Mechanisms of Functional Response and Resource Exploitation in Browsing Roe Deer

1. The functional responses of roe deer were examined using 11 plant species. A technique to discriminate between encounter- and handling-limited processes was used, and it can be concluded that the functional response applicable to patch browsing by roe deer is governed not by the rate of encounter but by the rate of oral processing. 2. The large differences between plant species were due to variations in both parameters of the functional response: h, the time lost in biting, and Rmax, the maximum processing rate. Removing the thorns from three of the species affected these parameters differently, according to the size and density of thorns. 3. Animals took larger bites from larger patches (branches), and bite mass declined as patch exploitation progressed, implying that animals were selecting the larger items to eat first. It was demonstrated experimentally that depletion of the larger bites does occur first, and it was concluded that prey selection is an important component of herbivore foraging behaviour. 4. The gain curves for deer feeding on the different plant species are calculated as being virtually linear. Patch depression did not, in general, occur because increasing bite rate compensated for declining bite mass. Our mechanistic approach is contrasted with other approaches to describing the gain curve in the literature. 5. A priori and empirical grounds are presented for rejecting the hypothesis that resource exploitation by browsing mammals is governed by optimal patch use. Diet optimization, involving a trade-off between diet quality and quantity, offers a better explanation of herbivore foraging behaviour.