INTERACTIONS BETWEEN COOT (FULICA-ATRA) AND SUBMERGED MACROPHYTES - THE ROLE OF BIRDS IN THE RESTORATION PROCESS

Grazing by herbivorous birds is often cited as an important factor in suppressing macrophyte development in shallow lakes undergoing restora tion, thus delaying the attainment of the stable clear water state. De velopment and succession of macrophyte communities and size, diet and grazing pressure of coot (Fulica atra) populations upon macrophytes, w ere monitored over the seasonal cycle at ten shallow lakes of varying nutrient status, in the Norfolk Broads in eastern England. In spring, territorial breeding birds were at relatively low density and included only a small proportion of macrophytes in their diet, resulting in lo w grazing pressure on macrophytes. In summer, there was a significant relationship between macrophyte cover and bird density, illustrating t he importance of macrophytes in the dispersion phase for birds followi ng breeding. Macrophytes comprised the bulk of bird diet where they we re available and the consumption of macrophytes was up to 76 fold high er than in spring. However, losses to grazing in both periods were neg ligible when compared to potential growth rates documented in the lite rature. Grazing experiments at two biomanipulated lakes confirmed that birds were not responsible for limiting macrophytes during the spring colonisation phase or in the summer growth period. During the period of autumnal senescence and over the winter months where some macrophyt e species remain available, e.g. as developed individuals or dormant b uds, grazing by birds may conceivably have an impact on the developmen t and structure of macrophyte populations in subsequent growing season s. The relative importance of bird grazing compared to other factors l imiting the development of macrophytes in shallow lakes is discussed i n the light of other experimental studies.