African Savanna herbivore communities: A model based on competition for food resources.

摘要

Availability of food and its quality may be a strong influencing factor in structuring African ungulate communities in terms of species richness, dominance, and diversity. This hypothesis has been explored in several cross-sectional ecosystem comparison studies, but it has rarely been addressed in any modelling studies. As a consequence, a resource-based, multi-species model was developed to include a large variety of herbivore species feeding in a complex heterogenous landscape, demarcated by habitats of differing forage quality. The spatial distribution of herbivores across a landscape is assumed to follow that described by the ideal free distribution (IFD). Two analytical methods are developed to describe the distribution of individuals across a range of habitats of differing size and intrinsic quality. The quality of the habitat is determined from the feeding energetic of herbivores. The energy budget model incorporates the effect of body size and forage quality on digestive constraints of herbivores and the effect of resource abundance on grazing rate. Interspecific competition occurs in habitats that are shared due to collectively grazing on the same resource. Whereas, intra-specific competition is modelled as reducing the grazing rate of an individual because of interference effects from conspecifics. The system of equations to describe the community dynamics was solved numerically to produce vegetation and population trajectories. The model provides insight as to how food competition, vegetation abundance and quality, collectively influence the structure and dynamics of large mammal communities in the African savannas. In the model, self-limiting is one factor that allows for the coexistence of more species than there are resources or habitats. Finally, simulations were designed to explore the implication of food availability and its quality on structuring ungulate communities. The results of the simulation show that ungulate biodiversity peaks at intermediate productive sites and decreases at low and high productive sites. These and other model results are compared to findings from cross-sectional ecosystem comparison studies. An extension of the model to incorporate other factors such as grazing succession, predation, interseasonal variation in rainfall is briefly discussed.