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 seaso n in a shallow eutrophic natural freshwater ecosystem, Lake Grand-Lieu, wes tern France. The bird population was counted, the proportion of petioles cu t by bird was estimated and the density of the four main invertebrate group s on the leaves was measured just after the breeding season, in July and th en in September 1997 in the whole N. alba area (739 ha divided into 15 sect ions). 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 thi s part of the lake. A positive relationship was recorded between the propor tion of cut petioles and coot density during the first period (excluding se ctions where N. alba was mixed with another plant species, Trapa natans). I n September, coot abundance fell to 5000 individuals and the impact decreas ed to 0.3 +/- 0.2% of leaves, new generations of leaves having meanwhile gr own. The comparison of the mean density of invertebrates on cut and intact leaves showed a massive depletion of the beetle, Galerucella nymphaeae, pre sent on the upper side of the leaves. The selection of this specific food i tem might explain this marked interaction, which was nevertheless insignifi cant in September because of high turn over rates of the plant. (C) 2001 El sevier Science B.V. All rights reserved.