HYDROLOGICALLY DRIVEN HIERARCHICAL COMPETITION–COLONIZATION MODELS: THE IMPACT OF INTERANNUAL CLIMATE FLUCTUATIONS

Abstract

A hierarchical competition–colonization model extensively used in ecological dynamics is driven by the stress conditions resulting from the annual fluctuations in rainfall in water-limited ecosystems. The results show that ecosystem composition is very sensitive to the inclusion of realistic amounts of interannual rainfall variability. The evolutionary dynamics of competing trees and grasses exhibit scale-invariance characteristics (i.e., a power law spectra) for the temporal changes in the relative density of the species. This temporal scaling results from the internal dynamics of the competition–colonization process, which is driven by Markovian annual rainfall amounts. The model was also implemented to approximately describe the conditions of two ecosystems characterized by grass–tree competition, one near La Copita, Texas, and one in Nylsvley, South Africa. Long-term simulations run for many different characteristics of the annual rainfall amounts show that the diversity of dynamic states in the tertiary grass/tree/bare soil system (i.e., its information entropy) is at a maximum near historically observed rainfall characteristics. This suggests that, under commonly observed interannual rainfall fluctuations, water-controlled ecosystems tend to self-organize in a manner that maximizes the richness of relative species abundances. Corresponding Editor: W. K. Lauenroth.