The Middle and Lower Yangtze floodplain (MLY), with a wetland area of 7.7 × 10⁶ ha, accounts for 17% of the gross wetland area in China (SFA, 2014). It is one of the most densely distributed lake clusters in China, stretching over 1,000 km inland from the Yangtze estuary at Shanghai and spanning five other provinces, namely, Jiangsu, Anhui, Jiangxi, Hubei and Hunan. The floodplain wetlands primarily consist of shallow lakes, and include globally important waterbodies such as Poyang Lake and Dongting Lake. In total, there are more than 600 lakes with areas greater than 1 km², representing 37.2% of the total area of freshwater lakes in China (Wang and Dou, 1998). At present, the total area of provincial and national nature reserves intended to protect wetlands and waterbirds in the Yangtze River floodplain is 2.07×10⁶ ha, which accounts for 26.1% of the total wetland area (7.7×10⁶ ha; SFA, 2014) of the region. Sixteen National Nature Reserves make up 39.1% of the total protected area. There are also 38 Provincial Nature Reserves, accounting for a further 42.4% of the total protected area, and 57 other nature reserves, making up 18.5% of the total protected area (China’s Nature Reserve specimen resource sharing platform, http://www.papc.cn/). These protected areas include ten Ramsar Sites which are designated to protect waterbirds and their habitats: (1) East, (2) South and (3) West Dongting Lake in Hunan, (4) Poyang Lake National Nature Reverse and (5) Nanji Wetland National Nature Reverse in Jiangxi, (6) Chongming Dongtan in Shanghai, (7) Honghu Lake, (8) Chenhu Lake and (9) Wanghu Lake in Hubei and (10) Shengjin Lake in Anhui (RCS, 2010; https://www.ramsar.org/wetland/ china). It is also a priority region of the East Asian-Australasian Flyway Partnership (EAAFP) (Cao et al., 2009) (Figure 1).

Most data used in this study are sourced from coordinated waterbird surveys undertaken in the MLY floodplain in 2004 (Barter et al., 2005), 2005 (Barter et al., 2006), 2011 (Lei et al., 2011) and 2015 (Tao et al., 2015). In addition, in order to comprehensively map species distributions over different decades, we combined waterbird data collected before the 2000s, which were mostly obtained from the literature and site surveys, such as waterbird research in China (Waterbird Specialist Group of Chinese Ornithological Association, 1994) and the Status of Waterbirds in Asia 1987-2007 (Li et al., 2009). Citizen science data, such as the annual China Bird Report (2003-2007) and bird survey records from the China Bird Watching Network website (2008-2015; http://www.chinabirdnet.org/) were also combined in this study. Wintering waterbirds typically start arriving at the MLY in October and do not depart until the following March, thus waterbird records during this period were used in this study. The dataset includes waterbird species, site name, site location (latitude and longitude), number of individuals and the survey date. Coordinated waterbird surveys in some large areas are usually divided into different counting units according to, e.g., a physical geographic boundary or a specific survey boundary. At Poyang Lake, sub-lakes such as Hanchi Lake and Zhuhu Lake which are separated from Poyang Lake in winter and the Ramsar site, Poyang Lake National Nature Reserve were treated as individual survey units (or sites). Thus, some large areas were divided into several sites and survey data were analysed at the survey unit scale to compare between sites. The dataset contains 15,700 records of 89 species from 309 sites. The waterbird survey sites were located by latitude and longitude using ArcGIS 10.5 and then mapped. Information on protected areas within the MLY floodplain, including location data, was collected from China’s Nature Reserve specimen resource sharing platform (http://www.papc.cn/).

Identifying priority sites using feeding guild as a determinant, rather than single species, may provide more meaningful information for policy makers and managers to promote conservation activities (Zhang et al., 2018). Therefore in this study, we have classified the waterbird species in the MLY floodplain into five such guilds according to their main foraging habitats and diets (Wang et al., 2013; Appendix A provides a complete species list). The five guilds were: (i) tuber eaters (seven species, including cranes and swans that inhabit shallow water and wet mudflats), (ii) short grass foragers (six geese species that inhabit wet meadows), (iii) seed and aquatic vegetation eaters (18 species, mainly dabbling ducks that inhabit shallow water), (iv) invertebrate eaters (33 species, mainly shorebirds that inhabit muddy areas and the shallowest water), and (v) fish eaters (25 species including storks, egrets and herons).

The criteria and population thresholds used to identify priority sites in the MLY floodplain are shown in Table 1, which refers to the identification criteria for Ramsar Sites (RCS, 2010), and Important Bird and Biodiversity Areas (IBA, Fishpool and Evans, 2001). Sites which met at least one Ramsar or IBA identification criteria were considered as priority sites (criterion listed in Table 1).

The Importance Index (I) (Xia et al., 2017a) was used to quantify the relative priority of sites (Equation 1). It can also indicate a site’s importance and conservation value. A higher value of I represents a higher priority of a site for conservation:

where n_i denotes the population of the ith waterbird species at the survey site, N denotes the flyway population of the ith species, according to Waterbird Population Estimates Fifth Edition (WPE5) (Wetlands International, 2015), and s denotes the number of species at the site.

We also established a Conservation Effectiveness Index (C) to quantify the relative degree of protected area coverage, which was the proportion of the Importance Index under protection, calculated as Equation 2:

$C={{C}_{p}}/{{C}_{t}}$

${{C}_{p}}=\sum{_{i=1}^{s}}{{I}_{p}};\ {{C}_{t}}=\sum{_{i=1}^{n}}{{I}_{t}}$

where C_p denotes the sum of the importance indices of the sites under protection by provincial and national nature reserves in the MLY floodplain or specific provinces. C_t denotes the sum of the importance indices of all the sites in the MLY floodplain or specific provinces. Higher C index indicates the main waterbirds or their habitats were under protection.

Globally threatened species were identified according to the International Union for Conservation of Nature and Natural Resources (IUCN) Red List (http://www.iucnredlist.org/). The peak counts of each species per site were used to identify whether or not a species met the Ramsar 1% criterion. The average total count of waterbirds during the 2011-2015 period at each site was used to determine the sites’ waterbird abundance. The importance of each site was quantified using the Importance Index, determined from Equation 1. The peak count of each species in different provinces/regions (Jiangxi, Hunan, Hubei, Anhui, Jiangsu and Shanghai) or time periods (1980-2000, 2001-2005, 2006-2010 and 2011-2015) was used when we calculate their Importance Index. This was then summarized according to province or time period as the total Importance Index, which may indicate the overall waterbird abundance. A list of protected areas in the MLY floodplain and their locations was collected from the China’s Nature Reserve specimen resource sharing platform (http://www.papc.cn/). Most vector files of the boundaries of the protected areas were sourced from Resource and Environment Data Cloud Platform, Chinese Academy of Sciences (http://www.resdc.cn/data.aspx?DATAID=272), while others were vectorized manually. Individual priority sites were then overlain with the protected areas distribution map. Sites outside the protected areas were network recognized as gap areas.

Waterbird data from 309 sites throughout the MLY floodplain were analyzed. Overall, 140 sites (45% of the total) were identified as priorities, 120 of which (39% of the total) supported at least one species listed as globally threatened on IUCN’s Red List). In total, 13 globally threatened species were recorded (Table 2), including two Critically Endangered species (CR), three Endangered species (EN), and eight Vulnerable species (VU).

One hundred and fifteen sites (37% of the total) met the Ramsar 1% criterion, and eight of these sites (namely Poyang Lake NNR, Nanji wetland NNR, Duchang PNR, Zhuhu Lake and Dalianzi Lake in Jiangxi; East Dongting Lake NNR in Hunan, Shengjin Lake NNR in Anhui, Chenhu Lake in Hubei) had more than 15 species which met the 1% criterion. Twenty-seven sites (8% of the total) supported 5000 or more waterbirds and seven sites supported 20,000 or more waterbirds (criterion 3).

In total, 140 sites met either the Ramsar criteria or IBA criteria, while only 10 of these sites of them are currently designated as Ramsar sites and 54 are identified as or included in IBAs (Appendix B). The spatial distribution of these sites is shown in Figure 2. Eight sites have an Importance Index of more than 200, namely Poyang Lake NNR, Duchang PNR, East Dongting Lake NNR, Nanji wetland NNR, Zhuhu Lake, Shengjin Lake, Longgan Lake and Hanchi Lake, while 12 sites with an Importance Index between 100 to 200, and seven sites between 50 to 100. The sites are listed according to their Importance Index value in Appendix B.

The total Importance Index values from various time periods (1980-2000, 2001-2005, 2006-2010 and 2011-2015) were calculated to present waterbirds abundance changes, as well as waterbirds spatial patterns in the MLY floodplain (Figure 3). The results showed that the Importance Index of the whole floodplain decreased slightly from 2001-2005 to 2011-2015, which is suggestive of a decrease in total waterbird abundance during this period. The Importance Index was also low before 2000, however this is likely to be an underestimate due to data limitations. The overall spatial pattern is asymmetric. Jiangxi Province, with 62 priority sites, stands out as the most prominent region for waterbird conservation. Its Importance Index is twice the sum of the other regions, which collectively hold 78 priority sites.

We also analyzed the spatial patterns of the Importance Index for various feeding guilds. The results of this analysis show that waterbirds are most dependent on wetland habitats in Jiangxi Province, which had the highest Importance Index for all five feeding guilds (Figure 4). Notably, it accounted for more than 50% of the total Importance Index for tuber eaters and invertebrate eaters, and more than 40% for seed eaters and fish eaters. Hunan and Anhui accounted for 38% and 15%, respectively, for grass eaters. Hunan and Shanghai accounted for nearly 20% of seed eaters.

The total protected areas in MLY floodplain is 2.07×10⁶ ha, which account for 26.1% of the total wetland areas in this region; most are natural wetlands. After comparative analysis with the distribution of protected areas, we found that 87 of the 140 priority sites (62.1%) identified in this study are located outside of the current protected area network in the MLY.

The status of protected area coverage, along with the size of the protected areas in each region of the MLY, is shown in Table 3. Overall, we found a relatively high C index in the MLY with an average value of 75.6%. Anhui and Hunan had the highest C index values (98.3% and 97.5%, respectively), meaning most of the important waterbird sites in those provinces are legally protected. Hubei and Jiangsu, with C indices of about 80% (Hubei 84.6% and Jiangsu 79.1%), also have relatively high levels of legal protection. Jiangxi, however, has a relatively low C index (69.1%) despite its high Importance Index (twice the total sum of all other provinces). Shanghai had the lowest C index (56.9%). We can build new protected areas or expand the land areas of existing protected areas to cover all priorities sites to fill conservation gaps, this may however be unrealistic to implement.

To strengthen the protected area network in the MLY, we suggest enhancing the conservation effectiveness by increasing the total Importance Index value for protection and assigning a greater priority to sites with high individual importance values. After further analysis, six sites are proposed as the best for increasing the C index in the whole MLY to more than 85% (Figure 5). The analysis showed that this could be achieved solely by selecting six sites in Jiangxi, namely Dalianzi Lake, Linchong Lake, Qihu Lake, Chihu Lake, Chengjiachi and Dawu Lake suggesting Jiangxi Province should be the focus when developing a blueprint for a stronger protected area network in the MLY.

A spatial prioritization method was applied in this study, and wetland priorities, as well as the C index in the Yangtze River floodplain were determined from waterbird survey data obtained from multiple sources. Internationally accepted criteria and standards were used to identify sites qualified as Important Bird and Biodiversity Areas (IBA; Fishpool and Evans, 2001) or wetlands of international importance (RCS, 2010). We referred to the assessment framework and data processing procedures in Xia et al. (2017a) . C index is an important indicator for the most effective and efficient design of protected area networks. In previous studies, the extent of site protection is usually measured through conservation gap rates, that is the ratio of unprotected sites (numbers or areas) to the total sites (numbers or areas) (Xia et al., 2017a). However, when calculated in this way, it may not fully represent the rationale of why protected areas are needed and, therefore, may not be an appropriate guide for the planning of protected area networks. To investigate ways to improve on this, we evaluated the conservation efforts using a slightly different approach which gives a more ecologically meaningful result. We calculated the C index as the ratio of the sum of the Importance Index of sites covered by protected areas to the total Importance Index in the region. A higher C index means that a greater proportion of all waterbirds in a region occur at protected sites. We introduced the concept of ‘feeding guild’ into the identification of priority sites, rather than only focusing on individual species. The dynamics of feeding guilds inherently implies there is variation in habitat types in these areas. Feeding guilds can also help guide future management options within protected areas by recognizing that habitat ‘ownership’ based on clarifying the requirements of specific feeding guilds, promotes habitat conservation activities through a cost-effective way (Zhang et al., 2018).

Citizen science data and coordinated waterbird survey data were compiled in this study to maximize the accuracy and comprehensiveness of the results. Coordinated waterbird survey data are only available after 2004 in the MLY River and such waterbird survey data, supported by WWF and WWT, from 2004, 2005, 2011 and 2015 were the main data source in this study. Citizen science data (e.g. China Birdwatching Association, CBA) were particularly useful for identifying occurrences of globally threatened species. In the future, we anticipate that a large quantity of data will be generated through citizen-science approaches and rigorous data validation processes will be required during data collation and any follow-up studies (Cohn, 2008). The criteria for Ramsar Sites and Important Bird and Biodiversity Areas state that the site should ‘regularly’ support more than 1% of the population or 20,000 or more individuals. We used the average number of individuals in 2011 and 2015 when assessing the population abundance for individual sites, since no other recent synchronously collected data were available. For some species, the peak count from a single survey was used to calculate the Importance Index. These peak count values were verified by bird experts or local investigators to ensure reliable results. The observed increase in conservation values from 1980-2000 to 2001-2005 should be treated with a degree of caution, since systematic and insufficient data before 2000 may lead to underestimation. In addition, we focus on wintering waterbirds, particularly Anatidae, along the EAAF and most of the data are sourced from coordinated surveys during mid-winter. The Yangtze Estuary, e.g. Chongming Dongtan in Shanghai, is an important stop over site for shorebirds on migration in spring and autumn, hence its conservation value may be an underestimated by this study. Furthermore, we suggest that future coordinated surveys in MLY should be expanded to include migration periods (spring and autumn) so that the importance of each site for stop over migratory waterbirds is better understood, which in turn can ensure a more comprehensive and better managed protected area network is developed.

Waterbirds along the EAAF faithfully utilize a number of traditional wintering sites and studies have shown that the MLY floodplain has been a long-standing wintering habitat for waterbirds along the flyway since at least the 1960s (Cao et al., 2009). Our study suggested that the total Importance Index in the MLY floodplain declined after 2001-2005, which implies that waterbird abundance is decreasing in this region due to declines in some waterbird populations in the EAAF. However, what cannot be ignored is that as habitat degradation occurs elsewhere in China, the MLY floodplain becomes more and more critical for wintering waterbirds if they cannot find alternative habitats.

Meanwhile, the study also showed that there are 140 sites met either Ramsar or IBA criteria or both, while only ten of them are designated as Ramsar sites and 54 of them identified as or included in current IBAs, which indicates that the value of the MLY floodplain is currently underestimated.

Furthermore, this study also highlighted the unbalanced distribution pattern in the MLY. Jiangxi and Hunan account for almost 60% of the Importance Index for all five feeding guilds. For tuber eaters and invertebrate eaters, they account for in excess of 70% (Figure 4). The spatial distribution of these waterbirds, which was wide in historical periods, is beginning to show a more concentrated pattern, partly because of reduced seasonal inundation as a result of dams affecting hydrological regimes (Wang et al., 2017; Xia et al., 2017b). Moreover, a mismatch between the locations of protected areas and the provincial conservation values cannot be ignored; Jiangxi Province, which has the highest conservation value, has a relatively low C index (Table 3) in turn can ensure a more comprehensive and better managed protected area network is developed. Nevertheless, wetland conservation across the whole floodplain still needs to be further strengthened along with its important role in the flyway and potential habitat degradation in the MLY floodplain.

Scientific conservation planning coupled with effective governance is essential to achieve successful waterbird conservation (Brooks et al., 2006; Amano et al., 2018). Currently, protection of the Yangtze River floodplain has the attention of the Chinese central government who advocated ‘the integrated conservation of the Yangtze River’ in early 2016. How to allocate protected wetland areas and build a green ecological corridor is now becoming a critical issue. The approach to identify priority sites for waterbirds provides a useful example. For successful conservation planning, a comprehensive system for key habitat identification including regular review and adaptation is necessary (William et al., 2016). Waterbird habitats in the MLY floodplain have undergone dramatic changes due to factors such as climate change, habitat loss/degradation and operation of the Three Gorges Dam. The formulation of a comprehensive basin-scale conservation plan should consider the above factors, whilst having a regular review process in the future. For example, we suggest enhancing conservation and management on some sub-lakes in Poyang Lake and Dongting Lake, which have hydrological interactions with the Yangtze River during the wet season and are independent systems in the dry season. Some of these sub-lakes are already identified as important sites for waterbirds, and it is therefore easier to undertake habitat management and restoration action. However, we acknowledge that this method is based on protection at the site level, and building a protected network of wetlands flyway-wide is more important. Therefore, we should connect waterbird conservation efforts in the MLY floodplain with that of the EAAF network. Firstly, it is necessary to undertake regular systematic and coordinated monitoring at the flyway and national level. In addition, whilst flyway waterbird population estimates directly affect the evaluation of a site’s importance, the accuracy of these population estimates varies considerably from species to species. For some species, e.g. Siberian Crane Grus leucogeranus, the population estimate is quite accurate, whereas the estimated population size or population trend of Common Coot Fulica atra is quite coarse or even non-existent. Therefore, organizing internationally coordinated monitoring for all waterbird species to understand population trends, support flyway population estimates and facilitate the setting of the population 1% criterion should be urgently strengthened. Furthermore, the value and use of data collected outside of coordinated surveys (e.g. citizen science data) should also be enhanced. For example, collect as much information as possible about species distribution, as well as population trend through citizen science data to complete the protected area networks especially for the identification of important waterbird sites at all periods of the annual life cycle. To improve the overall quality and accuracy of related studies, it is vital to strengthen capacity on data integration, setting standards for multi-source data collection, collation and quality control. Furthermore, economic factors rather than conservation values tend to influence local governments when setting the location and boundaries of protected areas, which has had an adverse impact on biodiversity (Zhao et al., 2013; Ma et al., 2019). Although the situation in MLY floodplain is acceptable, there is an unbalanced pattern with low conservation indices in Jiangxi and Shanghai. Therefore, we propose that conservation planning and the process of setting areas and boundaries for protected areas should be led by central government with local government involvement. This would optimize the allocation of conservation resources, improve the efficacy of protection and circumnavigate local government desire for economic development.