Bird assemblages as bio-indicators of water regime management and hunting disturbance in natural wet grasslands

Responses of the waterbird community to the management regime of a wet grassland system (primarily small changes in the water regime but also hunting disturbance) in a large floodplain lake, Grand-Lieu, were studied during three consecutive years (winter and spring in 1998-2000). These temporarily flooding grasslands (1100 ha) constituted a very important feeding area by supporting large flocks of waterbird species (52 regular spp., totalling up to 14?250-22?850 birds). A global similar seasonal pattern in the presence of waterbird species was recorded whatever the year. The March-May period was a key phase with the maximum species richness (42-48 spp.) and the maximum abundance (30-40% of the total according to year, essentially Anseriformes). Late in spring Ciconiiformes and to a lesser extent some waders exploit this feeding area. Nevertheless some changes in bird assemblages were exhibited according to the management regime (a decreasing spring water level from 1998 to 2000, and an exceptional hunting ban in January 2000, instead of an usual stop in late February). The hunting disturbance clearly limited diurnal accessibility of ducks to wet grasslands in January and February 1998 and 1999. Also, the duck population in flooding grasslands increased by 55-65% in January 2000 when hunting was prohibited and in the same time the richness species grew from 14-19 spp. up to 23 spp. in 2000. The overall impact of a high spring water level on waterbirds as in 1998 and to a lesser extent in 1999 (with respectively 54 and 35 cm of mean water level against 25 cm in 2000) was an increase in diving fish-eating birds and larger flocks of resting gulls. Conversely these conditions, notably a reduced period with a spring water level under 25 cm in spring (20 days in 1998-1999 instead of 40 days in 2000) were adverse to numerous ground-feeding waterbirds (Ciconiiformes and waders) that require shallow waters. This study showed that limitation of human disturbance and moderate flooding of wet grasslands can increase their attractiveness for numerous species of high conservation status.