Book Review: Trophic Cascades

Trophic Cascades, Edited by John Terborgh & James A. Estes, 2010 Island Press, WashingtonDC

On my bookshelf I have an original edition of Aldo Leopold’s 1933 classic Game Management.  Now, how I came into possession of this seminal volume in applied ecology might be considered unethical by some. I prefer to think of it as a “rescue effort” conducted to avoid its eventual deterioration in the bowels of a forgotten storage garage. Regardless of the nefarious track by which it found its current home, I value this volume as it represents the moment when a paradigm shift occurred in our understanding of wildlife management and the maintenance of the biological integrity of ecosystems. Trophic Cascades, a collection of essays detailing evidence of the critical importance of apex predators in ecosystem functioning, will be given a home on the same shelf as Game Management. While it may not yet represent a watershed intellectual moment, it is an important contribution that advances our collective understanding.

Trophic Cascades demolishes the entrenched bias that currently exists within ecological professions favoring the idea of “bottom up” regulation of ecosystems. Bottom up regulation is the idea that the raw productivity of an ecosystem is the ultimate shaper of the system structure. This viewpoint emphasizes that the physical parameters of the environment, coupled with the photosynthetic capacity, are of primary importance in determining what shape an ecosystem takes and that, as you move up the food chain, the impact on the system structure diminishes. I can still remember one of my undergraduate forestry instructors deriding his wildlife biologist colleague with the opinion that animals were just parasites living off “his plants.” Academics can be so cruel.

Of course, as in all things ecological, the true story is just a bit more complex. While basic variables such as temperature, moisture, and photosynthetic production potential have broad impacts on the distribution and abundance of species, top-down forces in the form of predation have substantial influence over these same attributes. The importance of top-down forces becomes evident when apex predators are effectively eliminated from the system, resulting in a chain reaction, or “cascade” of effects down to the lowest level of the trophic ladder. The resulting system ends up supporting less biodiversity and is a mere shadow of its pre-disturbance grandeur. How do we know this? Well, since the first European set foot in North America and aimed his blunderbuss at those terrifying wolves and bears, we have been conducting the same crazy experiment across the continent.  Heck, we still have folks like Sarah Palin participating in regional-level, trophic manipulation (I suspect she may not have finished reading her copy of Trophic Cascades). The evidence is all around us as to the changes in the dynamics of ecosystems due to the removal of apex predators but our short cultural memory, coupled with the inherent logistical difficulty of experimentally manipulating predator populations in a controlled setting, has made it hard to identify and study.

This is where the value of Trophic Cascades becomes apparent. This resource brings together, for the first time, a wealth of evidence from a variety of ecosystems detailing the (usually unpredictable) consequences of apex predator elimination. The material is absolutely fascinating to anyone with even a modicum of ecological expertise. A few examples will help to highlight the information contained within this important volume:

• Over fishing of sharks has damaged shellfish populations in East Coast estuaries through the ecological release of cownose rays.
• Balsam fir growth is indirectly regulated by wolf predation of moose on Isle Royale in Michigan.
• Over fishing of cod in the Baltic Sea has resulted in algal blooms via a multilevel trophic cascade. Algal bloom.jpg
• Forest regeneration of oak and hemlock has halted in vast areas of Pennsylvania woodland due to high deer densities.
• High human visitation in ZionCanyon in Utah has resulted in a reduced cougar presence and subsequent higher deer herbivory on cottonwoods in riparian forests.
• Microbial populations harmful to reef-building corals increase due to labile sugars released by fleshy algae. Algae that are, in themselves, released due to over-fishing.

Trophic Cascades effectively builds the case that the faunal truncations associated with apex predator removal can result in new, alternative ecosystem states that are difficult to restore due to positive feedback loops. From stream morphology to mesopredator (think raccoons and skunks) release and disease vectoring, the loss of the top tropic layer results in profound, unintended consequences across the ecosystem. In most cases these ripple effects are not well-understood and the temporal lag makes the determination of ultimate causality difficult to trace. What we end up with are biologically impoverished systems that are considered “native” by lay observers who have no collective memory or direct experience of the original baseline conditions. As in the classic example of the frog who boils to death when the temperature rises gradually but jumps out if placed directly into scalding water, our species only reacts to sudden, catastrophic change. We fail to connect the dots and end up losing the biodiversity that both enriches and sustains our existence.

While the majority of this book is devoted to the science associated with the cascade effect, it does make some salient policy suggestions and observations based upon the unrecognized importance of this issue. For instance, is the high-profile nature of climate change diverting important resources away from other immediate challenges such as ecological cascades, invasive species, and habitat loss?  Is “adaptive management” an appropriate tool given the frequently nonlinear, sudden phase shifts that these systems can display? Shouldn’t the focus of restoration ecologists ultimately be on restoring apex predators to each system and creating the landscape matrix that allows for their viable reproduction?

While not a volume to be casually consumed by the general public, Trophic Cascades is accessible to anyone with a basic understanding of biology and ecological science. The material is presented in a straightforward, logical format according to the evidence for this effect in various ecosystems. It is truly an illuminating resource that should be on the bookshelf of anyone concerned with conservation and ecological restoration. Just make sure to procure your copy through legal means.