Embedding food webs in ecological landscapes

Polis, G.A., Power, M.E. & Huxel, G.R. (eds) (2004) Food webs at the landscape level. The University of Chicago Press, Chicago, USA. xviii + 548 pp., figs, tables, line diagrams, halftones, index. Hardback: price $95.00, ISBN 0-226-67325-1. Paperback: price £25.50, $36.00, ISBN 0-226-67327-8. Food webs at the landscape level arose from an initiative by the late Gary Polis to synthesize landscape ecology and food web ecology. Polis’ groundbreaking work on the role of energy subsidies across ecotones forms the basis for several chapters and serves to remind us of the importance of his contributions. These chapters are complemented by a range of quantitative and modelling studies that provide both an overview of previous work and an abundance of unanswered questions to be addressed by the next generation of studies. The book is a significant advance, in that it is the first to acknowledge and illustrate the important role that flows of energy, materials or organisms from one habitat to another have on the structure and dynamics of food webs. Subsidies are shown to stabilize and destabilize food webs, depending on the nature of the subsidy and what components of the food web are subsidized. Influences of subsidies on species’ coexistence are shown through mechanisms such as apparent trophic cascades, where energy subsidies to a predator in an adjacent habitat increase top-down control of grazers and protect vegetation from grazing pressure. Structurally, the book is ordered in four parts. The first describes the fluxes of materials between habitats in a variety of systems and includes excellent contributions in terms of quantitative studies, summaries of methodologies and theoretical advances. The overview is elegant and concise, and would be perfect reading for a graduate student starting out in this area. Schindler and Lubetkin's section on the use of stable isotopes is one of the best I have read, effectively reviewing the field to date and indicating some exciting new directions for the technique. Robert Holt's modelling chapter is another highlight — using simple community models modified to incorporate external inputs of materials from adjacent ecosystems. The second section of the book concentrates on food web dynamics at the land–water ecotone, where the majority of the work on subsidies has taken place. Any one of these chapters as a stand-alone illustration of the importance of cross-system subsidies is an excellent read. Collectively, however, there is little new that emerges with each subsequent chapter. While this is likely to be a book that people ‘dip into’ rather than read cover-to-cover, I found myself searching for new developments by halfway through the section. Nevertheless, the range of studies clearly illustrates the importance of external subsidies to food-web structure in a variety of aquatic habitats. Much more satisfying in terms of contributing new ideas is the third section, which moves up to larger (regional and global) scales. Here the role of migratory species in transferring energy between ecosystems is addressed, and the strong interconnections between arctic and temperate regions (mediated by snow geese) and marine and freshwater systems (mediated by anadromous fish) are explored. Effects of agricultural development on the global distribution of energy constitute an emerging theme from this section, as does the role of barriers to animal movements. These themes are drawn out further in the fourth, synthesis section. Again, each of the syntheses would be an ideal introduction to subfields dealing, respectively, with physical processes, pulsed productivity, sustainable harvest and conservation and global change. The book is conceptually based on the insights that Polis has given us — largely that subsidies can act across ecotones to influence food webs. Unfortunately the book is also limited to that context — there is little or no mention of dispersal and dispersal limitation, processes which obviously influence local food web structure, and which are central to hot topics within community ecology. Perhaps a chapter addressing the consequences of neutral processes or lottery effects on food web structure could have yielded a range of potential directions for future studies. Equally, while some mention is made of metapopulation dynamics in some chapters, there is no exhaustive treatment of the way in which this rich area of landscape ecology can be integrated with food web ecology. The consequences of food webs comprising source and sink populations are potentially fascinating, but are not addressed in the book. While no book can do everything, given the extremely detailed coverage of the role of subsidies, there should really have been space made available for these other landscape-level processes. Gary Polis asked community ecologists to ‘stop looking at their feet’ and move from studying food webs at a local scale to studying them within a landscape context. This book shows how his exhortation is beginning to be realized and illustrates a new appreciation of the true nature of food webs — complex trophic networks that are embedded within a dynamic landscape. Food webs at the landscape level is an important contribution, and provides a sound basis for a broader synthesis of landscape and food-web ecology.

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