Is the density of redshank Tringa totanus nesting on saltmarshes in Great Britain declining due to changes in grazing management?
Abstract
Salt marsh habitats support about 50% of the population of redshank breeding in Britain. Between 1985 and 1996, breeding densities declined by 23%. This paper tests the hypothesis that this decline resulted from changes in the extent of important salt marsh habitats for nesting redshank, and/or a change in the intensity of grazing. Breeding redshank densities, the extent of salt marsh habitats, and the intensity of grazing were surveyed on a sample of 77 salt marsh sites around the coast of Britain in 1985 and 1996. From these data, statistical models were constructed 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. 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 salt marsh. 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 (Elymus pycnanthus), 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 salt marsh 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. 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. Assuming that the grazing intensity data were representative of grazing management on salt marshes throughout Britain, then it is estimated that 1665 ha of salt marsh 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 salt marsh that are expected to be lost to erosion over the next 20 years. It is also estimated that 6388 ha, or 14.6%, of salt marsh in Britain was heavily grazed in 1996. An analysis of the redshank survey data, together with these figures, suggested that heavy grazing is a significant threat to salt marsh habitats and its breeding redshank, on a national scale at present. It is considered that a detailed assessment of the grazing management of salt marshes in Britain and of grazing management in relation to agricultural policy is required as a precursor for the introduction of provisions to ensure that the decline in breeding redshank does not continue.