Functional Response of Browsers: Tree Exploitation by Moose

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Abstract
The partial consumption of plants (i.e. ramets) shown by many browsers and other mammalian herbivores implies that a mechanistic functional response model for such systems must account for the response with respect to both ramet attack rate and the biomass consumption rate. We tested such a functional response model together with three 'classical' models using moose feeding in artificial stands of birch in winter. The moose showed a decelerating functional response to both increasing tree density and total biomass available. In both cases, the 'random patch model' best described the data, as judged from the model parameter estimates by non-linear least square regressions. It is concluded that functional response models that assume constant handling time and/or attack coefficients with varying prey densities (such as the 'disc equation', the 'random predator equation' or the 'random parasitoid equation') obviously cannot be appropriate for browsers. Rather, "patch" models involving optimization of handling time for each ramet appears to be more appropriate.
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