Ecological Stoichiometry Another Currency

Ecological stoichiometry, an emerging branch of ecology (Sterner and Elser, 2002; Anderson et al., 2004) has been variously described as "the study of the balance of energy and multiple chemical elements in ecological interactions" (Hessen and Elser, 2005, p. 3), "the study of the balance of elements in ecological processes" (Moe et al., 2005, p. 29), "the study of the balance of energy and materials in living systems" (Kay et al., 2005, p. 6), and "dealing with the balance of energy and chemical elements in ecological interactions and especially in trophic relationships" (Anderson et al., 2004, p. 884). The field developed primarily from pelagic, freshwater studies (Hessen and Elser, 2005). One field of concentration has explored how an imbalance of elements and energy can place strong constraints on individual organism growth and reproduction (Bruning, 1991; Sterner and Schultz, 1998; Aerts and Chapin, 2000). Another approach (Kay et al., 2005) has examined stoichiometry in an evolutionary context across multiple scales, exploring the reciprocal interactions between evolutionary processes and the elemental composition of organisms and their resources, and relating elemental ratios in organisms to phenotypic and genetic variation upon which selection can act. Yet another approach has expanded the ideas of a stoichiometric approach to biogeochemical cycles to address the sustainable acquisition of ecosystem services (Ptacnik et al., 2005). Schade et al. (2005) have provided a conceptual framework for thinking about ecosystem stoichiometry. Importantly for this book, ecological stoichiom-etry has implications for understanding temporal explicitness in resource quality and its influence on terrestrial populations in fragmented landscapes.

Ecological stoichiometry is well established in aquatic ecology but not yet in terrestrial ecology. Two recent papers on stoichiometry (Anderson et al., 2004, Moe et al., 2005) argue persuasively that ecologists interested in animal population response to resource availability need to consider the currency with which they examine plant-animal interactions. They argue that ecological stoichiome-try provides a multiple currency approach to understand the effects of resource quality. By multiple currency, they mean that rather than "abstracting populations as aggregations of individuals or biomass," organisms are represented by carbon (C), phosphorous (P), and other trace elements that allow "key feedbacks, such as consumer-driven nutrient recycling" processes (Anderson et al., 2004 p. 884). The argument is that both food quantity and quality can be incorporated into a single framework. The concept of "currency" here has two related parts: one meaning refers to the difference between the effects, or explanatory variables, being measured, viz., quality versus quantity; the other meaning refers to the metrics used. The term "multiple currency," therefore, can be interpreted to refer to measuring not only quantitative but also qualitative aspects of the resource using quantitative metrics. The message is that measuring only quantity is insufficient; quantification of the qualitative aspects of the resource base is needed. Owen-Smith (2005, p. 613) reinforced this idea whenhe stated, "the numerical approach to population dynamics is seductive, but potentially misleading through overlooking the material basis for changes in N." These papers suggest a conceptual basis for some of the observations that ecologists have made concerning plant quality and its importance to herbivore response. An understanding of ecological stoichiometry can be gleaned from these papers as well as from other papers from a workshop called "Woodstoich2004" sponsored by the Center for Advanced Study at the Norwegian Academy of Sciences and Letters and published in 2005 in volume 109 of Oikos. An additional group of papers appeared in volume 85(5) of Ecology 2004 as a Special Feature edited by D.O. Hessen and called Stoichimetric Ecology.

Box 1.1. What is stoichiometry?

Stoichiometry is the accounting, or math, behind chemistry. Traditional textbooks in chemistry explain that stoichiometry is used to calculate masses, moles, and percents within a chemical equation. While it is beyond the purpose of this chapter to delve into this in detail (readers are encouraged to look at a basic chemistry textbook for a full explanation) the following is given to provide background to understand the developing field of ecological stoichiometry. The balanced chemical equation 8Al + 3Fe3O4 ^ 4Al2O3 + 9 Fe contains aluminium (Al), iron (Fe), and oxygen (O). The numbers 8, 3, 4, and 9 are coefficients that show the relative amounts (molecules or moles) of each substance present, and can represent either the relative number of molecules, or the relative number of moles. A mole is equal to Avogadro's number (6.023 x 1023) of molecules. A mole is simply a term to denote an amount. For example, if have a half dozen apples, you have six of them. If you have a mole of apples, you have 6.023 x 1023 apples. If no coefficient is shown, a one (1) is assumed. Given the equation above, we can tell the number of moles of reactants and products. Hence we have an accounting system to work with chemical formulas. Ecological stoichiometry is extending this basic accounting system to ecological systems. Essentially, the accounting considers both the quantitative as well as the qualitative relationships involved; here the quantity and quality of the resource base are considered important and incorporated into analysis of their influence on heterotroph population response (UNC Chapel Hill Chemistry Fundamental Program 2006).

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