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III.15 Ecological Stoichiometry R. W. Sterner and J. J. Elser OUTLINE 1. What is ecological stoichiometry? 2. Major patterns of nutrient content in organisms 3. Influence of stoichiometry on animal growth and community structure 4. Nutrient cycling in ecosystems 5. Influence of stoichiometry on species dynamics 6. Whole-lake food web experiments 7. Light:nutrient ratios and the ecology of Australia Ecological stoichiometry examines how the nutrient content of organisms shapes their ecology. Although the chemistry of living things is constrained by their need to have a certain representation of major biomolecules such as DNA, RNA, proteins, lipids, etc., there is enough flexibility in these allocations that different species have nonidentical chemical contents. Thus, community structure is related to the portioning of elements in ecosystems. Stoichiometric considerations play a role in the rate of growth of animals, in the rates of recycling of elements by food webs, in the rate of mineralization of nutrients from organic matter, and in many other ecological phenomena. Stoichiometric models often have complex dynamics not seen in models lacking explicit treatment of stoichiometry, which suggests that stoichiometry is an important force shaping ecological dynamics . GLOSSARY autotroph. An organism that converts inorganic carbon to organic carbon and thus does not need to ingest or absorb other living things. Green plants (including certain algae and cyanobacteria) are photoautotrophs because they use light energy to make this conversion. ecological stoichiometry. The balance of multiple chemical substances in ecological interactions and processes , or the study of this balance. geophagy. The eating of dirt. This behavior may be used to balance mineral intake for animals living in low-food-quality environments. growth rate hypothesis. Differences in organismal C:N:P ratios are caused by differential allocations to RNA necessary to meet the protein synthesis demands of rapid biomass growth and development. heterotroph. An organism that relies on organic carbon for energy. Heterotrophs include herbivores, carnivores , and detritivores as well as omnivores that may feed on more than one trophic level. homeostasis. Maintenance of constant internal conditions in the face of externally imposed variation. In ecological stoichiometry, homeostatic regulation of organism nutrient content causes some species to have narrower bounds to their chemical content than others. nullcline. A set of points in an ecological model where the rate of change of one species is zero (it is at equilibrium). In community models, intersections of nullclines indicate points where more than one species is at equilibrium. nutrient content. The quantity of an element in an organism ’s biomass. May be measured as moles or grams per organism, as the percentage of mass made up by a given element, or as the X:C ratio, where X is a nutrient such as N or P. threshold element ratio. The nutrient ratio of an organism ’s food where that organism switches from limitation by one of those elements to limitation by another. For example, in the case of C:P, when food is above the TER, that organism will be limited by P, and when food is below the TER, that organism will be limited by C. 1. WHAT IS ECOLOGICAL STOICHIOMETRY? Some branches of ecology are oriented toward understanding the dynamics of individual species, and others focus on the fluxes of matter and energy among collections of species in ecosystems. Ecological stoichiometry fits between these two approaches because it deals with the patterns and processes associated with the chemical content of species. Numerous ecological phenomena from the success or failure of populations to the carbon storage of whole ecosystems have a stoichiometric component. The term ecological stoichiometry is relatively recent, but the field is based on some of the most classic of ecological studies. Formally defined, ecological stoichiometry is ‘‘the balance of multiple chemical substances in ecological interactions and processes, or the study of this balance .’’ In addition, ecologists interested in stoichiometry often consider the availability of solar or chemical energy relative to the availability of one or more chemical substances. Ecological stoichiometry is concerned with the contents of multiple elements in living and dead organic matter. There are approximately 90 naturally occurring elements, of which 11 predominate in living organisms. Only four of these (C, H, O, and N) make up about 99% of living biomass; the other seven (Na, K, Ca, Mg, P, S, and Cl) are essential to all living things. About 10 others, metals and nonmetals, are required by most but not necessarily all species. Finally, about eight other elements are required by more limited...


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