Inherent density-dependency of wet-season range even at the extreme of nonequilibrium environments

Abstract

There has been considerable recent progress in integrating equilibrium and nonequilibrium rangeland ecologies on the basis of landscape heterogeneity. This follows from the argument that small key resource areas provide spatial buffer effects that sustain animal populations and are prone to animal–vegetation density-dependency, whereas wet-season ranges are density-independent. Although dryland spatial heterogeneity generally occurs through a combination of heterogeneous precipitation and landscape heterogeneity, the former is rarely considered in the development of such theories. Here, we used a published multi-agent model to examine how both types of heterogeneity interact. We found that a certain level of density-dependency occurred in wet-season range, even under extreme nonequilibrium conditions. During a drought, most of the key resources and part of the wet-season range that was not subject to drought simultaneously exhibited density-dependency, reflecting the impact of rainfall heterogeneity. With decreased key resource area, the density-dependency of the wet-season range after livestock population recovery—a factor included in current theory—decreased dramatically, whereas that during drought—a factor neglected in current theory—did not decrease markedly. Therefore, our results extend current theory by suggesting that the contradiction stemmed from neglecting the spatial buffer effect of heterogeneous precipitation, which is responsible for density-dependency of wet-season ranges during drought.