Waterbird Responses To Pond Abandonment At The Early Stage And Their Diet Analysis At Chongming Dongtan, China

Aquatic farming is developing rapidly in the world for the increasing population. A large number of aquaculture ponds are being constructed, and ponds are being frequently abandoned at the same time. The main reasons for pond abandonment include the occurrence of disease-causing microorganisms that force aquatic farming to stop, water contaminated by chemical toxic substances, bad management that leads to a low income, and the tangle of ownership and usufruct of ponds.A prevalent view to wetland abandonment is that it begets the decline of species, or abandoned sites are invaded by low value species. Generally speaking, wetland abandonment is disadvantage with the holistic function of species habitats. Various farming activities, such as cutting, grazing, firing, and aquatic farming, terminate in the abandoned sites. These farming activities are beneficial to the maintenance of species diversity in the wetland, because these traditional or low intensity farming activities play the role of intermediate disturbance. Species in the wetland are dependent of the farming activities to some extent.Chongming Dongtan (N 31°25'-31°38',E 121°50'-122°05') is located on the east end of Chongming Island, and the aquaculture ponds inside 98 levee of Chongming Dongtan has been abandoned since May 2008. Farming activities, such as forage casting and water management terminated with the stop of aquatic farming in the ponds, which simultaneously implies the stop of the managements and regulations to these ponds from fishing farmers. This pond abandonment event may beget the decline of waterbird diversity. Chongming Island is located in the migrant bird flyway of "East Asia-Australia", and it is the important stopover and overwintering site for migrant birds of Asia Pacific zone, and it is listed as one of the most sensitive ecological zone internationally by WWF. Furthermore, Chongming Dongtan Nature Reserve is national class nature reserve, and it was listed in the international important wetland of "Ramsar Convention" and was designated as "international important wetland". The aquaculture ponds inside 98 levee of Chongming Dongtan are the buffer area for the nature reserve. Therefore, it is undoubted that the protection to the waterbirds and their habitats of Chongming Island is of most importance.Diet is one of the most important ecological factors that affect waterbird diversity and distribution. It is of especially importance to study waterbird diet at Chongming Island, because Chongming Island is located in the middle of migrant bird flyway, and Chongming Island is the important food compensatory site for migrant bird. We tried to probe into the response of waterbird to pond abandonment in the aquaculture ponds inside 98 levees of Chongming Dongtan, and we tried to know the status of waterbird diet (macrobenthos as delegate). We conducted bird survey at 10-18 day intervals in the aquaculture ponds and adjacent park (as controlled zone) between May 2007 and May 2009, i.e. we conducted 5 surveys in summer and 6 surveys in other three seasons in the aquaculture ponds and in the park each year. At the same time, we sampled macrobenthos once each season in the aquaculture ponds and in the park in the two years, and we got the dry weight and ash free dry weight of macrobenthos. The aquaculture ponds have been abandoned since May 2008. In order to describe clearly, May 2007-May 2008 and May 2008-May 2009 were demarcated as pre-abandonment and post-abandonment in the ponds, respectively; and were demarcated as 1st time-period and 2nd time-period in the park, respectively.The results and conclusions are as follows:1). The diversities of both waterbird and macrobenthos in the ponds pre-abandonment are significantly higher than those in the ponds post-abandonment.We recorded a total of 14057 waterbird observations in the pre-and post-abandonment ponds. Of these,12154 (86.5%) waterbirds were recorded pre-abandonment and 1903 (13.5%) post-abandonment. A linear regression model showed a significant higher waterbird abundance pre-than post-abandonment in the ponds, based on the waterbird number recorded in each sample line as a count (R= 0.598, P= 0.003). We recorded a total of 60 waterbird species in the pre-and post-abandonment ponds. Of these,54 species were recorded pre-abandonment and 38 post-abandonment. A linear regression model showed a significant higher waterbird richness pre-than post-abandonment in the ponds, based on the waterbird species recorded in each sample line as a count (R= 0.661, P= 0.001). An analysis of COMDYN model showed that both the observed number of species and the estimated number of species were substantially higher pre-than post-abandonment for all seasons, and the relative species richness was obviously higher pre-than post-abandonment. The disparity in relative species richness between pre-and post-abandonment was greatest in summer and lowest in winter. Only 6 species were observed exclusively post-abandonment during our surveys. In contrast,22 species were recorded exclusively pre-abandonment. There were 4,3,6, and 2 species for summer, autumn, winter, and spring, respectively, which had significantly higher abundances pre-than post-abandonment (P< 0.05). In contrast, only 0,1,3, and 1 species had significantly higher species abundance post-than pre-abandonment. Waterbirds in taxonomic order Anseriformes varied most both in abundance and richness between pre-and post-abandonment ponds. Anseriformes was 19 times the abundance and 10 more species were found pre-vs. post-abandonment.The ecological factors, such as diet abundance and water level decline, may the main reasons that led to the reduction of waterbird diversity when the ponds were abandoned. In fact, more artificial disturbances were disadvantage with waterbird pre-than post-abandonment. These artificial disturbances, such as forage casting that may flush waterbird, and low water quality arising from rotten organic materials by casting organic forage and arising from chemical toxic substances by pond disinfection. All these activities were disadvantage with waterbird, but higher abundance and richness of waterbird were recorded pre-than post-abandonment. In brief, waterbird has adapted to the disturbance arising from aquatic farming in the ponds of Chongming Dongtan, and waterbird was even dependent of seasonal farming activities. The pond abandonment event affected negatively the waterbird diversity and was disadvantage with the holistic function of species habitats.Macrobenthos were categorized into Mollusks, Subphylurm crustaceea, Annelida, Insect larva and Others in our study. We compared density, dry weight and ash free dry weight of the forementioned five categories in corresponding seasons between pre-and post-abandonment. The results indicated that four categories, including Mollusks, Subphylurm crustaceea, Annelida and Insect larva, almost were higher pre-than post-abandonment in corresponding seasons in density, dry weight and ash free dry weight, but category Others was of no significant difference in most corresponding seasons.A large number of spiral shell and Chironomidae larva alive was casted into the aquaculture ponds aperiodically as forage by fishing farmers pre-abandonment, which formed a temporal high density of macrobenthos in the ponds pre-abandonment. This may be one of the reasons that led to density of macrobenthos significant higher pre-than post-abandonment. Furthermore, the dry period post-abandonment may also be the reason that led to density of macrobenthos significant higher pre-than post-abandonment.2). Pond abandonment event begot a new distribution of waterbird in the ponds and park (controlled zone).Variations of waterbird diversity were most obvious in taxonomic orders Anseriformes, Charadriiformes and Lariformes between pre-and post-abandonment ponds. The abundance and richness of waterbirds in Anseriformes and Lariformes declined when the ponds abandoned, but the abundance and richness of waterbirds in Charadriiformes increased. This result may correlate to water level and food available in the ponds. During pre-abandonment, the high water level were helpful to increase diversity of waterbirds in Anseriformes (mainly ducks), and fish harvesting activities begot a temporal assemblage of birds in Lariformes in winter, but high water level were disadvantage with waterbirds in Charadriiformes to prey. Therefore, there was a lower density macrobenthos post-than pre-abandonment, but there was a higher diet available in the post-abandonment ponds instead, which led to an increase of waterbirds in Charadriiformes.All the relative richness of waterbird approached to 1:1 in the corresponding seasons between 1st time-period and 2nd time-period in the park. Waterbird abundance increased significantly during 2nd time-period than during 1st time-period in the corresponding winters, but there was no significant difference in the other three seasons between the two time-periods. The increment of waterbird in the park in winter substantially consisted of waterbird in Anseriformes (mainly ducks). That waterbird in Anseriformes was abundant in the park in the two time-periods may correlate to high water level.Different waterbird responded distinctively to pond abandonment by comparing waterbirds in the ponds and waterbirds in the park. For example, abundance and richness of waterbirds in Anseriformes declined in the post-abandonment ponds, but they increased in the park during 2nd time-period. Abundance and richness of waterbirds in Charadriiformes increased in the post-abandonment ponds, but they have no obvious difference in the park between the two time-periods. Abundance and richness of waterbirds in Pelecaniformes occurred only in the pre-abandonment ponds, but they were never recorded in post-abandonment ponds and in the park during the two time-periods. The distinctive responses of waterbird indicated that pond abandonment have different impacts on waterbirds adaptive to different habitats, leading to waterbirds with ecological different habits have great disparity on pond abandonment in niche re-choice.3). Densities of macrobenthos were all higher in the ponds than in the park during the whole experimentA large number of spiral shells and chironomidae larvas alive were casted into the aquaculture ponds aperiodically as forage by fishing farmers pre-abandonment, which formed a temporal high density of macrobenthos in the ponds pre-abandonment. This may be one of the reasons that led to density of macrobenthos significant higher pre-than post-abandonment. Dry period post-abandonment led to a sharp reduction in density of macrobenthos in the ponds, but the density of macrobenthos in the post-abandonment ponds were still higher than that in the park. The reason that there was a low density of macrobenthos in the park may be as follows. Firstly, topsoil with abundant mcorbenthos and nutrition was carried away when the park was constructed in recent years, and the new riverbed was made of deep soil, which may lead to a low abundance of macrobenthos. Secondly, sandiness riverbed in the park could inhibit macrobenthos development. 4). Diet was not the key factor that begot decline of waterbird diversity in the abandoned aquaculture ponds at the early stage.There were no significant differences between the ponds pre-abandonment, the park 1st time-period and the park 2nd time-period, but they were all significantly higher than the ponds post-abandonment, when comparing waterbird abundance in the ponds and park. Variations of waterbird diversity differed completely from the distribution of macrobenthos, which indicated that diet was not the key factor that begot decline of waterbird diversity during the early stage of abandoned aquaculture pond. Habitat preference of waterbird in an area may affected by combined habitats, and such a complex variation course of waterbird diversity may be inexplicable by a single factor.Our study about waterbird responses to pond abandonment at the early stage and diet analysis may need a longer time. Our study is a phase work for the whole study about waterbird responses to pond abandonment, and we could offer the whole study with basic prophase databases and study idea.