Chemical composition is part of an organism's phenotype. It is therefore molded by natural selection and other evolutionary forces like any other aspect of an organism. Thus, the role of the organism in nutrient fluxes in ecosystems is shaped in evolutionary time. As we saw in our discussion of resource competition theory, ecological success or failure, and therefore evolutionary fitness, is tied directly to chemical content. Organism stoichiometry affects fitness. There are also more complex connections between stoichiometry and evolutionary pressures. Because of the distinct stoichiometry of structural matter, evolutionary pressures on body size or major body plans involving structure will have stoichiometric implications. In fact, anywhere organism function has some kind of distinct stoichiometric signature, evolution will be steered by and will in turn alter stoichiometry. Ecological stoi-chiometry can help us better understand some of the many selective forces on organism function. A good example is in life histories. A life history for a given species is its schedule of vital rates including growth, reproduction, and mortality. Life-history theory typically considers these schedules to be phenotypic traits exposed to selection and shaped by ecological forces. However, these vital schedules, though measurable, do not exist independent of the physical, inorganic world. Because certain vital rates have distinct chemical signature, ecological stoichiometry can illuminate key aspects of life-history evolution.
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