Ecological stoichiometry (more broadly referred to asbiological stoichiometry) considers how thebalance of energy and elements influences living systems. Similar tochemical stoichiometry, ecological stoichiometry is founded on constraints ofmass balance as they apply to organisms and their interactions inecosystems.[1] Specifically, how does the balance of energy and elements affect and how is this balance affected by organisms and their interactions. Concepts of ecologicalstoichiometry have a long history inecology with early references to the constraints of mass balance made byLiebig,Lotka, andRedfield. These earlier concepts have been extended to explicitly link the elementalphysiology oforganisms to theirfood web interactions and ecosystem function.[2][3]
Most work in ecological stoichiometry focuses on the interface between an organism and its resources. This interface, whether it is between plants and theirnutrient resources or largeherbivores andgrasses, is often characterized by dramatic differences in theelemental composition of each part. The difference, or mismatch, between the elemental demands of organisms and the elemental composition of resources leads to an elemental imbalance. Considertermites, which have a tissuecarbon:nitrogen ratio (C:N) of about 5 yet consumewood with a C:Nratio of 300–1000. Ecological stoichiometry primarily asks:
Elemental imbalances arise for a number of physiological andevolutionary reasons related to the differences in the biological make up of organisms, such as differences in types and amounts ofmacromolecules,organelles, andtissues. Organisms differ in the flexibility of their biological make up and therefore in the degree to which organisms can maintain a constant chemical composition in the face of variations in their resources. Variations in resources can be related to the types of needed resources, their relative availability in time and space, and how they are acquired. The ability to maintain internal chemical composition despite changes in the chemical composition and availability of resources is referred to as "stoichiometric homeostasis". Like the general biological notion ofhomeostasis, elemental homeostasis refers to the maintenance of elemental composition within some biologically ordered range.Photoautotrophic organisms, such asalgae andvascular plants, can exhibit a very wide range of physiological plasticity in elemental composition and thus have relatively weak stoichiometric homeostasis. In contrast, other organisms, such as multicellularanimals, have close to strict homeostasis and they can be thought of as having distinct chemical composition. For example, carbon tophosphorus ratios in the suspended organic matter inlakes (i.e.,algae, bacteria, anddetritus) can vary between 100 and 1000 whereas C:P ratios ofDaphnia, acrustaceanzooplankton, remain nearly constant at 80:1. The general differences in stoichiometric homeostasis between plants and animals can lead to large and variable elemental imbalances between consumers and resources.
Ecological stoichiometry seeks to discover how the chemical content of organisms shapes their ecology. Ecological stoichiometry has been applied to studies ofnutrient recycling,resource competition,animal growth, and nutrient limitation patterns in whole ecosystems. TheRedfield ratio of the world's oceans is one very famous application of stoichiometric principles to ecology. Ecological stoichiometry also considers phenomena at the sub-cellular level, such as the P-content of aribosome, as well as phenomena at the whole biosphere level, such as theoxygen content ofEarth's atmosphere.
To date the research framework of ecological stoichiometry stimulated research in various fields of biology, ecology, biochemistry and human health, includinghuman microbiome research,[4] cancer research,[5] food web interactions,[6] population dynamics,[7]ecosystem services,[7] productivity of agricultural crops[7] and honeybee nutrition.[8]
The study of elemental ratios (i.e., C:N:P) within the tissues of organisms can be used to understand how organisms respond to changes in resource quality and quantity. For instance, in aquatic ecosystems, nitrogen and phosphorus pollution within streams, often due to agricultural activities, can increase the amount of N and P available to primary producers.[9] This release in the limitation of N and P can impact the abundance, growth rates, andbiomass of primary producers within the stream.[10] This change in primary production can trickle through the food web via bottom-up processes and impact the stoichiometry of organisms, limiting elements, and biogeochemical cycling of streams. In addition, bottom-up changes in elemental availability can influence the morphology, phenology, and physiology of organisms that will be discussed below. The focus of this article is on aquatic systems; however, similar processes related to ecological stoichiometry can be applied in the terrestrial environment, as well.
The demands for carbon, nitrogen and phosphorus at specific ratios byinvertebrates can change at different life stages within invertebrate life history. The growth rate hypothesis (GRH) addresses this phenomenon and states that the demands for phosphorus increase during active growth phases to produce P-richnucleic acids in biomass production and are reflected in the P content of the consumer.[11][12] During early growth stages, or earlierinstars, invertebrates may have higher demands for N and P enriched resources to fuel the ribosomal production of proteins andRNA. At later stages, the demand for particular elements may shift as they are no longer actively growing as rapidly or generating protein rich biomass. Growth rates of invertebrate organisms can also be limited by the resources that are available to them.