Stoichiometry of Consumer-driven Nutrient Recycling

How does consumer physiology and life history respond to them? And (3) What are their consequences for ecological processes in ecosystems? Elemental imbalances are defi ned by a mismatch between the elemental demands of a consumer and those provided by its resources. For example, carbon-tophosphorus (C:P) ratios in the suspended organic matter in lakes (i.e., algae, bacteria, and detritus) can vary between 75 and 1,500, whereas C:P ratios of Daphnia, a crustacean zooplankter, remain nearly constant at 80:1. This excess of carbon can impose a direct element limitation (in this case by P) on the consumer, as it is unable to consume, extract, and retain enough of the limiting element to achieve maximal growth and reproduction. A key concept in ecological stoichiometry is stoichiometric homeostasis, the degree to which organisms maintain a constant chemical composition in the face of variations in their environment, particularly in the chemical composition and availability of their food. As in the general biological notion of homeostasis (e.g., for body temperature in homeotherms), elemental homeostasis involves processes of regulation of elemental assimilation and retention that keep elemental composition within some biologically ordered range. Photoautotrophic organisms, such as vascular plants and especially algae, can exhibit a very wide range of physiological plasticity in elemental composition and thus are said to have relatively weak stoichiometric homeostasis. In contrast, other organisms—multicellular animals, for example—have a nearly strict homeostasis and thus can be thought of as having distinct chemical composition.