A spatial depletion model of the responses of grazing wildfowl to the availability of intertidal vegetation

1. This paper describes the development of a spatial depletion model to predict the distribution and abundance of grazing brent geese (Branta bernicla) and wigeon (Anas penelope) in response to changes in the availability of their food on the intertidal mudflats of the Lindisfarne National Nature Reserve, north-east England. 2. The model incorporated the numbers of the two grazers, their feeding rates, the abundance of Zostera noltii, Z. angustifolia and Enteromorpha sp. (the three main plant taxa on which they feed), the non-grazing loss of vegetation and the birds' feeding preferences. 3. It successfully predicted the distribution of both grazers in both years of the study, despite considerable differences in environmental conditions between the two years. 4. The maximum number of brent geese that could be supported was sensitive to changes in the biomass of the food plants, but relatively insensitive to changes in the numbers of wigeon. The maximum wigeon carrying capacity, however, was sensitive to changes in brent goose numbers. 5. Such spatial depletion models could have general applicability in predicting the consequences of systems in which depletion is important, and the possible effects of habitat loss and environmental change.