INTERTIDAL HABITAT LOSS AND WILDFOWL NUMBERS - APPLICATIONS OF A SPATIAL DEPLETION MODEL

1. A spatial depletion model of the responses of grazing wildfowl to t he availability of intertidal vegetation at Lindisfarne National Natur e reserve, north-east England, was used to investigate the capacity of the site's beds of Zostera and other intertidal vegetation to support brent geese Branta bernicla hrota and wigeon Anas penelope. 2. Recent total winter counts of brent geese and wigeon were both only 40% of t he maximum that the food supply at the site could theoretically suppor t. Other factors must have been restricting their numbers. Earlier arr ival of brent geese at the site could increase the number of brent goo se-days which could be supported, but would have only a slight negativ e effect on the wigeon-days. 3. The model was used to examine three co nservation issues: encroachment of Spartina anglica, sea level rise an d loss of food plants from the whole site (which could result from inc reased autumn storms or plant disease). Loss from the top of the shore through encroachment by Spartina anglica had the greatest effect on t he site's capacity to sustain geese and wigeon. Loss from the bottom o f the short, as would occur through sea level rise, had less impact. I ncreased loss of vegetation over the whole site would have an intermed iate effect. 4. This work has important implications for the managemen t of the site. Factors such as hunting, that may be restricting curren t numbers below those that could be supported by the food supply, requ ire urgent investigation. Model predictions indicate that encroachment of Spartina is likely to depress local populations of brent geese and wigeon under current conditions only if it results in the complete lo ss of Zostera from the top 500 m of the shore.