Resource Competition in a Variable Environment: Phytoplankton Growing According to the Variable-Internal-Stores Model

Abstract

Many studies in population ecology and competition theory are based on models in which the consumption rate of a resource is some function of resource availability and in which the yield of new consumers is constantly proportional to the amount of resource consumed. For such models, the best competitors at equilibrium have low resource requirements, and the best competitors in nonequilibrium habitats have high maximal growth rates. In this study, I abandon the assumption of constant yield and allow consumption rates to be a function of the consumer's internal state with respect to a resource, as well as a function of external resource availability. Specifically, I study functions and parameters describing the competition of planktonic algae for a dissolved nutrient, phosphorus, that is supplied in periodic pulses. In this model, trade-offs between competitive abilities in equilibrium and nonequilibrium habitats can arise in several ways. The most important trade-offs are likely to involve the capacity to store nutrient and the relation between consumption rate and internal nutrient state. It is possible that a species may have unimpressive rates of growth and consumption as functions of resource availability but nevertheless be a superior competitor. It is also possible that no trade-offs between competitive abilities in equilibrium and nonequilibrium habitats arise: that the same species is the best competitor in all habitats. Which of these possibilities is prevalent in natural phytoplankton can only be determined empirically, by adequate knowledge of the constraints on and correlations among the physiological properties of algae.