Ecological Stoichiometry: The Biology of Elements from Molecules to the Biosphere. Robert Warner Sterner and James J. Elser. Princeton University Press, Princeton, NJ, 2002. 440 pp., illus. $75.00 (ISBN 0691074909 cloth).
Ecology struggles to understand the problems of complex “middle-number” systems. In these systems, the objects of study are too numerous to be treated as individuals (individuals in a population, leaves on a tree, microbes in the soil) but not numerous enough, and existing in too many configurations, for simple statistical models to apply. As a result, much of the theory seeks to reduce the dimensionality of problems and find some axis of variation along which the system's behavior becomes predictable. This approach results in some relatively simple independent variable that is linked to response syndromes that may then be characterized and organized. Examples of this include climate and vegatation pattern (e.g., the Holdridge system, which predicts biogeographic pattern as a consequence of climate) or Rosenzweig's work linking productivity to climate. Other notable examples include succession (where the organizing variable becomes time since disturbance) and the stability–diversity hypothesis. Techniques to reduce the dimensionality of complex systems to the point where they may be understood are characteristic of disciplines studying middle-number systems. Such techniques are widely used in meteorology, fluid dynamics, organic chemistry, and other fields. The development of this type of technique in ecology has been slow, however, because there are so many axes of variation to choose among.
Or perhaps the field did not yet have a theory developed enough to suggest where to look. Sterner and Elser make a strong case that the place to begin looking is in the stoichiometry, or element ratios, of organisms and their environment. Their argument—if I may take the liberty of simplifying its dimensionality—is that organisms are composed of elements, and the ratios of those elements both determine key characteristics of the organism and define its resource requirements from its environment.
The organism-based view links to three other key points. First, evolution clearly acts to affect the element ratios of organisms—few things are more basic than composition—so the stoichiometric theory can link to the theory of evolution. Second, element ratios are a strong and strongly differentiated aspect of the environment, varying with underlying geology, atmospheric deposition, and many other factors. Third, organisms exert some control over the stoichiometry of their environment by consuming and releasing elements in ratios different from those found in their bulk environment. This fact leads to complex feedbacks between environmental and organismal stoichiometry, and, if is a mismatch, can trigger both population and evolutionary responses.
The stoichiometric approach links three aspects of ecology: evolutionary constraints, organismal characteristics, and large-scale ecosystem processes. Sterner and Elser's book is a splendid exposition of the ties among the evolutionary, organismal, and ecosystem levels of organization.
Historically, the stoichiometric approach was first elaborated in marine and aquatic ecology and is associated with the pioneering work of Alfred Redfield (although the authors point to a true origin in Liebig's law of the minimum, which links limiting resources to biological productivity). In the early 1930s, Redfield observed that the ratios of several key elements in marine phytoplankton were identical to the composition of seawater, with both seawater and phytoplankton having (classically) ratios of 1:15:105 between phosphorus, nitrogen, and organic carbon. Redfield's ratios form the basis of marine biogeochemistry, and they led to the realization that much of Earth's geochemistry is controlled by life processes. From Redfield's starting point, studies of ecological stoichiometry have gone in many directions. Sterner and Elser follow these threads in many directions and in many different ecosystems.
The structure of the book is logical, building from an initial conceptual road map chapter. This chapter lays out the foundations of the idea, building strongly on a classic exposition of the role of stoichiometry by Bill Reiners (1986) by illustrating the central role element ratios play in physiology, evolution, and global cycles. The section also introduces the role of homeostasis, as element ratios are interesting only if they are actively controlled to particular levels by homeostatic processes. This section makes the point that element ratios are so central to life and life so pervasive that it takes only a very simple structure of axioms and theorems to deduce that life must have altered the global geochemical cycles from a basic knowledge of the chemical composition of biomolecules. Following this approach, Sterner and Elser conclude that it is stunning “how few steps it takes to get from statements about cellular allocation to statements about the largest spatial and temporal scales relevant to the Earth's biota.” While the balance of the book delves into more detailed and less cosmic connections, the intellectual continuity from the thermodynamics of life to the global biogeochemical cycles via the theory of evolution lies at the heart of this approach's appeal.
Following this powerful introduction with its appeal to the unity of ecology, the authors review the chemical composition of life. By reviewing the major classes of biological compounds systematically, they illustrate how both central tendencies and variability in stoichiometry originate in the chemistry of life. This pulls together a combination of very basic biochemistry found in textbooks with some fascinating tidbits. I found it particularly interesting that selection has apparently operated at the finest scales and that enzyme systems designed to capture a particular limiting element tend to have low demand for that element—e.g., the enzymes of the nitrogen fixation pathway are low in nitrogen-rich protein.
The next section, a synthesis of the role of element ratios in regulating autotrophic growth, succeeds in unifying concepts from marine, aquatic, and terrestrial ecology. To my mind, this is the deepest and most authoritative chapter in the book; it should be required reading for all ecologists. The vehicle the authors use is a tour de force. They show the essential similarities between growth of phytoplankton and growth of higher plants by demonstrating the equivalence of the Droop (phytoplankton) and Ågren (land plants) models for growth as a function of nutrient availability.
The authors take an unadvertised second major step in this chapter. By linking nutrients and light, they also illustrate that the stoichiometric theory is complementary to ecological energetics, a theme they expand on in subsequent chapters. The next few chapters deal with the stoichiometry of animals and explore how the stoichiometry of specific organisms affects communities and ecosystems. These chapters are fascinating, and they begin to build the paradigm toward larger scales. They are predominantly concerned with aquatic and marine examples.
The chapter on the “stoichiometry of consumer-driven nutrient recycling” misses the terrestrial literature associated with McNaughton, Detling, Hobbs, Holland, and Chapin and colleagues on the role of terrestrial herbivores in nutrient recycling almost entirely, providing an opportunity for an enterprising synthesizer to apply Sterner and Elser's ideas in the context of macroscopic animals. Chapters 4, 5, and 6 are largely reviews and lay the groundwork for another breakaway synthesis in chapter 7.
The authors build a predictive basis for modeling species interactions derived from stoichiometric theory. They advance into this area with examples of competition, predation, and mutualism drawn from terrestrial and aquatic settings and then tackle a more general problem. They place the problem of predicting species interaction squarely in the context of complexity theory, showing how multiple stable states, nonlinear transitions, and chaotic behavior can be predicted from simple stoichiometric rules. They also use a sophisticated idea of prediction, clearly arguing that in complex systems, prediction means more than a set of rules for deriving the state of the system at time t from its state at time t–1. Rather, they argue that the first step in prediction is to establish the types of behavior that should be expected (thresholds, alternate equilibria, etc.) from the underlying description of the system. This chapter should elevate the discussion of prediction in ecology and lies in a direct line of descent from Robert May's seminal monograph on stability and complexity in model ecosystems (2001).
Chapter 8 returns to the ecosystem level. The return is needed because, the authors note, many of the seminal studies of stoichiometry were done at the ecosystem level. This chapter recaps some exemplary issues at a wide range of time and space scales, from stand-level ecosystem processes to global change. Although this chapter samples only a tiny subset of possible work, it does provide a provocative taxonomy of stoichiometric situations in terms of their dynamical systems properties. This chapter is less an attempt at synthesis and theory than the structurally similar chapters 3 and 7; it is more an opening to the outstanding questions of today's research.
The global change section was painfully brief for me, as this is my own area of research. The insightful way the authors framed the problems left me wanting to take their text to my own computer and expand it to monograph length, preserving the structure of their argument but filling in the details and ideas from the rapidly expanding literature in this area. I think many readers will have this same urge—to take Sterner and Elser's structure and starting point and then expand their own areas of interest within the paradigm to see how they fit. I think often the fit will be good, and the inevitable instances of lack of fit will lead to further insight.
So who should read this book? This is one of those books that should be well thumbed and on the shelf of every ecologist. It has something to offer to evolutionary, physiological, behavioral, ecosystem, or global ecologists. In its broad perspective, it will inform any reader. The authors are both aquatic ecologists, and the book's depth (sorry) is greater in aquatic coverage than in terrestrial material. The review of issues for “wet” systems is very nearly comprehensive—as might be expected given the role these two authors have played in the field— and the terrestrial material is exemplary. Despite the relative underemphasis on terrestrial systems, any ecologist will find the discussions of terrestrial stoichiometry stimulating and provocative; these discussions may well inspire new investigations.
The approach of building explanation at multiple levels of organization and of building from one level to the next should be inspiring to those tired of debates about reductionism versus holism. The authors move with complete facility from biochemical and evolutionary mechanism to emergent properties of complex dynamical systems. Insights from dynamical systems thinking are sprinkled through the stoichiometric and biogeochemical literature, but Sterner and Elser place those insights at the center of the story, and they show how simple axioms about the composition of biomolecules imply complex systems-level behavior. Buy this book!