Scientists are facing a confluence of challenges with regard to preserving biodiversity and ecosystem function. Climate change, habitat destruction, introduced species, biodiversity loss, loss of topsoil, and increased additions of nutrients and pesticides are all recognized as major drivers of global ecological change. However, the loss of top-down forcing that results from the decline of apex predators (and megaherbivores) has largely been neglected both by ecologists who aim to explain the functioning of the natural world and conservation practitioners who try to preserve what remains. Fortunately, this book effectively synthesizes the knowledge needed to appreciate the role of top-down forcing as a ubiquitous force in nature at a critical time when many systems are experiencing habitat degradation, species loss and, at times, ecosystem collapse as a consequence of predator extirpation.
In this book John Terborgh, James Estes and the many contributors convincingly demonstrate that the Hairston, Smith and Slobodkin (HSS) hypothesis or the ‘green world hypothesis’, which proposes that the collective regulatory action of predators prevents herbivores from depleting vegetation, characterizes many natural systems. A trophic cascade is a term used to describe instances in which predator additions or deletions induce effects that cascade down food chains or food webs and affect biomass of organisms at least two trophic levels below (e.g. predator effects on primary producers). Unfortunately, the current trend in most ecosystems is predator removal by human activities, herbivore release, and reductions in primary producer biomass and diversity, leading to drastic changes in the structure and function of ecosystems. Therefore, preserving/restoring many ecosystems requires us to reverse this trend and effectively conserve apex predator populations.
Trophic Cascades is a comprehensive, clearly written work divided into four main sections and has an extensive section of cited literature. The first section synthesizes the evidence for trophic cascades in a variety of aquatic systems: lake and many marine habitats, including intertidal, near-shore and open-ocean. Starting with aquatic ecosystems is logical as aquatic systems provide the classic examples of trophic cascades, including starfish effects on rocky intertidal communities, sea otter influence on kelp forests, and the role of predatory fish in lakes. The second section dispels the myth that trophic cascades are all wet by providing numerous examples of trophic cascades in terrestrial ecosystems from the trophics to high latitudes. The third section needs to be read by all conservation practitioners. It examines the effects of meso-predator release, trait mediated effects, body size and behaviours of both predators and prey, and interactions among predators, herbivores and fire in structuring vegetation, and also includes a crucial chapter on alternative states in ecosystems. I found the fourth or synthesis section to be somewhat redundant, reiterating many of the previously provided examples. However, this section is important to emphasize that top-down regulation should be universally accepted by professional scientists and conservationists.
Because climate change dominates much of the current scientific discourse it would have been nice to see a chapter that modelled and quantified the effect of predator presence or absence on carbon sequestration in various ecosystems. It is clear that ecosystems are typically ‘greener’ (i.e. have more plant biomass) in areas with apex predator populations at ecologically effective sizes but to what extent? This addition would have added strength to the argument that apex predators should be preserved and have possibly attracted a broader readership to this issue.
Altogether, Trophic Cascades is an important addition to both the ecological and conservation literature. No doubt it will be widely discussed because the HSS hypothesis and the concept of trophic cascades has received uneven acceptance among ecologists working in different ecosystems. This, as the book often points out, is probably a result of the difficulty in conducting rigorously controlled experiments on apex predators that require large ranges to survive. Nevertheless, the wealth of knowledge presented in this book indicates that declines in biodiversity and ecosystem services typically follow the loss of apex predators. As such, we need to adopt the advice of Michael Soulé (Chapter 20) and ‘never remove or discourage native predators’.
