A dynamic model of herbivore–plant interactions on grasslands

Abstract

A model of plant and large herbivore dynamics is presented in which individual animal intake is limited by either diet digestibility (DIG) or forage availability and in which grazing can result in sward heterogeneity. The model is used to illustrate the interaction between animal density, plant dynamics, sward heterogeneity and diet selection strategy and how this subsequently feeds back to affect animal intake and sward heterogeneity. Two contrasting diet selection strategies were simulated; selection for maximum instantaneous intake of mass (IR) and selection for DIG. For both diet selection strategies, individual animal intake initially increased with increasing animal density, as intake was limited by DIG and the greater frequency of defoliation maintained the sward in a more juvenile and digestible state. As animal density increased further, a point was reached at which intake became limited by forage availability and individual animal intake fell thereafter. The DIG strategy resulted in a higher daily intake than the IR strategy when grazing pressure was low. Here, intake was limited by digestibility and the DIG strategy led by repeated defoliation, to the maintenance of a small proportion of the sward in a state of high digestibility. The model was further used to show that selection for a strategy of long-term optimisation of sward structure through modulation of bite depth is unlikely. The model highlights the potential plasticity of the grazing process and the need to include plant dynamics in models of large herbivore–plant interactions.