SEASONAL BIRD ASSEMBLAGES IN A MEDITERRANEAN PATCHY WETLAND: CORROBORATING THE INTERMEDIATE DISTURBANCE HYPOTHESIS

Abstract

ABSTRACT: The structure of bird as-semblages recorded in breeding and wintering period in a Mediterranean wetland of Central Italy was studied in order to evaluate their sea-sonal changes and the influence of land cover on their parameters. We identified three habitat macro-types, according to a man-disturbed gra-dient (from natural to man-made macro-types). Bird assemblages showed differences in relation to season and habitat type, either at quantitative (species richness, diversity index) or qualitative level (species composition and turnover, simi-larity). In winter, we obtained the higher values of mean species richness, Margalef richness, Shannon diversity, and β-diversity. The dendro-gram of similarity showed a major clear-cut divi-sion between seasons, and a secondary division among habitat macro-types. In winter, assem-blages were more rich and diverse, with a high turnover among point counts. The semi-natural habitat macro-types showed always the higher values of these indexes, with the exception of the β-diversity, which showed the highest value in winter but the lowest in the breeding period. The high mean values of richness (α-diversity) of the semi-natural habitat macro-types, especially in the breeding period could be mainly explained by the ‘intermediate disturbance hypothesis’; on the contrary, differences in turn-over (β-diversity) between the seasons are mainly due to seasonal changes in habitat heterogeneity of this habitat macro-type (from an arid homogeneous pasture to a patchy flooded one). A pattern of intermedi-ate disturbance was evident at mean species rich-ness level (significant values) but not at diversity index level (not significant values). These results could imply specific strategies for the manage-ment of small wetlands, focused on maintaining a regime of natural (flooding) and man-made (grazing) disturbances.