A SPATIAL DEPLETION MODEL OF THE INTERACTION BETWEEN BEAN GEESE AND WIGEON WITH THE CONSEQUENCES FOR HABITAT MANAGEMENT

1. Depletion models describe the behaviour of predators (or herbivores) distributed between patches of prey (or vegetation) in the absence of interference. We show how such a model can be used to describe the distribution of two herbivore species in relation to the vegetation. 2. The Yare Valley, Norfolk, England, is the major wintering site for bean geese in Britain and the population is of considerable national conservation interest. Within the Yare Valley, Buckenham marsh has traditionally been an important site for this species. 3. Field data on habitat preference, depletion rates and seasonal changes in the numbers of both wigeon and bean geese were incorporated, along with data on grass productivity, into a depletion model. This was used to determine the number of bean geese that can be sustained in the fields at Buckenham. A sensitivity analysis showed that changing the intake rates of the two species resulted in the greatest change in the number of geese that could be sustained. 4. The predicted number of bean geese that can be sustained varies with the stock grazing regime and wigeon numbers. The actual number observed on Buckenham each year varies in the manner expected from the model. The model is used to predict the consequences of severe winters, an increasing wigeon population and changes in stock management.