The Research On The Characters Of Migratory Waterbird Communities And The Habitats Restoration Strategy At The Yangtze River Mouth

The mouth of the Yangtze River is one of the most ecologically sensitive zones in the world with the indigenous bio-diversity and increasing degenerative pressure, comprising abundant coastal mudflats including the Chongming Dongtan Ramsar wetland, mudflats in the southern Yangtze estuaries and north Hangzhou Bay, Jinshan islands, Changxing & Hengsha islands.Over two million Shorebirds migrate to Australia along the East Asian-Australasian Flyway that stretches from central Siberia and western Alaska south through eastern Asia to Australia and New Zealand. The location of the mouth of the Yangtze River suggests that this area may be the important stopover site for the migrants during the migratory seasons. However, as one of rapidest economic developing metropolises of China, Shanghai which located in the Yangtze River Mouth has about 18 million people and a territory of 6500 km~2, with the highest density of population in the world. Within last 20 years, about 400 km~2 coastal intertidal mudflats were reclaimed. Such an excessive reclamation has nothing benefit on the wildlifes and the habitats.Wetland waterbirds group concludes shorebirds and swimming birds, the research takes the shorebirds as the main objective with some swimming birds as sub-objective to understand: (1) Shorebird use of wetlands at the mouth of the Yangtze River during spring and fall migration; (2) The effects of reclamation on shorebirds; (3) The community characters and habitats selection of the waterbirds at the degenerative and the artificial wetlands; (4) Carrying capacity of shorebirds at new-formed wetland during the migratory seasons. Based on which, the study aimed to discuss some elementary suggestions for wetlands protection, ecosystem restoration, birds' habitats optimization and sustainable managing.The brief of research process as follows:1. From 1984 - 2004, we conducted censuses, monitored hunting activities, and banded birds at the mouth of the Yangtze River to understand how shorebirds used the study area. Shorebird counts and banding data showed that the ratios of shorebird numbers between northward migration period (spring) and southward migration period (fall) varied from 1.5:1 to 7.2:1 at different sites from 1984 - 2005, and ratios between seasons of 1991 and 1992 based on hunted birds were about 2:1. The most common species observed during spring (northward) migration were Great Knots (Calidris tenuirostris), Red Knots (Calidris canutus), Bar-tailed Godwits (Limosa lapponica), Sharp-tailed Sandpipers (Calidris acuminatd), and Red-necked Stints (Calidris ruficollis). In spring 2003-2004, 96.98% of the shorebirds observed were adults, and 3% were after hatching year birds. In contrast, almost all (94.73%) of birds counted in fall were identified as juveniles. These findings indicate that adults either use a different migration route in the fall or pass by the mouth of the Yangtze River without stopping. Juveniles, however, may depend on coastal stopovers for feeding during their first southward passage.2. From March 2004 to January 2005, we conducted seasonal shorebirds censuses in ten coastal habitats along the south Yangtze River mouth and North Hangzhou Bay, examining the relative seasonal abundance of shorebirds and their spatial distribution simultaneously. A total of 25 species were identified, the seasonal dominant species were Great Knot (Calidris tenuirostris), Sharp-tailed Sandpiper (Calidris alpine) and Red-necked Stint (Calidris ruficollis) in spring; Kentish Plover (Charadrius alexandrinus), Common Greenshank (Tringa nebularia) and Lesser Sand Plover (Charadrius mongolus) in summer; Kentish Plover, Red-necked Stint and Common Greenshank in autumn; Dunlin (Calidris alpine), Kentish Plover and Marsh Sandpiper (Tringa stagnatilis) in winter. These species accounted for more than 85% of all shorebirds counted. The numbers of shorebirds counted were highest in spring and then in autumn, winter and summer consequently. Among the four seasons, there were little significant differences of bird species abundance between the sites outside seawall (natural mudflat) and the sites inside seawall (artificial wetland), but the average density of shorebirds had obvious difference. The habitat-selection analysis of the environmental-factor (outside and inside the seawall) impacting on the shorebird community was made in the ten study sites with Canonical Correspondence Analysis. The study results indicated that: (1) Outside the seawall, total intertidal the widths of intertidal mudflat and bare mudflat were the key factors affecting the shorebirds; the proportion of bulrush (Scirpus × riqueter) covering and supertidal mudflat width had the positive correlation with the bird abundance, the human disturbance and the proportion of reed (Phragmites communis) / smooth cord-grass (Spartina alterniflora) covering had the negative impacts on the bird contribution; (2) Inside the seawall, mudflat with sallow water and the proportion of mudflat were the key factors to increase the bird abundance; the proportion of bulrush covering should benefit to the shorebird appearance. The habitats with heavy human disturbance, dense reed and smooth cord-grass bed or high water table were not good to be used by shorebirds. 3. Until now the influence of intensive reclamation on the shorebirds utilizing the shoreline was little known. The shorebird distribution in ten fixed sample sites in 1984/85 and 2004/05 and the physical changes in inter-tidal wetland and coastal land-use at these sites were investigated. In 2004/05, 25 shorebirds species were counted. The statistical results show that the seasonal abundance of shorebirds present was similar to those recorded in the 1980s, but the bird species counted in both north and south migration periods declined from 1984/85 to 2004/05. During 1984-2004, taking the area of intertidal mudflat in 1984 as the reference point, there has been an overall accretion of between 430 km~2 - 550 km~2 of intertidal mudflat in the south Yantze River estury (SYRE) and north Hangzhou bay (NHB) areas over the last two decades. By 1998, approximately 143 km~2 (SYRE) and 32 km~2 (NHB) of intertidal mudflats had been reclaimed and there was approximately 247 km~2 (SYRE) and 106 km~2 (NHB) intertidal mudflats remaining. In 2003, the two areas of further reclaimed land increased to approximately 266 km~2 (SYRE) and 98 km~2 (NHB). After carrying out fieldwork along the shoreline using GPS points based on the satellite map of 2003, we found that the mudflats that were visually classified from the map were areas that had been reclaimed but were currently undeveloped. These areas maintained natural characteristics of intertidal mudflat, as the tide could still access the area providing nutrients for the macrobenthos. These reclaimed, yet undeveloped areas covered about 151 km~2 in SYRE and 40 km~2 in NHB, which could be considered as relatively available habitats for the shorebirds of high value. Most shorebirds were squeezed into those residual reclaimed but unused mudflats and the spatial distribution of shorebirds has changed significantly, because the physical characteristics of the mudflats were transformed since the decades. The width of mudflats and upper intertidal zones were the key variables affecting the distribution of shorebirds. Some implications for conservation on reclamation rules/guidelines and land-utilization procedures for artificial wetlands were proposed.4. From December 2004 to May 2005, an avian study was undertaken in the artificial wetlands of 200-hm~2 area behind the 1998's seawall in Chongming Dongtan, Shanghai. A total of 56 bird species of 8 orders and 15 families were counted in the winter of 2004, and the dominant species were waterbirds and shorebirds; 55 species of 10 orders and 19 families were counted in the spring of 2005, which were dominated by shorebirds. Meanwhile, the environmental factors including water level, water area, vegetation coverage, macrobenthos density, fishing status and human disturbance were measured quantitatively. Based on multi-regression analysis between bird community and environmental variables, the study results indicated that in winter, the vegetation coverage positively correlated with bird species abundance; the water level, water area and fishing status impacted on the bird number and diversity of species, genus and family; and the macrobenthos density affected the number and evenness of birds. In spring, the water area had positive relationship with bird species abundance, but the water level became a negative factor on the bird species abundance and number; the birds species diversity and evenness were affected by water level, water area and vegetation coverage; macrobenthos density correlated with bird diversity positively. The research could not prove that the fishing activities produced a significant impact on the bird community. 5. The carrying capacity of the food supply was measured in Jiuduansha wetland, a stopover site of Yangtze River Mouth, for migrating shorebirds during the migratory season (spring and autumn). From March-April and September-October 2005, the macrobenthos resource of Jiudhansha wetland was investigated, and the most macrobenthos species were defined as appropriate foods for the shorebirds. The results showed that the total food resource was 4541.20 kg AFDW (Ash Free Dry Weight) in spring and 2279.64 kg AFDW in autumn. Another calculation was undertaken only within available habitats (Intertidal Bare mudflat and Bulrush zones), the food resource of available habitats in this area was 3429.03 kg AFDW in spring and 1700.92 kg AFDW in autumn. Based on the shorebird body size classification (lean weigh, basic metabolic rate and body length) and energy depletion model, the maximum of carry capacity in term of the all food supply in Jiuduansha wetland can support 3.5 million shorebirds during spring and 1.75 million during autumn. The carry capacity of shorebirds in term of the available food was 2.6 million during spring and 1.3 million during autumn. Considering the effect of intake rate, the real carrying capacity was 0.175-0.35 million shorebirds in the study area.In conclusion, based on the research results, we take the Jiuduansha wetland as planning objective to bring forward some suggestions for wetlands protection, ecosystem restoration, birds' habitats optimization and sustainable managing, which as follows:· The seasonal dominant birds group is shorebirds in spring and fall; the finches are most in summer; the swimming birds are the dominant species in winter.· The width of mudflats and upper intertidal zones are the key factors affecting the shorebirds distribution. The area with high human disturbance, high water level and dense reed/Spartina plants cover produced a negative impact on the shorebirds.· In the artifical wetland, the habitat variables of water area, water level and vegetatable density play different roles for shorebirds and swimming birds.· The main factor of restriction on shorebirds number in Jiuduansha wetland was the scarcity of available habitats rather than food supply, and we recommend restoring wading pools in the reed or Spartina areas as management strategy to improve the use efficiency of the resource for the shorebirds.· Acorrding to the dominant birds groups during spring/fall and winter, the wetland physical characters could be resized and controlled to make use of the nature resource reasonably and sustainably.