A key aspect to ecological stoichiometry is the degree of variation of elemental composition of an organism or species. Is elemental composition a fixed trait characteristic of that species, or is it a flexible parameter that largely reflects local resource and environmental conditions? This contrast is captured in a quantitative way in the concept of 'stoichiometric homeostasis', the degree to which the elemental composition of an organism maintains a strict chemical composition (like a glucose molecule) despite variation in the chemical composition of resources. Stoichiometric homeostasis can be parametrized in the form of a homeostasis coefficient, H (eta), as:
where m is the slope of a plot of the elemental composition of the consumer under consideration against the elemental composition of the resource being consumed. Both the x- and j-variables should be log-transformed in this measure. In this approach, if the consumer shows no variation in its elemental composition despite wide variation in its resource supply, then the slope of such a line (m) approaches zero and H approaches infinity (strict homeostasis). On the other hand, if the consumer's elemental composition exactly tracks that of its resource supply, then m takes the value of 1 and H has a value of 1 (no homeostasis, complete plasticity).
Data for real organisms range between these extremes and can depend on the experimental conditions imposed. Strict homeostasis does not appear to be reached in many situations, although strong homeostasis appears to be the rule for many metazoans. For example, in experiments in which the P-content of the algae on which it is raised varies from 1% to 0.05%, the P-content of the crustacean herbivore Daphnia varies little (only from 1.5% to 0.9%, corresponding to H of ~7, strong but not strict homeostasis). In contrast, the green alga Scenedesmus, when grown on inorganic nutrient supplies with N:P ratios ranging from 5 to 80, shows almost a 1:1 correspondence between its biomass N:P ratio and that of its medium. In this case of weak homeostasis, H approaches a value of 1.
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