Because some individual organisms maintain a constant or approximately constant body nutrient content, ecological interactions can be studied using mass-balance principles of stoichiometry. Animals are more homeostatic in their chemical content than autotrophs such as algae, and macroelements including nitrogen and phosphorus are more closely regulated than microelements. Phosphorus is a key element in understanding trophic interactions and growth rates in planktonic and benthic ecosystems. Phosphorus content in animals relates to both body size and phylogeny. Evolutionary pressures on element content include selection for boniness in fish and selection for high levels of RNA in rapidly growing microbes and invertebrates. Some herbivores have reduced growth when carbon:phosphorus ratios in their food are high. Similarly, the rates and ratios of nutrients recycled by consumers vary stoichiometrically with food and consumer chemistry. At the ecosystem level, relative availability of light vs. nutrients regulates C:P stoichiometry and therefore ecosystem functioning.
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