Is the density of redshank Tringa totanus nesting on saltmarshes in Great Britain declining due to changes in grazing management?

Authors


Abstract

1. Saltmarsh habitats support c. 50% of the population of redshank Tringa totanus breeding in Britain. Between 1985 & 1996, breeding densities declined significantly by 23%. This paper tests the hypothesis that this decline resulted from changes in the extent of important saltmarsh habitats for nesting redshank, and/or a change in the intensity of grazing.

2. We surveyed breeding redshank densities, the extent of saltmarsh habitats, and the intensity of grazing on a sample of 77 saltmarsh sites around the coast of Britain in 1985 and 1996. From these data, we constructed statistical models that described breeding densities in relation to a range of habitat and grazing variables for each of the surveys, and examined changes in breeding density between the surveys, in relation to changes in the important habitat and grazing variables included in these models.

3. During both surveys, breeding densities were lowest on heavily grazed plots, and there was some evidence, from the larger number of survey sites for which data were available in 1985, that breeding densities tended to be highest on lightly grazed saltmarsh. Multiple regression modelling, incorporating a range of habitat variables and grazing intensity, also showed this effect, although in 1996 interpretation of the relationship between breeding density and grazing intensity was complicated because both grazing intensity and a habitat variable accounted for a similar component of the variance in breeding density. These models also showed that certain habitat variables were significant correlates of breeding density, particularly the extent of sea-couch grass, which was positively correlated with breeding density in both survey years. During 1985, breeding densities were also correlated with the extent of a number of other saltmarsh habitats, which did not significantly correlate with breeding densities in 1996. In addition to the measured habitat and grazing variables, densities also showed significant regional variation in Britain during both surveys.

4. Of the habitat and grazing variables included in the multiple regression models of breeding density, only the intensity of grazing changed between 1985 and 1996, showing a significant increase. Breeding densities declined most markedly on sites that had experienced an increase in the intensity of grazing from ungrazed/lightly grazed to moderate/heavily grazed. This suggests that an increase in the intensity of grazing was the most likely explanation for the decline in breeding densities observed between 1985 and 1996. Causal explanations for the increase in grazing intensity are discussed.

5. Assuming that the grazing intensity data were representative of grazing management on saltmarshes throughout Britain, then we estimate that 1665 ha of saltmarsh experienced an increase from ungrazed/light grazing to moderate/heavy grazing over the 11 years between 1985 and 1996. This is comparable to the 2100 ha of saltmarsh that are expected to be lost to erosion over the next 20 years. We also estimate that 6388 ha, or 14·6%, of saltmarsh in Britain was heavily grazed in 1996.

6. Our analysis of the redshank survey data, together with these figures, suggest that heavy grazing is a significant threat to saltmarsh habitats and its breeding redshank, on a national scale at present. We urgently need a detailed assessment of the grazing management of saltmarshes in Britain, and how grazing management is affected by agricultural policy, as a precursor for the introduction of provisions to ensure that the decline in breeding redshank does not continue.

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