Intertidal habitat loss and wildfowl numbers: applications of a spatial depletion model.
A spatial depletion model of the responses of grazing wildfowl to the availability of intertidal vegetation at Lindisfarne National Nature reserve, north-east England, was used to investigate the capacity of the beds of Zostera (Z. noltii and Z. angustifolia) and other intertidal vegetation to support brent geese Branta bernicla hrota and wigeon Anas penelope. Recent total winter counts of brent geese and wigeon were both only 40% of the maximum that the food supply at the site could theoretically support. Other factors must have been restricting their numbers. Earlier arrival of brent geese at the site could increase the number of brent goose-days which could be supported, but would have only a slight negative effect on the wigeon-days. The model was used to examine three conservation issues: encroachment of Spartina anglica, sea level rise, and loss of food plants from the whole site (which could result from increased autumn storms or plant disease). Loss from the top of the shore through encroachment by Spartina anglica had the greatest effect on the capacity of the site to sustain geese and wigeon. Loss from the bottom of the shore, as would occur through sea level rise, had less impact. Increased loss of vegetation over the whole site would have an intermediate effect. This work has important implications for the management of the site. Factors such as hunting, that may be restricting current numbers below those that could be supported by the food supply, require urgent investigation. Model predictions indicate that encroachment of S. anglica is likely to depress local populations of brent geese and wigeon under current conditions only if it results in the complete loss of Zostera from the top 500 m of the shore.