AN EVALUATION OF INTERTIDAL FEEDING HABITATS FROM A SHOREBIRD PERSPECTIVE - TOWARDS RELEVANT COMPARISONS BETWEEN TEMPERATE AND TROPICAL MUDFLATS

Macrozoobenthic communities of intertidal soft sediments are reviewed worldwide from the perspective of a mollusc-eating shorebird species. Based on 19 sites, total biomass figures varied between 5 and 80 g AFD M per m2(average 24 g AFDM per m2); no latitudinal trends are apparent . The contribution made by bivalves and gastropods varies between 1% a nd 99%, north-temperate intertidal flats having relatively more mollus cs than tropical flats. Intertidal flats in the tropics contain a grea ter variety of taxa, with brachiopods in Indonesia and echinoderms in northwest Australia contributing significantly to biomass only there. Limits to the occurrence of avian predators of intertidal benthos are set by the harvestable fraction of the biomass on offer and the costs of living at a particular site. No systematic differences in the harve stable fraction of the total mollusc-biomass for a worldwide occurring shorebird species specializing on molluscs (knots Calidris canutus) w ere apparent between temperate and tropical intertidal areas, in spite of large differences in maintenance metabolism incurred by these bird s. The harvestable fractions of bivalves in the two West African areas (Banc d'Arguin, Mauritania and Guinea-Bissau) tended to be high (23-8 4% of total biomass in six species), they were relatively low (2-52% i n five species) in the temperate Wadden Sea and the tropical northwest Australian site. Harvestable biomass determines the intake rate of sh orebirds, as illustrated by functional-response curves of knots feedin g on two bivalves species. We argue that the collection of information on size-depth relationships along with faunal and biomass surveys at a range of sites is bound to greatly increase our understanding of bot h the biology of tidal-flat invertebrates and the resource base under- pinning the spectacular seasonal migrations of shorebirds.