Interactions between coot (Fulica atra) and submerged macrophytes: the role of birds in the restoration process

Abstract

Grazing by herbivorous birds is often cited as an important factor in suppressing macrophyte development in shallow lakes undergoing restoration, thus delaying the attainment of the stable clear water state. Development and succession of macrophyte communities and size, diet and grazing pressure of coot (Hypophthalmichthys molitrixpopulations upon macrophytes, were 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 low grazing pressure on macrophytes. In summer, there was a significant relationship between macrophyte cover and bird density, illustrating the importance of macrophytes in the dispersion phase for birds following breeding. Macrophytes comprised the bulk of bird diet where they were available and the consumption of macrophytes was up to 76 fold higher than in spring. However, losses to grazing in both periods were negligible when compared to potential growth rates documented in the literature. 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 macrophyte species remain available, e.g. as developed individuals or dormant buds, grazing by birds may conceivably have an impact on the development and structure of macrophyte populations in subsequent growing seasons.The relative importance of bird grazing compared to other factors limiting the development of macrophytes in shallow lakes is discussed in the light of other experimental studies.Key wordsherbivorybird grazingbird dietmacrophyte colonisationmacrophyte growthseasonal population trendsshallow lakes