Carrying capacity of shorebirds at Jiuduansha Wetland during the migratory seasons
九段沙湿地鸻形目鸟类迁徙季节环境容纳量

The carrying capacity of food supply was measured in a stopover site at the Yangtze River Mouth for the migrating shorebirds during the two migratory seasons (spring and autumn). From March to April and September to October 2005, the macrobenthos resources of Jiudhansha wetland were investigated, and the most macrobenthos species in the new-formed shoal were defined as the appropriate foods for shorebirds. The results of biomass measurement showed that the total food resources were 4541.20 kg AFDW (Ash Free Dry Weight) in spring and 2279.64 kg AFDW in autumn. Another calculation was undertaken only in the available habitats (Intertidal Bare mudflat and Bulrush zones) for the shorebirds, the food resources in the available area was 3429.03 kg AFDW in spring and 1700.92 kg AFDW in autumn. Based on the classification of shorebird community (by lean weigh, basic metabolic rate and body length) and the energy depletion model, the whole food supply in Jiuduansha wetland can support 3.5 million shorebirds during spring season and 1.75 million during autumn season theoretically. The carry capacities of shorebirds in term of the available food were 2.6 million and 1.3 million during the two migratory seasons. Considering the effect of intake rate, the potential carrying capacity was 0.13-0.26 million shorebirds in the study area. The geographic information indicated that the main restrictive factor on shorebirds number in this area was the scarcity of available habitats when high tide periods rather than food supply. We recommend restoring some wading pools in the dense reed or Spartina sp. zones for shorebirds as the management strategy, by which to improve the utilization efficiency of the resources in Jiuduansha wetland for the shorebirds.