Resource Competition in a Variable Environment: Phytoplankton Growing According to Monod's Model

Abstract

Theoretical models of interspecific competition often assume equilibrium in population and resource dynamics, an assumption that is often criticized. Departures from equilibrium are hypothesized to reduce interspecific competition and facilitate coexistence. When competition between two algae for one resource is modeled using Monod equations, the species that wins at equilibrium does not usually win when resources are supplied as periodic pulse, if the second competitor has the higher maximal growth rate. For model parameters typical of nitrogen and phosphorus limitation, this selection for species with high maximal growth rates is strong. A review of laboratory studies of algal competition is largely inconclusive concerning this prediction. Theory suggests, however, two reasons to doubt that selection for species with high maximal growth rates occurs by this mechanism in nature. First, when variability in resource supply occurs as sinusoidal fluctuations, rather than as pulses, selection for species with high maximal growth rates is weak. Second, when intracellular storage of resources occurs, selection for species with high maximal growth rates is also weak. Under either or- these conditions, it is common for algae that are superior competitors at equilibrium to be superior competitors under nonequilibrium conditions also. Under all conditions tested, coexistence of two competitors on one resource is less common than competitive exclusion. Lack of equilibrium may, however, be a likely explanation for species diversity in the more complicated ecosystems with more than one resource, or more than one trophic level, that are nearly universal in nature.