Is the Concept of Carrying Capacity Useful in Variable Environments?

Carrying capacity in plant-herbivore systems generally refers to the equilibrium density that can be attained by a herbivore. In this paper I have summarised and analysed models and methods for calculating the carrying capacity of mammalian herbivores. I examined the models for their ability to objectively and quantitatively estimate carrying capacity in deterministic and stochastic environments. Of the seven model types reviewed, only the interactive model was suitable for calculating carrying capacity in both types of environment. The interactive model relates plant biomass to the rate of increase and food intake of herbivores. The carrying capacity predictions of the model were tested using a red kangaroo-shrubland assemblage. I adjusted the degree of environmental stochasticity (rainfall) for the assemblage and modelled the resulting plant-herbivore dynamics. The results indicated that the concept of carrying capacity could be validly applied to deterministic, or low variance, environments but in stochastic environments that are characterised by a high degree of unpredictable environmental variance the concept is not useful for describing plant-herbivore dynamics.