Predicting site quality for shorebird communities: A case study on the Wash embayment, UK

Conservation managers responsible for estuaries are often required to monitor their site to ensure that the conservation status of any bird species for which the site is considered important is not affected by deterioration of their habitat or by disturbance of the birds themselves. Here, we use an individuals-based model to predict the quality of the Wash embayment, UK, defined in this case as overwinter survival rate, for eight shorebird species. We use the model to predict how site quality would be affected by changes in the types of prey available, prey density, mudflat area and the rate at which birds are disturbed. The results suggested that Macoma, Hydrobia and Corophium had relatively little influence on site quality for any species modelled except black-tailed godwit, despite being the preferred prey for some bird species. Arenicola marina, other annelids and Cerastoderma edule were found to be important influences on site quality. Birds began to starve, when autumn, estuary-wide food biomass density was below about 5 g AFDM m−2 and survival rates fell below 90% at 4 g AFDM m−2. One possible conservation objective for the Wash estuary would be to monitor whether the 99% confidence limit of biomass density falls below one of these limits, to determine whether site quality is being maintained. The system as a whole was predicted to be relatively insensitive to habitat loss. Black-tailed godwits were the most sensitive species, but their survival was not affected until 40% of the feeding grounds were removed. The survival of all species in the model remained high at fewer than 20 disturbances/hour. Although disturbance rates on the Wash were not measured during this study it is unlikely that present-day rates of disturbance on the Wash represent a threat to the survival of the bird species modelled. Our results show how an individuals-based model can assess present-day site quality and how it may change in the future. The model predicted prey biomasses below which survival rate decreased, which shorebird species were most vulnerable to changes in site quality, and that prey density was a more important factor in shorebird survival than habitat area on the Wash. They also show such models can be used to set maximum disturbance rates for each species by predicting how disturbance rates influence shorebird survival.