Grasping complex matter : large herbivore foraging in patches of heterogeneous resources

Key words: herbivory; functional response; grazing; food intake; food quality; selectivity; cattle; savanna The functional response, defined as the relationship of forage intake with forage availability, is the principal link between a forager and a forage resource, and as such connects the trophic levels of consumers and producers. This thesis deals with the effects of forage resource structure on the shape of the functional response curve. Special attention was paid to the effects of the proportion of high quality forage within a heterogeneous forage resource, and of the spatial arrangement of high quality forage items within a matrix of low quality forage items. African Nguni cattle grazing stemmy, semi-arid grasslands were used as a model for large herbivores foraging in patches of heterogeneous forage resources. Decreasing the proportion of high quality forage decreased the asymptotic rate of forage intake and changed the shape of the functional response curve from a linear type I over an asymptotic type II to a dome-shaped type IV. Increased clustering of high quality forage items compensated for this decrease in forage intake to some extent. It is proposed that these effects result from the physical interference of low quality forage items with the cropping process in selective herbivores, thus connecting plant growth form with foraging behaviour. The generally negative relationship of forage availability with forage quality in natural grasslands suggests a dome-shaped functional response curve in the natural foraging situation, with low forage intake at high forage availability. However, because of pronounced spatial variation in forage availability and quality, foragers could utilise local foraging opportunities and achieved high forage intake and high diet quality even when on average forage availability was high and forage resource quality was low. Simulation models showed that such a system of a forager and a forage resource with variation in forage quality can reach different stable states, the major difference between these states being the exclusion or persistence of foragers in the system. It is further proposed that the spatial variation of forage availability and quality partially results from the local concentration of foraging efforts during times of plenty in a seasonally variable forage resource. This de facto resource partitioning can form the basis of species co-existence in herbivore assemblages, and can be useful in the management of species diversity in natural grasslands