Foraging: Behavior and Ecology
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Foraging: Behavior and Ecology
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Overview
Foraging is the first comprehensive review of the topic in more than twenty years. A monumental undertaking, this volume brings together twenty-two experts from throughout the field to offer the latest on the mechanics of foraging, modern foraging theory, and foraging ecology. The fourteen essays cover all the relevant issues, including cognition, individual behavior, caching behavior, parental behavior, antipredator behavior, social behavior, population and community ecology, herbivory, and conservation. Considering a wide range of taxa, from birds to mammals to amphibians, Foraging will be the definitive guide to the field.
Product Details
| ISBN-13: | 9780226772653 |
|---|---|
| Publisher: | University of Chicago Press |
| Publication date: | 09/15/2008 |
| Sold by: | Barnes & Noble |
| Format: | eBook |
| Pages: | 576 |
| File size: | 6 MB |
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Read an Excerpt
Foraging
Behavior and EcologyThe University of Chicago Press
Copyright © 2007 The University of ChicagoAll right reserved.
ISBN: 978-0-226-77264-6
Chapter One
Foraging: An OverviewRonald C. Ydenberg, Joel S. Brown, and David W. Stephens
1.1 Prologue
Hudson Bay in winter is frozen and forbidding. But, at a few special places where strong tidal currents are deflected to the surface by ridges on the seafloor, there are permanent openings in the ice, called polynyas, that serve as the Arctic equivalent of desert oases. Many polynyas are occupied by groups of common eiders. When the current in the polynya slackens between tide changes, these sea ducks can forage, and they take advantage of the opportunity by diving many times. With vigorous wing strokes they descend to the bottom, where they search though the jumbled debris, finding and swallowing small items, and occasionally bringing a large item such as an urchin or a mussel clump to the surface, where they handle it extensively before eating or discarding it. (Readers can take an underwater look at a common eider diving in a polynya at www.sfu.ca/eidervideo/. These videos were made by Joel Heath and Grant Gilchrist at the Belcher Islands in Hudson Bay.)
This foraging situation presents many challenges. Eiders must consume a lot of prey during a short period to meet the high energy demand of a very cold climate. Most available prey are bulky and of low quality, and the ducks must process a tremendous volume of material to extract the energy and nutrients they need. They must also keep an eye on the clock, for the strong currents limit the available foraging time. Throughout the winter, individual ducks may move among several widely separated polynyas or visit leads in the pack ice when the wind creates openings. Foxes haunting the rim of the polynyas and seals in the water below create dangers that require constant wariness. In this unforgiving environment, the eider must meet all these challenges, for in the Arctic winter, a hungry eider is very soon a dead eider.
1.2 Introduction
Twenty years ago, Dave Stephens and John Krebs opened their book Foraging Theory (1986) with an example detailing the structure of a caddisfly web. The example showed how the web could be analyzed as a trap carefully constructed to capture prey. The theme of the book was that foraging behavior could also be looked at as "well-designed." In it, they reviewed the basic theoretical models and quantitative evidence that had been published since 1966. In that year, a single issue of The American Naturalist carried back-to-back papers that may fairly be regarded as launching "optimal foraging theory." The first, by Robert MacArthur and Eric Pianka, explored prey selection as a phenomenon in its own right, while the second, by John Merritt Emlen, was focused on the population and community consequences of such foraging decisions. This book gives an overview of current research into foraging, including the offspring of both these lines of investigation.
The reader will discover that foraging research has expanded and matured over the past twenty years. The challenges facing common eiders in Hudson Bay symbolize how the study of foraging has progressed. Some of these problems will be familiar to readers of Foraging Theory (which items to eat?), but their context (diving) requires techniques that have been developed since 1986. Eiders work harder when they are hungry, so their foraging is state-dependent. The digestive demand created by bulky prey and the periodicity in prey availability mean that their foraging decisions are time-dependent (dynamic). Predators are an ever-present menace, and eiders may employ variance-sensitive tactics to help meet demand. Furthermore, the intense foraging of a hundred eiders throughout an Arctic winter in a small polynya must have a strong influence on the benthic community as these prey organisms employ their own strategies to avoid becoming food for eiders.
All these topics have been developed greatly since 1986. This book argues that foraging has grown into a basic topic in biology, worthy of investigation in its own right. Emphatically, it is not a work of advocacy for a particular approach or set of models. The enormous diversity of interesting foraging problems across all levels of biological organization demands many different approaches, and our aim here is to articulate a pluralistic view. However, foraging research was originally motivated by and organized around optimality models and the ideas of behavioral ecology, and for that reason, we take Stephens and Krebs's 1986 book as our starting point. We aim to show that the field has diversified enormously, expanding its purview to look at topics ranging from lipids to landscapes.
A colleague recently asked when we would finally be able to stop testing the patch model. Our answer was that there is no longer a single patch model, anymore than there is a single model of enzyme kinetics. The patch model and the way it expresses the concept of diminishing returns is so useful that it plays a role in working through the logic of countless foraging contexts. Hence, it often helps in developing hypotheses—which is what we are really interested in testing. In exactly analogous ways, working scientists everywhere use the conceptual structure of their discipline to develop and test hypotheses. If their discipline is healthy, it expands the concepts and methods it uses, just as we feel has been happening in foraging research.
We have aimed the text at a hypothetical graduate student at the outset of her career, someone reading widely to choose and develop a research topic. This book is best used in an introductory graduate seminar or advanced undergraduate reading course, but should be useful to any biologist aiming to increase his familiarity with topics in which foraging research now plays a role. We begin with a chapter-by-chapter comparison with Stephens and Krebs (1986) to give a brief overview of how the field of foraging research has developed over the past two decades, identify the main advances, and introduce students to the basics.
1.3 A Brief History of Optimal Foraging Theory
Interest by ecologists in foraging grew rapidly after the mid-1960s. Scientists in areas such as agricultural and range research already had long-standing interests in the subject (see chap. 6 in this volume). Entomologists, wildlife biologists, naturalists, and others had long been describing animal diets. So what was new? What generated the excitement and interest among ecologists?
We believe that the answer to this question is symbolized by a paper published by the economist Gordon Tullock in 1971, entitled "The coal tit as a careful shopper." Tullock had read the studies of Gibb (1966) on foraging by small woodland birds on insects, and he suggested in his paper that one could apply microeconomic principles to understand what they were doing. (We do not mean to suggest that Tullock originated this approach, merely that his paper clearly expressed what many ecologists were thinking.) The idea of using an established concept set to investigate the foraging process from first principles animated many ecologists. This motivation fused with developing notions about natural selection (Williams 1966) and the importance of energy in ecological systems to give birth to "optimal foraging theory" (OFT). The new idea of optimal foraging theory was that feeding strategies evolved by natural selection, and it was a natural next step to use the techniques of optimization models.
Although the terminology differs somewhat among authors, the elements of a foraging model have remained the same since the publication of Stephens and Krebs's book. At their core, models based on optimal foraging theory possess (1) an objective function or goal (e.g., energy maximization or starvation minimization), (2) a set of choice variables or options under the control of the organism, and (3) constraints on the set of choices available to the organism (set by limitations based on genetics, physiology, neurology, morphology, and the laws of chemistry and physics). In short, foraging models generally take the form, "Choose the option that maximizes the objective, subject to constraints." A specific case may be matched with a detailed model (e.g., Beauchamp et al. 1992), or a model may conceptualize general principles to investigate the logic underlying foraging decisions, such as whether an encountered item should be eaten or passed over in favor of searching for a better item.
We now regard the rubric "optimal foraging theory," used until the mid- 1980s, as unfortunate. Although optimality models were important, they were not the only component of foraging theory, and the term emphasized the wrong aspects of the problem. "Optimality" became a major focus and entangled those interested in the science of foraging in debates on philosophical perspectives and even political stances, which, needless to say, did more to obscure than to illuminate the scientific questions. A few key publications will enable the reader to appreciate this history and the intensity of debate. Stephens and Krebs (1986) reviewed the issues up to 1986 (see Pyke et al. 1977; Kamil and Sargent 1981; and Krebs et al. 1983 for earlier reviews). Perry and Pianka (1997) provided a more recent review, and showed that while the titles of published papers dropped the words "optimal" and "theory" after the mid-1980s, foraging remained an active area of research. Sensing opprobrium from their colleagues, scientists evidently began to shy away from identifying with optimal foraging theory. If the reader doubts that this was a real factor, he or she should read the article by Pierce and Ollason (1987) entitled "Eight reasons why optimal foraging theory is a complete waste of time." In a more classic (and subtle) vein, Gould and Lewontin (1979) criticized the general idea of optimality in their famous paper entitled "The spandrels of San Marco and the Panglossian paradigm: A critique of the adaptationist programme" (later lampooned by Queller [1995] in a piece entitled "The spaniels of St. Marx"). Many other publications have addressed these and related themes.
A persistent source of confusion has been just what "optimality" refers to. Critics assert that it is unreasonable to view organisms as "optimal," using biological arguments such as the claim that natural selection is a coarse mechanism that rarely has enough time to perfect traits, or that important features of organisms may originate as by-products of selection for other traits. These arguments graded into ideological stances, such as claims that use of "optimality" promotes a worldview that justifies profound socioeconomic inequalities. It is difficult to disentangle useful views in this literature from overheated rhetoric, a problem exacerbated by careless terminology and glib applications on both sides. Our view is that most of this debate misses the point that "optimality" should not be taken to describe the organisms or systems investigated. "Optimality" is properly viewed as an investigative technique that makes use of an established set of mathematical procedures. Foraging research uses this and many other experimental, observational, and modeling techniques.
Nor does optimality reasoning require that animals perform advanced mathematics. As an analogue, a physicist can use optimality models to analyze the trajectories that athletes use to catch a pass or throw to a target. However, no one supposes that any athlete is performing calculus as he runs down a well-hit ball (see section 1.10 below).
The word "theory" was also a stumbling block for many ecologists, who regarded it as a sterile pursuit with little relevance to the rough-and-tumble reality of the field. Early foraging models were very simple, and their explanatory power in field situations may have been oversold (see, e.g., Schluter 1981). Ydenberg (chap. 8 in this volume), for example, makes clear the limitations of the basic central place foraging model put forward in 1979. But, informed by solid field studies (e.g., Brooke 1981), researchers identified the holes in the model and developed theoretical constructs to address them (e.g., Houston 1987). Errors in the formulation of the basic model were soon corrected (Lessells and Stephens 1983; Houston and McNamara 1985). This historical perspective shows how misrepresentative are oft-repeated claims such as, "Empirical studies of animal foraging developed more slowly than theory" (Perry and Pianka 1997). As in most other branches of scientific inquiry, theory and empirical studies proved, in practice, to be synergistic partners. Their partnership is flourishing in foraging research, and theory and empiricism in both laboratory and field are important parts of this volume.
If the basics of foraging models have remained unchanged since the publication of Stephens and Krebs's book (1986), the range and sophistication of objective functions, choice variables, and constraint sets has expanded. Mathematics has spawned new tools for formulating and solving foraging models. And advances in computing have permitted evermore computationally intensive models. The emphasis of modeling has expanded from analytic solutions to include numerical and simulation techniques that require mind-boggling numbers of computations. The last two decades have seen a pleasing lockstep among empirical, modeling, mathematical, and computational advances.
New concepts have also emerged. Some of the biggest conceptual advances in foraging theory have come from the realization that foragers must balance food and safety (see chaps. 9, 12, and 13 in this volume), an idea that ecologists had just begun to consider when Stephens and Krebs published their book in 1986. Box 1.1 outlines the history of this important idea.
A second profoundly important concept is "state dependence," the idea that the tactical choices of a forager might depend on state variables, such as hunger or fat reserves. This concept developed in ecology in the late 1970s and 1980s and is described in sections 1.8 and 1.9 below. Stephens and Krebs (1986) used the idea of state dependence in two chapters and anticipated the still-growing impact of this concept.
A third important conceptual advance not considered at all in Stephens and Krebs (1986) lies in social foraging games and the consequences of foraging as a group. Foraging games between predator and prey represent an extension of both game theory and foraging theory. Here the objective function of the prey takes into account its own behavior as well as that of the predator, and the predator's objective function considers the consequences of its behavior and that of its prey. We anticipate that these models will find application in a variety of basic and applied settings.
1.4 Attack and Exploitation Models
The second chapter of Stephens and Krebs (1986) develops the foundational models of foraging, the so-called "diet" and "patch" models. The treatment is clear and rigorous, and the beginning student is encouraged to use their chapter as an excellent starting point. In addition to the classic review articles listed above, one can find recent reviews of the published tests of these models in Sih and Christensen (2001; 134 published studies of the diet model) and Nonacs (2001; 26 studies of the patch model).
The significance of these two models lies in the types of decisions analyzed. The terms "diet" and "patch" are misnomers in the sense that the decisions are more general than choices about food items or patch residence time. Stephens and Krebs (1986) termed these models the "attack" and "exploitation" models to underscore this point, but these terms have never caught on.
The diet model analyzes the decision to attack or not to attack. The items attacked are types of prey items, and the forager decides whether to spend the necessary time "handling" and eating an item or to pass it over to search for something else. The model identifies the rules for attack that maximize the long-term rate of energy gain. Specifically, the model predicts that foragers should ignore low-profitability prey types when more profitable items are sufficiently common, because using the time that would be spent handling low-profitability items to search for more profitable items gives a higher rate of energy gain. The diet model introduced the principle of lost opportunity to ecologists, who have since used the concept in many other settings (e.g., "optimal escape"; Ydenberg and Dill 1986). The diet model considers energy gain, but the same rules apply in non-foraging situations of choice among items that vary in value and involvement time.
(Continues...)
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Table of Contents
ForewordJohn Krebs and Alex Kacelnik
Acknowledgments
1 Foraging: An Overview
Ronald C. Ydenberg, Joel S. Brown, and David W. Stephens
Box 1.1 Prehistory: Before Foraging Met Danger
Peter A. Bednekoff
Box 1.2 Diving and Foraging by the Common Eider
Colin W. Clark
Box 1.3 A Two-Player, Symmetric, Matrix Game
Box 1.4 A Two-Player Continuous Game
Part I Foraging and Information Processing
2 Models of Information Use
David W. Stephens
3 Neuroethology of Foraging
David F. Sherry and John B. Mitchell
Box 3.1 Glossary
Box 3.2 A Nobel Prize in the Molecular Basis of Memory
Box 3.3 Neural Mechanisms of Reward
Peter Shizgal
4 Cognition for Foraging
Melissa M. Adams-Hunt and Lucia F. Jacobs
Box 4.1 Learning in the Laboratory
Part II Processing, Herbivory, and Storage
5 Food Acquisition, Processing, and Digestions
Christopher J. Whelan and Kenneth A. Schmidt
Box 5.1 Modeling Digestive Modulation in an Ecological Framework
Christopher J. Whelan
Box 5.2 More than a Matter of Taste
Frederick D. Provenza
6 Herbivory
Jonathan Newman
Box 6.1 Herbivory versus Carnivory: Different Means for Similar Ends
David Raubenheimer
Box 6.2 Animal Farm: Food Provisioning and Abnormal Oral Behaviors in Captive Herbivores
Georgia Mason
7 Energy Storage and Expenditure
Anders Brodin and Colin W. Clark
Box 7.1 Neuroendocrine Mechanisms of Energy Regulation in Mammals
Stephen C. Woods and Thomas W. Castonguay
Box 7.2 Energy Stores in Migrating Birds
Åke Lindström
Box 7.3 What Current Models Can and Cannot Tell Us about Adaptive Energy Storage
Alasdair Houston and John McNamara
Part III Modern Foraging Theory
8 Provisioning
Ronald C. Ydenberg
Box 8.1 Effects of Social Interactions at Resource Points on Provisioning Tactics
Box 8.2 Provisioning and Spatial Patterns of Resource Exploitation
Box 8.3 Variance-Sensitive Provisioning
9 Foraging in the Face of Danger
Peter A. Bednekoff
Box 9.1 Allocation of Foraging Effort when Danger Varies over Time
Box 9.2 Three Models of Information Flow in Groups
10 Foraging with Others: Games Social Foragers Play
Thomas A. Waite and Kristin L. Field
Box 10.1 The Ideal Free Distribution
Ian M. Hamilton
Box 10.2 Genetic Relatedness and Group Size
Box 10.3 The Rate-Maximizing Producer-Scrounger Game
Part IV Foraging Ecology
11 Foraging and Population Dynamics
Robert D. Holt and Tristan Kimbrell
Box 11.1 Basic Concepts in Population Dynamics
12 Community Ecology
Burt P. Kotler and Joel S. Brown
Box 12.1 Isolegs and Isodars
13 Foraging and the Ecology of Fear
Joel S. Brown and Burt P. Kotler
Box 13.1 Stress Hormones and the Predation-Starvation Trade-off
Vladimir V. Pravosudov
Box 13.2 Giving-up Densities
Joel S. Brown
14 On Foraging Theory, Humans, and the Conservation of Diversity: A Prospectus
Michael L. Rosenzweig
Contributors
Literature Cited
Index