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 factorin suppressing macrophyte development in shallow lakes undergoingrestoration, thus delaying the attainment of the stable clear waterstate. Development and succession of macrophyte communities andsize, diet and grazing pressure of coot (Fulica atra)populations upon macrophytes, were monitored over the seasonalcycle at ten shallow lakes of varying nutrient status, in theNorfolk Broads in eastern England. In spring, territorial breedingbirds were at relatively low density and included only a smallproportion of macrophytes in their diet, resulting in low grazingpressure on macrophytes. In summer, there was a significantrelationship between macrophyte cover and bird density,illustrating the importance of macrophytes in the dispersion phasefor birds following breeding. Macrophytes comprised the bulk ofbird diet where they were available and the consumption ofmacrophytes was up to 76 fold higher than in spring. However,losses to grazing in both periods were negligible when compared topotential growth rates documented in the literature. Grazingexperiments at two biomanipulated lakes confirmed that birds werenot responsible for limiting macrophytes during the springcolonisation phase or in the summer growth period. During theperiod of autumnal senescence and over the winter months where somemacrophyte species remain available, e.g. as developed individualsor dormant buds, grazing by birds may conceivably have an impact onthe development and structure of macrophyte populations insubsequent growing seasons.The relative importance of bird grazing compared to other factorslimiting the development of macrophytes in shallow lakes isdiscussed in the light of other experimental studies.