Interaction between coot ( Fulica atra) and waterlily ( Nymphaea alba) in a lake: the indirect impact of foraging

A previously unrecorded indirect impact is described of coot ( Fulica atra) on a floating-leaved species ( Nymphaea alba) during the plant growing season in a shallow eutrophic natural freshwater ecosystem, Lake Grand-Lieu, western France. The bird population was counted, the proportion of petioles cut by bird was estimated and the density of the four main invertebrate groups on the leaves was measured just after the breeding season, in July and then in September 1997 in the whole N. alba area (739 ha divided into 15 sections). Up to 6±1% of leaves were cut in July, with differences between sections (from 0.3±0.1 to 16±6%) when about 10,000 coots fed in this part of the lake. A positive relationship was recorded between the proportion of cut petioles and coot density during the first period (excluding sections where N. alba was mixed with another plant species, Trapa natans). In September, coot abundance fell to 5000 individuals and the impact decreased to 0.3±0.2% of leaves, new generations of leaves having meanwhile grown. The comparison of the mean density of invertebrates on cut and intact leaves showed a massive depletion of the beetle, Galerucella nymphaeae, present on the upper side of the leaves. The selection of this specific food item might explain this marked interaction, which was nevertheless insignificant in September because of high turn over rates of the plant.