traditionally been used to explain ecological dynamics. In Trophic Cascades, John Terborgh and James Estes reason that predation’s proper place as an ecological force is as a ‘‘coequal with physical forcing.’’ This edited volume offers the ‘‘universal importance’’ of trophic cascades as the mechanism elevating predation to a ‘‘law of nature as essential and fundamental to ecology as natural selection is to evolution.’’ Cristina Eisenberg’s popularly written counterpart, The Wolf’s Tooth, narrates the same thesis with interviews (including ones with Terborgh and Estes) and insight from her gray wolf (Canis lupus) research, concluding that ‘‘wolf presence affects everything.’’ Given predation’s far-reaching effects through trophic cascades, these authors advocate restoration and stewardship of predators as necessary to maintain ecosystem function and resilience, to increase
Corresponding author: Bump, J.K. ([email protected]).
554
biodiversity, to preserve grandeur, to create what Eisenberg calls ‘‘landscapes of hope.’’ A trophic cascade, in which a perturbation at one trophic level propagates through lower levels with fluctuating positive and negative effects, is an iconic metaphor of ecological theory, being historical, contentious, charismatic, and consequential. Early ideas about predatordriven, top–down forcing are found in the writing of Charles Elton and Aldo Leopold and since then the strength and ubiquity of trophic cascades has been repeatedly debated. Standout examples involve well-loved species like sea otters and orcas, wolves, deer, sharks, and unfortunate human introductions such as rats and foxes. According to these books, acknowledgement of the relative importance of top–down forcing via trophic cascades is beyond important because the apex predator species that are most responsible for cascading ecological effects are also frequently most at risk of extirpation. Both books are fundamentally about relationships. What is the relationship between predators and ecosystems? What conditions strengthen or weaken this relationship? What shall we decide is our relationship with predators? According to the evidence presented by Terborgh and Estes, the jury is in on the importance of predator–ecosystem relationships. They explain why a fair and accurate perspective is one that acknowledges top–down forcing via trophic cascades as a determinant of species abundance and distribution within the broader constraints of species ranges and global productivity gradients. The first two sections (‘Aquatic Ecosystems’ and ‘Terrestrial Ecosystems’) in Trophic Cascades detail that across biomes, powerful ecological effects from top–down forcing have been found ‘‘almost wherever people have taken the
Update time to look very carefully.’’ The last two sections (‘Predation & Ecosystem Processes’ and ‘Synthesis’) explore the theoretical perspectives and conservation relevance of trophic cascades, demonstrating that there is both much research still to be done and results to be applied. Mechanisms of top–down forcing have been identified that extend beyond trophic cascades or, in an alternative view, expand the concept of trophic cascades. The terminology in the literature is a bit muddy here (e.g. trophic control, behaviorally-mediated cascades, trait-mediated effects or indirect interactions, disease-mediated trophic cascades), reflecting a swiftly moving area of ecological research. New mechanistic understanding continues to emerge [1,2], and with each discovery a nuanced appreciation for what is lost and gained with predator extirpation and restoration. Chapters in Trophic Cascades on alternative states in ecosystems (by Marten Scheffer) and theoretical perspectives (led by Robert Holt) present major research fronts exploring links between predation and ecological stability [3], spatial heterogeneity, and disease dynamics [4]. Within the book, these chapters balance the dramatic evidence illustrating why we should care about trophic cascades, asking how we can predict the nonlinear effects of trophic cascades and whether such effects can be reversed. Given their research programs, Terborgh and Estes are expert contributors and fine editors for the subject. In 1995, Estes contributed to Linking Species and Ecosystems [5], the only book that can be considered a potential predecessor to Trophic Cascades. In his chapter 15 years ago, Estes posed five questions concerning top-level carnivores and ecosystem effects. Most generally he asked: what are the ecological effects of top predators, how general are they, and under what conditions do they occur? The answers to the first two questions and an exploration of the third are largely provided in Trophic Cascades by an impressive list of contributors with extended expertise on the topic. A contribution from Oswald Schmitz on non-consumptive effects of predators is lacking and given Schmitz’s pioneering experimental work in the field, his perspective is a notable absence [6]. We are fortunate to have two complementary books suited to separate audiences. Eisenberg’s narrative is earnest and personal; great for students and nonscientists. Terborgh and Estes’ volume is comprehensive, synthetic, and urgent; geared toward researchers and resource managers. Eisenberg also interviewed two other contributors to Trophic Cascades, Robert Paine and Michael Soule´. Yet the overlap between these books is enriching and does not create an either/or choice for reading. With insistent commentary, Terborgh and Estes document dynamics of top– down forcing wherever we have looked. Eisenberg offers insight into how one might begin to look for predation’s influence individually. Learning how to look is not a trivial point. Lack of awareness and entrenched resistance to the importance of top–down forcing is still widespread. For example, Soule´’s chapter emphasizes the critical role of connectivity in facilitating the dispersal and migration of apex preda-
Trends in Ecology and Evolution
Vol.25 No.10
tors, a factor which is required for healthy top–down forcing. When inadequate subpopulation connectivity for genetic exchange was raised as an issue in a recent wolf delisting debate in the northern Rocky Mountains, Edward Bangs (wolf recovery coordinator for the U.S. Fish and Wildlife Service) said, ‘‘Connectivity can happen through a ride in the back of a truck [7].’’ Given this response in favor of ultra pragmatic solutions, Bangs’s stance against the mere suggestion to pragmatically use small populations of wolves for ecosystem restoration is inconsistent [8,9]. Such attitudes miss the comprehensive value of connectivity and reflect the challenge of applying top–down forcing restoratively in resource management (predator removal acknowledges top–down forcing, albeit as an undesirable force). As Eisenberg writes, ‘‘When it comes to resource management, it’s still largely a bottom–up world.’’ Resistance to conservation imperatives logically stemming from acceptance of predation’s full ecological importance might reflect extended bottom–up bias and failures of imagination, but it also reveals an important limitation of trophic cascades to influence human behavior and policy. No matter how well we understand and champion the ecological significance of predation, our endeavors to share resources and live successfully with competing predators requires an ethical discussion on coexistence. This much is explicitly sidestepped, unfortunately, by Terborgh and Estes, but can be found implicitly in Eisenberg’s prose. Do we really need to know more about the ecological significance of predators to do a better job of living alongside them? Can the fascinating story of trophic cascades create the ‘‘emotional salience’’ that Soule´ feels is necessary to create real conservation action? Eisenberg is decidedly hopeful; Terborgh and Estes less so, writing ‘‘only time will tell.’’ Acknowledgments I thank Amy J. Schrank for editorial comments and discussion.
References 1 Bump, J.K. et al. (2009) Wolves modulate soil nutrient heterogeneity and foliar nitrogen by configuring the distribution of ungulate carcasses. Ecology 90, 3159–3167 2 Schmitz, O.J. et al. (2010) Predator control of ecosystem nutrient dynamics. Ecol. Lett. DOI: 10.1111/j.1461-0248.2010.01511.x 3 Scheffer, M. (2009) Critical Transitions in Nature and Society, Princeton University Press 4 Holt, R.D. and Roy, M. (2007) Predation can increase the prevalence of infectious disease. Am. Nat. 169, 690–699 5 Jones, C.G. and Lawton, J.H. (1995) Linking Species and Ecosystems, Chapman and Hall 6 Schmitz, O.J. (2010) Resolving Ecosystem Complexity, Princeton University Press 7 Morrell, V. (2008) Wolves at the door of a more dangerous world. Science 319, 890–892 8 Licht, D.S. et al. (2010) Using small populations of wolves for ecosystem restoration and stewardship. BioScience 60,147–153; response commentary by Mech, L.D. et al. DOI:10.1525/bio.2010.60.2.9 9 Mech, L.D. et al. (2010) Restricting Wolves Risks Escapes. Bio Science 60, 485–486. DOI: 10.1525/bio.2010.60.7.19
0169-5347/$ – see front matter doi:10.1016/j.tree.2010.07.004 Trends in Ecology and Evolution 25 (2010) 554–555
555