Trophic Cascades: Predators, Prey and the Changing Dynamics of Nature

Bottom-up or top-down? The debate as to which force drives ecosystem food webs and population dynamics has been ongoing for decades. Are ecosystems stabilized from the top down as proposed by Hairston, Smith and Slobodkin? Or are the producers in control from the bottom up? John Terborgh and James A. Estes have sought to reinforce and solidify the argument for top-down control with their book, Trophic Cascades: Predators, Prey and the Changing Dynamics of Nature. This collection of chapters by leading scientists with either aquatic or terrestrial expertise offers convincing evidence for the existence of top-down control and trophic cascades in various habitats. Each chapter presents a different set of experiments, case studies, or general observations of predator-induced community stabilization in freshwater, marine, boreal, tropical, and other ecosystems. Though closure to this seemingly endless debate seems to have remained elusive to the editors, points were definitely awarded to the side of top-down control. This review is a collective critique by students in a graduate class; part of our goal was to assess the suitability of the book as a graduatelevel text. The first chapter is a general introduction to basic concepts central to the following chapters, introducing the ‘‘green world’’ hypothesis and providing definitions of top-down forces and trophic cascades. This initial chapter is followed by four parts with several chapters each. Part I Aquatic Ecosystems contains five chapters covering freshwater lakes, where the first empirical evidence for trophic cascades was gathered, as well as coastal, intertidal and open ocean ecosystems. In Chapter 2, Robert Paine of the famed Pisaster seastar experiments reviews the origins of the terms ‘‘keystone species’’ and ‘‘trophic cascades,’’ and describes his manipulation of seastar populations and their dramatic effects on lower trophic levels. Chapter 3 reviews the loss of sharks as apex predators and subsequent effects on the scallop industry, sea otters and their effect on the chemical defenses of kelp, and the impact of commercial cod harvest on the lobster fishery of Maine. The concept of ‘alternative states’ combined with case studies provided convincing evidence of the widespread existence of cascading effects present in these systems. Animal behavior, nutrient enrichment, terrestrial organic input, and the pros and cons of whole lake studies are discussed in Chapter 4. Though the authors’ own work is well described, experiments of others are not, leaving the reader to look elsewhere for clarity and further understanding. This chapter and others that follow mostly describe factors that contribute to trophic cascades rather than support the existence of top-down control in these systems. Multiple examples of three-level cascades in tropical nearshore ecosystems are presented in Chapter 5, covering a wide latitudinal range and giving insight as to why some cascades are stronger than others. These authors clearly and concisely present what is promised in their introduction. Unfortunately, most figures in this chapter are unnecessary and data poor, a trend seen throughout the book. Chapter 6 is well written and highlights the possibility of trophic cascades occurring in the open ocean without ignoring the hurdles to proving their existence. More clarity distinguishing open oceans from inshore or other marine systems would have been useful in this chapter. Overall, the first section of this text provides exceptional case studies and evidence for trophic cascades in a variety of aquatic ecosystems. Part II Terrestrial Ecosystems includes six chapters covering arctic scrubland, boreal and tropical habitats, islands, and below-ground trophic cascades. Chapter 7 references the importance of herbivores within terrestrial trophic cascades, but lacks solid case studies or detailed explanations of its referenced studies. It is at best a glance at trophic cascades in terrestrial systems, weak in structure but rich in references. An interesting case study of a flooded tropical landscape that resulted in hilltop islands is described in Chapter 8. The resulting trophic changes created interesting top-down effects, even at the soil nutrient level, with what is described as a ‘‘trophic bounce back. ’’ Though this was a fascinating experiment, the chapter lacked helpful descriptions of the cascades occurring within the systems. The authors argue that anticipating top-down effects from trophic Integrative and Comparative Biology, volume 51, number 4, pp. 644–652