Food webs, constituting the functional processes of energy flow and material cycling, are one of the most complex phenomena in modern biology. Integrating interspecific interactions in food webs has been a central organizing theme in ecology since its classical development. Ecological stoichiometry deals with the balance of energy and chemical elements in ecological interactions and especially in trophic relationships. It offers an integrative framework for such analyses, as all organisms are composed of the same major elements, whose balance affects production, nutrient cycling, and food-web dynamics. In ecological stoichiometry, animals maintain homeostasis in body nutrient composition.
Homeostasis, the resistance to change of the internal milieu of an organism compared to its external world, is a fundamental life process. Homeostasis in ecological stoichiometry is observable in the patterns of variation in nutrients in organisms relative to their external world, including the resources they eat. Homeostasis was defined by Kooijman as follows: "the term homeostasis is used to indicate the ability of most organisms to keep the chemical composition of their body constant, despite changes in the chemical composition of the environment, including their food."
Based on the vast range of empirical and theoretical studies, a number of reviews of ecological stoichiometry have recently appeared. These have highlighted that there is often a mismatch in the elemental composition of food compared to consumers, with notable implications for individual performance and nutrient transfer efficiency. Within the framework of homeostasis, the chemical composition of consumers is relatively homeostatic regardless of the chemical composition of their food and their divergent life history strategies set their stoichiometric requirements. According to stoichiometric theory, with considerable empirical support, consumer elemental composition and relative growth rate ultimately determines its stoichiometric requirements. Limiting nutrients are retained by the consumers at higher efficiencies, while other nutrients may be consumed in excess and egested or excreted. Consumers with high body N or P content and high growth rates require food with high N or P content, respectively, to maintain optimal growth. Consumers with a high nutrient demand (higher P: C or N: C ratio) are more susceptible to reductions in growth or fitness if their food resources are low in the required nutrients. On the other hand, consumers with low body N or P, or low relative growth rates have lower requirements and are less likely to suffer due to reduced food quality. Phytoplankton stoichiometric composition could vary widely with fluctuations in nutrient supply. However, zooplankton are thought to be strictly homeostatic, which suggests that zooplankton may approach a direct phosphorus limitation. The mismatch between phytoplankton and zooplankton in stoichiometric composition has important ecological consequences. Studies of stoichiometric homeostasis in zooplankton could increase understanding of energy flow and material cycling in aquatic ecosystems, and stoichiometric regulation in growth, reproduction, and metabolism of zooplankton. This paper reviews the definition of homeostasis in ecological stoichiometry and the framework of homeostasis in zooplankton. It aims to provide insight and a theoretical foundation for related studies in China.