Resource Utilization Pattern and Guild Structure in a Waterfowl Community

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Abstract
Resource utilization and guild structure was studied in a waterfowl community breeding in an eutrophicated lake in southeast Finland. The community was composed of 13 spp. [Podiceps cristatus, P. griseigena, P. auritus, Anas platyrhynchos, A. crecca, A. querquedula, A. penelope, A. acuta, A. clypeata, Aythya fuligula, A. ferina, Bucephala clangula and Fulica atra] with a density of 123 pairs km-2 and a biomass of 159 kg km-2. Resource utilization was analyzed in terms of feeding habitat and feeding method. Using cluster analysis 3 and 5 interpretable main guilds were recognized along these 2 dimensions, respectively. When the 2 resource axes were considered simultaneously, the overlaps between the species were usually very low and segregation became more apparent, breaking down the clear 1-dimensional guild structure. Both cluster analysis and the principal component analysis yielded 2 similar main guilds: diving ducks and grebes; dabbling ducks and coot. The species in the 1st guild were characterized by wide niche breadths along the feeding habitat dimension and by narrow niche breadths along the feeding method dimension, while among the species in the 2nd guilds the opposite was true. The segregation of the species in the 2-dimensional niche space cannot be interpreted as evidence for competition but merely reflects group-specific adaptations to utilize resources intrinsically in different ways. The comparisons between the observed and randomized resource use overlaps using morphologically very similar species pairs did not reflect any strong competitive interactions either. Competition does not play an important role in structuring the waterfowl community studied.