Predicting site quality for shorebird communities: a case study on the Humber estuary, UK

Abstract

The conservation importance of estuaries is often measured by bird numbers, but monitoring numbers is not necessarily a reliable way of assessing changes in site quality. We used an individual-based model, comprised of fitness-maximising individuals, to assess the quality of the Humber estuary, UK, for 9 shorebirds; dunlin Calidris alpina, common ringed plover Charadrius hiaticula, red knot Calidris canutus, common redshank Tringa totanus, grey plover Pluvialis squatarola, blacktailed godwit Limosa limosa, bar-tailed godwit L. lapponica, Eurasian oystercatcher Haematopus ostralegus and Eurasian curlew Numenius arquata. We measured site quality as predicted overwinter survival. The model accurately predicted the observed shorebird distribution (if non-starving birds were assumed to feed on any prey or patch on which intake rate equalled or exceeded their requirements), and the diets of most species. Predicted survival rates were highest in dunlin and common ringed plovers, the smallest species, and in Eurasian oystercatchers, which consumed larger prey than the other species. Shorebird survival was most strongly influenced by the biomass densities of annelid worms, and the bivalve molluscs Cerastoderma edule and Macoma balthica. A 2 to 8% reduction in intertidal area (the magnitude expected through sea level rise and industrial developments) decreased predicted survival rates of all species except the dunlin, common ringed plover, red knot and Eurasian oystercatcher. This paper shows how an individual-based model can assess present-day site quality and predict how site quality may change in the future. The model was developed using existing data from intertidal invertebrate and bird monitoring schemes plus new intertidal invertebrate data collected over 2 winters. We believe that individual-based models are useful tools for assessing estuarine site quality. KEY WORDS: Climate change · Estuary management · Behaviour-based model · Site quality · WadersResale or republication not permitted without written consent of the publisher Mar Ecol Prog Ser 305: 203-217, 2005 site depend not only on the conditions at the site, but also the conditions at other sites both within the nonbreeding and breeding seasons (e.g. Goss-Custard 1993, Goss-Custard et al. 1995b, Gill et al. 1997. A reduction in numbers at a site might be due to a decrease in the site's quality, but might also be due to an increase in the quality of other sites. A decrease in reproductive rate or an increase in overall mortality rate will reduce the overall population size, which could reduce the number of birds using the site even though its quality had not changed. Another drawback of assessments using bird numbers is that they are not predictive and so it is often too late to take remedial action by the time the change has been detected.Because bird numbers are not a reliable way of assessing the quality of a site, another method is required. In migratory shorebirds (Charadrii), population size is a function of the interaction between (1) the...

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