Ecological stoichiometry of assemblages: Physiological tradeoffs couple competitive ability and homeostasis

Background/Question/Methods

Imbalance between the chemical composition of organisms and that of their resources is central to consumer-resource ecology and is the foundation of ecological stoichiometry. The stoichiometry of populations and individuals has been examined for a diverse suite of organisms, but comparatively few studies have examined the response of whole assemblages to resource availability. Stoichiometric regulation is coupled to competitive ability and resource requirements through constraints in physiology, suggesting that resource-dependent shifts in species abundance could mediate homeostasis of assemblages. We performed a meta-analysis of stoichiometric data and used a stoichiometrically explicit model of consumer-resource interactions to determine how the elemental composition of assemblages might respond to imbalance differently than populations of a single species. Since the same physiological constraints that determine the competitive ability of species also produce strong dependence between elemental content and stoichiometric regulation, we used these constraints to scale single-species stoichiometric regulation to the stoichiometry of assemblages.

Results/Conclusions

Using data from published studies and experiments with bacterial cultures, we show that within broad phylogenetic and functional categories (zooplankton, phytoplankton, and heterotrophic bacteria), the strength of regulation (or homeostasis) is inversely proportional to the nutrient content of individuals. Homeostatic species maintain lower biomass carbon: nitrogen: phosphorus (C:N:P) ratios than species with flexible biomass stoichiometry. For each class of organisms, the regressions of biomass ratios versus resource ratios in multiple species converge around a pivot point, or a ‘fulcrum stoichiometry’. The fulcrum stoichiometry occurs at a biomass N:P of 15.7:1 for phytoplankton and a biomass C:N:P of 104:16:1 for bacteria. These ratios are close to the Redfield ratio (C:N:P = 106:16:1) and suggest a fundamental biomass composition for plankton when dissolved resources are approximately balanced. Furthermore, the biomass ratios in non-homeostatic species are confined to a narrow region of the possible patterns of regulation. We propose a conceptual framework of ‘stoichiometric strategies’ to align the gradient of stoichiometric regulation to generalized life history strategies. The stoichiometric strategies are incorporated into an assemblage-level consumer-resource model in which simple physiological tradeoffs in physiological parameters (e.g. P affinity versus C efficiency) lead to dynamic non-homeostasis of assemblages.