Introduction

Most ecological analyses have been constructed from single-currency descriptions, ones based on single dimensions such as biomass, carbon, nitrogen, or energy. For example, the carbon budget of a forest or a lake would include the inputs and outputs and relevant interchanges of C within the ecosystem. This means of analysis has enabled a great deal of progress. However, a multivariate approach that looks simultaneously at multiple dimensions can provide additional insight and predictive power. For example, both carbon and nitrogen can be studied at once and such a description would have to include knowledge of the C:N ratio of different ecosystem components. These multivariate approaches considering multiple currencies are useful because each individual currency interacts with others. Sometimes these interactions are simple, sometimes they are complex. Some pairs of measures are tightly linked in certain locations. For example, the Ca:P ratio in different fish species is almost exactly equal to 2.3, which is the same ratio as Ca:P in bone. Linkages such as these result from the fact that energy and multiple substances do not flow independently through ecosystems. In other cases, element ratios can exhibit a great deal of flexibility with and across species. Understanding these linkages is the goal of ecological stoichiometry.

'Ecological stoichiometry' is a relatively new term, but stoichiometric approaches were used in some of ecology's classic studies. Considerations of limiting factors, as in Liebig's law of the minimum, are inherently stoichio-metric. Resource ratio approaches to competition explicitly include ratios of nutrient supply and nutrient content of competitors, making these models stoichiometric. The classical view of the oceans as having N and P in balance with biotic demand, as first articulated by A. C. Redfield in the middle of the twentieth century, is a stoichiometric view. Ecological stoichiometric principles are involved in studies of food quality, nutrition, nutrient recycling, and others, which have long histories of investigation.

To be more explicit, ecological stoichiometry is defined as the study of the balance of energy and multiple chemical elements in ecological interactions. Most will be familiar with the concept of stoichiometry from introductory chemistry classes where it is used to analyze chemical reactions to identify, for example, the reactant that might limit the formation of some specific chemical product in a chemical reaction. In any chemical reaction when reactants combine to form products, mass must balance during these atomic rearrangements. Ecological stoichiometry seeks to apply this same line of reasoning to understand some of the factors regulating ecological processes such as trophic interactions (herbivory, predation, detritivory), competition, energy flow, and biogeochem-ical cycling. In considering the stoichiometry of a single chemical reaction, knowing the elemental composition of the reactants and products is essential. Similarly, in ecological stoichiometry, one must know the elemental composition of the organisms involved, and their abiotic world.

Consider the familiar example of the enzyme-catalyzed and light-driven reaction of photosynthesis:

This reaction involving carbon dioxide and water as reactants, and glucose, water, and oxygen as products in actuality is the net outcome of dozens of individual reactions. It summarizes the overall requirements for CO2 and H2O needed for the formation of a glucose molecule by this complex, multistep biochemical process. Our concern here is that the fixed chemical structure of glucose firmly establishes the chemical bounds of the system's behavior. If only one carbon dioxide molecule is added to the above, CO2 will be in excess, and no more glucose molecules can be produced due to limitation by the other reactant, water. Considering the stoichiometry of this reaction calls attention to the powerful ways in which chemistry imposes constraints on biology. All chemical elements on the left side of the reaction must be accounted for on the right, thus explaining a very important outcome of photosynthesis: the production of O2. Also, the specific chemical formula of the desired product determines what mixture of reactants can be optimally used.

Ecological stoichiometry takes the same approach. However, instead of considering one to perhaps dozens of reaction steps, it summarizes the chemical balance of ecological transactions that are the net outcome not of dozens but perhaps of tens of thousands of reactions that comprise an organism's entire metabolism (its 'metabolome'). Despite this leap in reaction number, the law of mass balance for all constituent elements must still be obeyed. Functional organisms cannot be constructed with arbitrary proportions of chemical elements. Thus, stoichiometric constraints impose order on ecological interactions same as they do on individual chemical reactions. Most ecological interactions involve some form of transfer of matter. These fluxes strongly control productivity and community structure both in terms of absolute magnitude (the fertility or richness of the habitat) and in terms of relative abundance (the ratios of limiting resources). Elements such as nitrogen and phosphorus act like limiting reagents in the highly complex set of chemical reactions that take place during organism assimilation, growth, and decay.

Two classical papers in ecological stoichiometry in particular provided groundwork for subsequent studies. The oceanographer A. C. Redfield noted the quantitative similarity in terms of N:P ratios between the mean chemical composition of deep oceanic waters and the chemical composition of active plankton in surface waters. He hypothesized that the similarity was not accidental. This 'Redfield ratio' of 16N:1P atoms was considered by Redfield to be evidence that the biota exerted a large-scale influence on the chemical composition of the sea, a new perspective relative to prevailing views that did not assign the biota with a major causative role in global chemical cycling. This stoichiometric process, operating over extremely large spatial and temporal scales, imposes a biotic fingerprint on the chemistry of the abiotic world. The influence of the Redfield ratio on oceanic biogeochemistry is difficult to overestimate but more recent developments have come to place this finding in a broader context.

The terrestrial biogeochemist W. A. Reiners proposed that the study of the chemical signatures of living things (such as their C:N:P ratios) and of their ecological coupling in nature provides a 'complementary' perspective on ecosystem dynamics, supplementing understanding derived from the then-dominant single currency bioener-getics perspective. He argued that, at the core of living things, what he called 'protoplasmic life', chemical composition was relatively constrained. However, around their protoplasmic core, living things deployed drastically different materials for structural support. These structural materials may have dramatically different elemental composition, for example, C-rich cellulose in plants, Ca-rich shells in mollusks, Si-rich frustules in diatoms, or P-rich bones in vertebrates. The evolution of these major structural adaptations, and the subsequent proliferation of biota bearing them, in turn had major impacts on ecological dynamics and ultimately on large-scale biogeochemical cycling. Reiners's argument highlights the importance of protoplasmic versus structural allocations in determining organismal elemental composition. While indeed this is important, it is also true that even 'protoplasmic' life can vary in elemental composition in important ways due to differences in biochemical allocations connected to growth status and life-history strategy (as described below).

These examples illustrate a general pattern in which photoautotrophic organisms (cyanobacteria, algae, vascular plants) are generally thought to exhibit great plasticity in elemental composition. In contrast, heterotrophic organisms (including bacteria but especially metazoans) regulate their elemental composition more strictly around particular values. The contrast of these physiological strategies has profound consequences for food web dynamics and energy flow and nutrient cycling in food webs.

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