Regulation of phosphorus stoichiometry and growth rate of consumers:: theoretical and experimental analyses with Daphnia

Initial theories of ecological stoichiometry were based on the assumption that the mass-specific content of key nutrient elements (such as P), changes little within a consumer species. However, evidence has shown that this content changes substantially according to feeding conditions. To clarify how the specific P content (S-P) of a consumer species depends on food conditions and relates to the growth rate, we constructed a multiple mass-balance model incorporating feeding and metabolic costs and stoichiometrically regulated releases for C and P. The validity of the model was then tested experimentally by examining the growth rates and S-P of Daphnia pulicaria under various food conditions. The experimental observation agreed qualitatively well with the model, showing that the S-P of consumers relates positively to growth rate at high food C:P ratios but negatively at low food C:P ratios. Thus, within a consumer species, individuals with high S-P do not necessarily grow at high rates. The concordance in results between the model and our observation suggests that maintenance costs for both P and C are substantial regardless of food conditions and play crucial roles in determining the relationship between the S-P and growth rate of consumers.