Materials and Methods
Study Area and Artificial Ecological Islands
The Sanjiang Plain, located in northeast China, is the largest concentrated distribution area of freshwater marshes in China. It is not only an important ecological resource and environmental protection barrier, but also an important stopover site for many Palaearctic-realm migratory waterbird species. In past years, the wetland resources in Sanjiang Plain have been seriously degenerated or lost due to long-term excessive and unreasonable utilization and development. From 2000 to 2015, the total area of wetlands in the Sanjiang Plain decreased by 2508.56 km2, the vegetation coverage of wetlands decreased from 91.8% to 74.0%, and the habitat area that is suitable for waterfowl decreased by 20.33%. These results indicate that the support capacity of waterfowl habitats in the Sanjiang Plain has decreased significantly in the past 15 years (Liu et al. 2018; He et al. 2017).
Fujin National Wetland Park (the park for short), which covers an area of 22 km2, is located in the hinterland of the Sanjiang Plain, Heilongjiang Province, Northeast China (E 131°41′02.8″-131°46′09.2″, N 46°53′18.8″-46°56′18.5″). (National wetland park refers to a specific area approved by the state forestry administration and protected and managed in accordance with relevant regulations for the purpose of protecting wetland ecosystem, making rational use of wetland resources, carrying out wetland publicity, education and scientific research) (National Forestry and Grassland Administration. 2017).This area has a temperate continental monsoon climate with distinct seasons. There is less rain in spring and more in summer, and the temperature drops sharply and differs in autumn. The annual precipitation is approximately 608.6 mm, and the average temperature is -20.4°C in January and 22.2°C in July.
Before 2004, the park’s cofferdams were crisscrossed and cultivated, the wetlands were almost all reclaimed, and the wetland resources were severely damaged. To fully protect the wetland ecosystem, the local government decided to strengthen wetland restoration projects in 2005, through water diversion, increasing vegetation diversity, the establishment of artificial ecological island and other measures to carry out ecological restoration of the wetland, which has become a successful example of China’s conversion of farmland to forest wetland.
Due to the park’s flat terrain and the relatively uniform distribution of various environmental factors, the vegetation is dominated by a single community, including Phragmites australis andTypha . The plant diversity is low, and can only provide a single resting and foraging habitat for animals; therefore, there are few animal species and waterfowl. To create good habitats for birds and other aquatic organisms, attract birds and increase the integrity of the wetland ecosystem of the park, the local government hired the Wildfowl and Wetlands Trust (WWT, The U.K.) and domestic experts to make a scientific plan and design for the park. The city also cooperated with the German government for technical and financial purposes regarding wetland biodiversity conservation and ecological environment restoration projects.
In this project, through the construction of ecological island, the purpose of increasing the micro-habitat types and enhancing the heterogeneity of various abiotic environmental factors such as hydrology and topography was achieved. These islands can provide habitats for more species of hydrophytes and increase the diversity of plants to improve primary production in wetlands. Wildlife diversity, such as benthic animals and fish, depends on plant growth; plant growth attracts birds that feed on them, and their settling achieves the purpose of having more biological species in a relatively smaller area. The project built 6 islands in the open waters of the park in 2011 and 2013, and 12 islands were created in total. To build the islands, canals were dug in the park; the canals were expanded, and the slope habitat was increased to strengthen the hydraulic connection among water patches. The original low-lying areas were dug to a depth of more than 2 metres according to the terrain, and as a result, the water levels were distributed in different layers and regions that were adapted to the requirements of different overwintering wildlife. At the same time, the excavated earth was designed according to the terrain and stacked on relatively higher ground to form soil substrate islands (SSIs) above the water. Pebbles were placed on some of the soil islands to form pebble substrate islands (PSIs). As a result, the original plateau is now more prominent, and there is always a certain area of land at the highest water level to achieve significant topographic differences. The island shape is the frustum of a cone, with the highest point of the island rising approximately 1 m above the water surface on average (Figure 2). The edge slope of each island is different, and the shallow water zone is very limited. The total island area was approximately 0.1 km2 after it was built. Due to island collapse, the size of each island that extends out of the water currently varies from 200 m2 to 5000 m2.
Topographic changes can affect the formation of landscape patterns in the park in a way that the water, soil and other conditions also change; thus the diversity and distribution range of plants and animals in the park have been greatly enriched. Ecological islands provide a place for birds to forage, reproduce and rest. These structures, which have a good field of vision and are inaccessible to mammalian predators, can attract waterbirds, increasing the birds’ nesting population and success rate. (Momose et al. 1998). Therefore, the structure and function of the wetland ecosystem has been gradually restored, and the biodiversity of the wetland has improved.
Methods
In order to observe birds of high abundance and variety, bird monitoring was conducted and macroinvertebrate samples were collecting in the summer of 2015-2017 (mid-August). In order to analyze the effect of human disturbance on bird distribution, we asked the park management center to provide the visitor data of the wetland park for three years as a reference. In order to investigate the heterogeneity of vegetation structure and composition, a field survey was conducted in July-August 2015.
Monitoring began a half hour before sunrise and sunset, lasting 3-5 hours (04:00–9:00, 15:00-18:00) for three successive days (days without heavy wind and rains). Due to domain and time limitations, we set one line transect (about 16.3 km) and chose four open water areas that were at fixed locations inside the park for bird spotting. Each site was visited at roughly the same time of day to reduce the noise from systematic patterns (e.g., regular diurnal bird movements) that would obscure the trends observed within the park (Figure 1). A 10X binocular (SWAROVSKI SLC 10x42 WB) was used to identify and count birds. Numbers and species of all birds present within 300 m on both sides of the transect were recorded. Birds flying forward were excluded, and only those feeding in and flying within the sampling areas were recorded, and the maximum observed value was used to calculate the abundance (Taft et al. 2002; John. 2000; Chang et al. 1995; Jing et al. 2007). We use field binoculars to identify bird areas (swimming, foraging, resting, etc.) between islands versus non-island areas.
Macroinvertebrate samples were collected in a 1 m2quadrat from 20 sites with D-frame kick nets (30 cm aperture, 500 mm mesh) and then sieved with water. The retained macroinvertebrates were transferred into pre-labelled polyethylene containers. The faunal samples were fixed using buffered formalin (4%) and subsequently preserved using 70% ethanol. The organisms were identified and counted to the ”species” level (Al-Sayed et al. 2008).
Due to the collapse of some islands, 9 islands (Figure 1) were selected. Survey sample points were set on those islands, and two depth gradients were established on each island, namely, a shallow water level area (depth <40 cm) and a deep water level area (40 cm< depth <80 cm). Samples were randomly collected at each depth gradient, and repeated sampling was conducted in three directions in the shallow water habitat ”around” the islands. To prevent disturbance caused by the migration of macroinvertebrates from the islands to the open water, 11 samples were randomly selected from an open-water area (50 cm < depth < 130 cm) that was far from the islands in the park. The distance between the open-water sampling points depends on the size of the floating raft between the sampling points, ranging from 100 to 300 meters. In total, 20 samples were taken (Figure 1).
At each sampling site, we calculated the mean macroinvertebrate abundance and recorded the species. For the benthic communities in each group identified by the cluster analysis, the average density and number of species (considering each taxon as a species) were determined. An initial multivariate analysis was performed using the standardized species matrix in a cluster analysis (Bray–Curtis hierarchical clustering), and non-metric multi-dimensional scaling (MDS) was performed using the similarity scores generated from the cluster analysis (Clarke. 1994). These analyses were performed to find any “natural groupings” based on the species matrix to check if the grouping was consistent with the artificial grouping results based on the species matrix (Butcher. et al. 2003).
We used Q-Q plots of the residuals in SPSS to compare the fit of the common distributions (normal, Poisson, negative binomial). The procedure indicated that the normal distribution fit the data well. We also calculated traditional measures of biodiversity, the Shannon-Wiener index (H’, log e) (Shannon. 1948), Margalef index (d) (Margalef. 1958) and Pielou evenness index (J) (Pielou. 1975) (species level). Due to the different sensitivities of different species of macroinvertebrates and waterfowl to environmental changes, one-way ANOVA was carried out at the order level to compare differences in the macroinvertebrates and waterfowl species abundance among the various sites in 3 years. The taxa abundances were log (x + 1) transformed to dampen the effects of the few most abundant taxa.
Analysis of similarities (ANOSIM) was used to evaluate the community similarity. Moreover, similarity percentage analysis (SIMPER) was used to determine the contributions of individual taxa towards the dissimilarity between and similarity within the groups identified by cluster analysis, both of which were included in the PRIMER V5.2 software package (Clarke. 1994; Clarke. 2006).
Pearson correlation test was carried out to prove the close relationship among waterfowl, plant and macroinvertebrate, both of which were included in the IBM SPSS Statistics 20 software package.