Ecological stoichiometry

Ecological stoichiometry considers how the balance of energy and elements affect and are affected by organisms and their interactions in ecosystems. Ecological stoichiometry has a long history in ecology with early references to the constraints of mass balance made by Liebig, Lotka, and Redfield. This research area in ecology has recently gained momentum by explicitly linking the elemental physiology of organisms to the their food web interactions and ecosystem function.

Most work in ecological stoichiometry focuses on the interface between a consumer and its food. This interface, whether it is between plants and their resources or large herbivores and grasses, is often characterized by dramatic differences in the elemental composition of each participant. Consider termites which have a body C:N of about 5 but consume wood with a C:N ratio of 300-1000. Ecological stoichiometry primarily asks: 1) why do elemental imbalances arise in nature? 2) how is consumer physiology and life-history affected by elemental imbalances? and 3) what are the subsequent effects on ecological processes in ecosystems?

Elemental imbalances are defined by a mismatch between the elemental demands of a consumer and that present in its resources. Mismatches often arise between grazers and their food because plants and their derived organic matter vary considerably in their elemental composition while metazoan consumers have less elemental flexibility. For example, carbon to phosphorus ratios in the suspended organic matter in lakes (i.e., algae, bacteria, and detritus) can vary between 100 and 1000 whereas C:P ratios of "Daphnia", a crustacean zooplankter, remain nearly constant at 80:1. There are a number of physiological and evolutionary explanations for these differences in elemental composition that are related to the types of needed resources, their relative availability in time and space, and how they are acquired.

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 resources, is referred to as "stoichiometric homeostasis". Like the general biological notion of homeostasis, elemental homeostasis refers to the maintenance of elemental composition within some biologically ordered range. Photoautotrophic organisms, such as algae and vascular plants, can exhibit a very wide range of physiological plasticity in elemental composition and thus have relatively weak stoichiometric homeostasis. In contrast, other organisms, multicellular animals for example, have close to strict homeostasis and they can be thought of as having distinct chemical composition.

Ecological stoichiometry seeks to discover how the chemical content of organisms shapes their ecology. Ecological stoichiometry has been applied to studies of nutrient recycling, resource competition, animal growth, and nutrient limitation patterns in whole ecosystems. The Redfield ratio of the world's oceans is one very famous application of stoichiometric principles to ecology. Ecological Stoichiometry equally considers phenomena at the sub-cellular level, such as the P-content of a ribosome, as well as phenomena at the whole biosphere level, such as the oxygen content of Earth's atmosphere.

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