The functional and aggregative responses of a herbivore: underlying mechanisms and the spatial implications for plant depletion

Summary 1.  The functional and aggregative responses of dark‐bellied brent geese Branta bernicla bernicla feeding on the green algae Enteromorpha intestinalis and Ulva lactuca are described. 2.  Bite rate showed maxima at both high and low algal biomass, although the functional response in terms of biomass intake rate was linear (type‐1). The differences in the shapes of these responses can be explained by the fact that bite size declines with decreasing algal biomass. 3.  There was little interference between the geese; the rate of aggression was only weakly related to goose density, and time spent feeding decreased only slightly with increasing density. 4.  Geese fed on all parts of the algal bed, showing weak, but significant aggregation in patches of higher algal biomass in both October and November. 5.  The observed pattern of aggregation was weaker than that predicted by the ideal free distribution with a low value of interference. 6.  Simulation models were used to explore four possible behavioural factors leading to this deviation from predictions: cost of travel between patches, constraints on the perception of patch quality, disturbance and short‐term resource guarding. It is concluded that resource‐guarding is likely to be the most important contributory factor.