A metaphysiological modelling approach to stability in herbivore–vegetation systems

The metaphysiological modelling approach relates aggregated population dynamics to biomass gained from resources consumed, relative to physiological attrition and mortality losses. A GMM (Growth, Metabolism and Mortality) formulation was applied to the interactive dynamics of herbivore–vegetation systems, taking into account seasonality in resource production, heterogeneity in resource quality, adaptive responses by consumers to this variability, and nutritional influences on mortality losses. In transforming the intake or functional response to changing resource availability to a biomass gain response, allowance was made for constraints on daily digestive capacity, and for changing diet quality as resources became depleted over the seasonal cycle. Because of the steeply saturating form of the intake response, the output dynamics was inherently unstable in a constant environment with a single, uniform resource, for realistic parameter values. Stabilization resulted when resources were heterogeneous and herbivores adjusted their resource selection adaptively over the course of the year. This mechanism caused the gain response to diverge from the intake response, due to changing diet quality over the annual cycle. In seasonally varying environments, no equilibrium between resource production and consumption persists. Nevertheless, dynamic stability can emerge from the adaptive responses of consumers to spatial and temporal variability in resource availability and quality.