Despite claims to the contrary, the science of ecology has a long history of building theories. Many ecological theories are mathematical, computational, or statistical, though, and rarely have attempts been made to organize or extrapolate these models into broader theories. The Theory of Ecology brings together some of the most respected and creative theoretical ecologists of this era to advance a comprehensive, conceptual articulation of ecological theories. The contributors cover a wide range of topics, from ecological niche theory to population dynamic theory to island biogeography theory. Collectively, the chapters ably demonstrate how theory in ecology accounts for observations about the natural world and how models provide predictive understandings. It organizes these models into constitutive domains that highlight the strengths and weaknesses of ecological understanding. This book is a milestone in ecological theory and is certain to motivate future empirical and theoretical work in one of the most exciting and active domains of the life sciences.
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About the Author
Samuel M. Scheiner is a theoretical biologist and has been on the faculty of Northern Illinois University and Arizona State University. Michael R. Willig is professor of ecology and evolutionary biology and director of the Center for Environmental Sciences and Engineering, both at the University of Connecticut.
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THE THEORY OF Ecology
The University of Chicago PressCopyright © 2011 The University of Chicago
All right reserved.
Chapter OneA General Theory of Ecology
Samuel M. Scheiner and Michael R. Willig
In the absence of agreed protocols and overarching theory, Ecology with its numerous subdisciplines, can sometimes resemble an amorphous, postmodern hotel or rabbit warren with separate entrances, corridors and rooms that safely accommodate the irreconcilable. Grime 2007
The development of theory in ecology is a lively and robust enterprise (Pickett et al. 2007). Despite claims to the contrary, the science of ecology has a long history of building theories that fruitfully guide research and deepen understanding. Our goal with this book is to reveal a selection of those theoretical structures. In doing so, our hope is that ecologists will better appreciate the theoretical frameworks within which they do research, and will more thoroughly engage those theories in designing observational, experimental, and modeling components of their research. Many theories in ecology contain unspoken or even subconscious assumptions. By bringing such assumptions to the forefront, we can understand their consequences, and discover new mechanisms, patterns, and linkages among theories. Theory sometimes seems to be distant or disconnected from everyday practice in ecology. By the end of this book, the relevance of theory to understanding in ecology and its role in advancing science should become clear.
In this chapter, we present a general theory of ecology that serves as the supporting framework—a conceptual infrastructure—for the constitutive theories that appear in subsequent chapters. Although those chapters span the disciplinary range of ecology, they are representative rather than comprehensive. We could not possibly synthesize the full richness of ecological theory in a single book without it becoming encyclopedic. We encourage others to continue the process of theory development in other venues, and to reengage theoretical discourse with ecological research (e.g., Pickett et al. 2007).
We do not claim novelty for the general theory of ecology that we put forward. Quite the contrary, the elements of the general theory have existed for at least 50 years. Many of its principles are implicit in the tables of contents of most ecology textbooks, although our previous treatise (Scheiner and Willig 2008) was their first formal explication. In this chapter, we expand our earlier discussion of the structure of theories and the framework that underlies theory in ecology, providing a foundation for the chapters that follow.
Importantly, we do not claim that the theory presented here is a final version. Rather, it should be considered provisional and ever changing, a general characteristic of theory that is oft en misunderstood by nonscientists. Indeed, the list of fundamental principles that we present will require additions, deletions, or refinements as ecological theory matures and is confronted by empirical evidence. Critically, this debate can occur only after explication of the theory. In the process of assembling this volume, we convened a workshop of the contributors at the Center for Environmental Sciences and Engineering of the University of Connecticut. At that workshop, a fundamental principle emerged that was not considered in our previous paper (Table 1.3, number 3 below). The theory of ecology is, in turn, embedded within an even broader theory that encompasses all of biology (Scheiner 2010). As that broader theory continues to evolve it may alter the structure of or our understanding of this theory.
The structure of theories
Before we present our general theory of ecology, we must describe the essence of theory and its structure (Tables 1.1 and 1.2). Theories are hierarchical frameworks that connect broad general principles to highly specific models. For heuristic purposes, we present this hierarchy as having three tiers (a general theory, constitutive theories, and models); however, we do not suggest that all theories fit neatly into one of these three categories. Rather, the framework will oft en stretch continuously from the general to the specific. The three tiers illustrate that continuum, and provide a useful way of viewing that hierarchy. The definitions and principles of the general theory are meant to encompass a wide variety of more specific constitutive theories, which in turn contain families of models. This view of constitutive theories as families of models is consistent with how theories are treated across all of biology and in other sciences (van Fraassen 1980; Giere 1988; Beatty 1997; Longino 2002; Pickett et al. 2007; Wimsatt 2007; del Rio 2008; National Research Council 2008).
Each theory or model applies to a domain. The domain defines the universe of discourse— the scope of the theory—delimiting the boundaries within which constituent theories may be interconnected to form coherent entities. Constitutive theories are oft en most fruitful when they focus on one or a few phenomena in need of explanation (e.g., Hastings Chapter 6; Sax and Gaines Chapter 10). Without such boundaries, we would be faced with continually trying to create a theory of everything.
Nonetheless, we recognize that domains are somewhat arbitrary conceptual constructs and that theories or models may have overlapping domains. Changing the domain of a model can be a fruitful avenue for juxtaposing phenomena or processes that had been considered in isolation. For example, microeconomic theory uses three concepts—utility, income, and price—to understand consumer choices (Henderson and Quandt 1971; Mansfield 1979). Choices are assumed to maximize utility, subject to income and price constraints. Behavioral ecologists study the economics of choice for nonhuman animals and have applied conceptual constructs and mathematical models from economics to understanding foraging ecology and space utilization (Stephens and Krebs 1986; see Sih Chapter 4). Recent examples of such borrowing of models across domains include the use in ecology of maximum entropy from thermodynamics theory (Harte et al. 2008; McRae et al. 2008) and connectivity models from electrical circuit theory (McRae et al. 2008).
All theories and models contain simplifying assumptions so as to focus other characteristics of a system. The problem with many assumptions is that they are unstated, even subconscious. Making such assumptions explicit sometimes may change the focus of the theory. For example, a fundamental principle of ecology is that ecological traits arise through evolution, but nearly always this is an unstated and ignored assumption. Models of community assembly usually ignore phylogenetic relationships among species. Recently, models that incorporate phylogenetic relationships have added substantially to our understanding of community assembly (e.g., Kraft et al. 2007).
Sometimes, such unstated assumptions can turn around and bite us. Most models of life history evolution assume that organisms can always adopt the optimal phenotype, instantaneously reallocating resources from growth to reproduction, and so ignoring evolutionary and developmental constraints. Ignoring this assumption led to predictions that were biologically improbable, e.g., an organism should allocate 100% of its resources to reproduction one day after devoting 100% of its resources to growth (Schaffer 1983), or an annual plant should switch multiple times between growth and reproduction (King and Roughgarden 1982).
Principles and propositions
When asked to describe a theory, we oft en think of a set of broad statements about empirical patterns and the processes that operate within a domain. For the sake of clarity, we use different terms to refer to those broad statements when we speak of general theories (fundamental principles) versus when we speak of constitutive theories (propositions). In part, fundamental principles are similar to propositions. Each can be a concept (labeled regularities) or a confirmed generalization (condensations of facts). They differ in that fundamental principles are broader in scope, oft en encompassing multiple interrelated patterns and mechanisms. Because constitutive theories are meant to guide the building of specific models, their propositions should be more precise statements that represent the potential individual components of those models.
Propositions can be laws: statements of relationship or causation. The propositions are where the fundamental principles of the general theory are integrated. For the general theory of ecology, some of the principles involve patterns, others involve processes, many involve both (see below). Thus, the causal linking of process and pattern, the lawlike behavior that we look for in theories, occurs through the propositions of the constitutive theories.
Laws reside within constitutive theories, and not as part of the general theory, because no single law is required for the construction of the models in all of ecology's subdomains. Several chapters show, however, that ecology is rich in laws that hold within more limited domains (see discussion in Willig and Scheiner Chapter 15). A brisk debate has occurred over whether ecology has any laws at the level of its general theory (e.g., Lawton 1999; Murray 2000; Turchin 2001; Berryman 2003; Simberloff 2004; O'Hara 2005; Pickett et al. 2007; Lockwood 2008), which is related to the debate about laws across all of biology (e.g., Beatty 1997; Brandon 1997; Mitchell 1997; Sober 1997). The continuing search for such laws is an important aspect of a theory's evolution.
The reaction of many to confirmed generalizations is, "Well, isn't that obvious?" In reality, the answer is no. Oft en such generalizations are obvious only after their explication. Generalizations serve as reminders about assumptions contained in lower-level theories or models. For example, a fundamental principle in ecology is that ecological processes depend on contingencies (see below). Yet many ecological theories and models are deterministic and ignore the role of contingency or stochasticity in molding patterns and processes in nature. Deterministic models are not wrong, just potentially incomplete. Sometimes ignoring contingencies has no effect on model predictions. At other times, the consequences can be profound. As the statistician George E. P. Box is reputed to have said, "Essentially, all models are wrong, but some are useful."
Fundamental principles keep prodding us to test assumptions. For example, one fundamental principle tells us that species are made up of individuals that differ in phenotype. Nonetheless, many ecological theories assume that species consist of identical individuals. Although this is a useful simplification in many instances, it is important to be reminded continually about this assumption and its consequences to predictive understanding. Similarly, many of the fundamental principles consider variation in the environment or species interactions, yet many constitutive theories or models average over that variation (Clark 2010).
Not all assumptions within a constitutive theory derive from the fundamental principles of its general theory. Some assumptions derive from other domains. If an assumption is taken unchanged from another domain it may be unspecified within a theory. For example, all constitutive theories in ecology take as given the conservation of matter and energy, fundamental principles from the domain of physics. We take as given the fundamental principles of any other general theory. As such, we recognize the general tenet of consilience: the entire set of scientific theories must be consistent with each other (Whewell 1858). The decision to explicitly include such assumptions as fundamental principles within the theory under consideration depends on whether those assumptions are subject to test within that theory. Since no theory in ecology would ever test the conservation of matter, it lies outside those theories.
Theories may clash, but such clashes indicate foci of research that advance understanding. In general, theories inhabiting different domains will not clash directly, although results from one domain can point to problems with theories in other domains. For example, studies of geographical distributions of clades of organisms within the domain of historical biogeography became important evidence for the theory of continental drift , a part of the domain of geology. In that instance, the need for a causal mechanism to explain distribution patterns was a factor that led to the development of new fundamental principles in another domain.
At the lowest level of our theory hierarchy are models. Models are where the theoretical rubber meets the empirical road. Many ecological theories are just such models. Although scientific theories encompass a wide variety of types of models, including physical models (e.g., Watson and Crick's ball and wire model of a DNA molecule), in ecology we generally deal with abstract or conceptual models. These models may be analytic, statistical, or computational.
Models are where predictions are made and hypotheses are tested. Those predictions can run the gamut from general qualitative predictions (e.g., increases in primary productivity will lead to increases in species richness) to very specific quantitative predictions (e.g., an increase in soil nitrogen of 5 ppm will result in an increase in average species richness of 4.3 species). The prediction can be a point estimate if the model is deterministic, or it can be a distribution of values if the model is stochastic. The models that make those predictions can be very simple (e.g., equation 7.1 in Holt Chapter 7) or highly complex (e.g., figure 12.4 in Peters et al. Chapter 12). A particular constitutive theory can encompass many different types of models. Because general theories consist of families of models, they very rarely rise or fall based on tests of any one model. Alternative or competing models exist within most theoretical constructs in ecology (e.g., Pickett et al. Chapter 9) allowing a single theory to encompass a diversity of phenomena.
Recognizing that what is oft en labeled as a theory is but one model within a larger theory can help to clarify our thinking. For example, Scheiner and Willig (2005) assembled an apparently bewildering array of 17 models about species richness gradients into a framework built on just four propositions. A similar process of clarification can be found in Chapter 8, where Leibold shows that all metacommunity theories can be captured within a single framework of just two characteristics: amount of interpatch heterogeneity and dispersal rate. Other chapters in this book provide further examples of model unification. This process of model unification has begun to take hold in other areas of ecology (e.g., McGill 2010). We disagree, however, with McGill's claim that to be unified a theory can contain just a single model. Rather, a strength of our approach to theory unification is the ability of a theory to embrace model diversity.
Because theories oft en consist of families of models, it is possible for models to be inconsistent or even contradictory. Sometimes, such inconsistencies point to areas that require additional empirical evaluation or model development. But sometimes contradictory models can be maintained side-by-side because they serve different functions or are useful under different conditions. For example, in some physics models, light is treated as a particle and in others as a wave. There is no need to insist that contradictory models always be reconciled or that one always prevail. Instead, this apparent contradiction is resolved at a higher level in the theory hierarchy by a more general theory, for example one that allows for both wave-like and particle-like behavior of light. The apparently contradictory models are built from differing sets of propositions arising from different assumptions and thus refer to different domains. In a similar fashion, constitutive theories can be contradictory if they are built with different assumptions.
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Table of Contents
FOREWORD James P. Collins
1. A General Theory of Ecology: Samuel M. Scheiner and Michael R. Willig
PERSPECTIVES ON THE ROLE OF THEORY IN ECOLOGY
2. Theory Makes Ecology Evolve: Jurek Kolasa
3. A General, Unifying Theory of Ecology?: Jay Odenbaugh
CONSTITUENT THEORIES OF ECOLOGY
4. Foraging Theory: Andrew Sih
5. Ecological Niche Theory: Jonathan Chase
6. Single Species Population Dynamics and Its Theoretical Underpinnings: Alan Hastings
7. Natural Enemy-Victim Interactions: Do We Have a Unified Theory Yet?: Robert D. Holt
8. The Metacommunity Concept and Its Theoretical Underpinnings: Mathew A. Leibold
9. Domain and Propositions of Succession Theory: Steward T. A. Pickett, Scott J. Meiners, and Mary L. Cadenasso
10. The Equilibrium Theory of Island Biogeography: Dov Sax and Steven D. Gaines
11. Theories of Ecosystem Ecology: Ingrid C. Burke and William K. Lauenroth
12. Perspectives on Global Change Theory: Debra P. C. Peters, Brandon T. Bestelmeyer, and Alan K. Knapp
13. A Theory of Ecological Gradients: A Framework for Aligning Data and Models: Gordon A. Fox, Samuel M. Scheiner, and Michael R. Willig
14. Biogeographical Gradient Theory: Robert K. Colwell
15. The State of Theory in Ecology: Michael R. Willig and Samuel M. Scheiner