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The most overarching and pervasive threat to farm ponds is linked to their small size and wide distribution. Anthropogenic disturbances and natural processes are the cause of farm pond degradation, while management and conservation need to involve policy, numerical assessment, public awareness, and cost-efficiency analysis (Table 2.2).
Table 2.2 Degradation threats and management challenges associated with farm ponds
Continued
The lack of rigor and consistency in planning small water conservancies is a widespread and chronic phenomenon (McGreavy et al., 2012; Poschlod and Braun-Reichert, 2017; UN-Water, 2018). Since ancient times, the construction and maintenance of farm ponds has been driven by the government prioritization of agriculture rather than assessment of the abovementioned ecosystem services. For example, in the golden age of the Han Dynasty, pond projects were launched nationwide, and more than 60% of the ponds were located in the agriculturally prosperous Yangtze and Huai River basins (Zhang,2009). When it came to the subsequent Three Kingdoms Period, however, the ponds were damaged to restrict agriculture in neighboring realms, with Quebei losing 60% of its water surface area due to warfare (Hang, 2001). Today, China is facing grand challenges in feeding 22% of the world's population and land uses have been largely intensified to maintain the current average annual growth rate in agricultural production to be as high as 4.6%. As a result, many farm ponds have been converted to paddy fields, orchards, or threshing grounds(Yin and Shan, 2001). Moreover, centralized irrigation based on reservoirs and canals has weakened the role of farm ponds in water supply, resulting in further negligence and mismanagement of the ponds and increased vulnerability to floods and habitat losses (Liu and Yang, 2012).
Although recent evidence indicates that fa rm ponds contribute disproportionately more to aquatic biodiversity than larger and more widely studied freshwater systems, such as lakes and rivers, environmental legislation and management strategies of these features is far behind the needs due to the ignorance by the government. In Europe, the EU Water Framework Directive (WFD; 2000/60/EC) was implemented to protect and improve the quality of ground and surface waters, but, in practice, it only covers rivers and standing waters with a surface water area larger than 0.5 km2, therefore excluding the vast majority of temporary streams and wetlands (Lassaletta et al., 2010). Similarly, legislation in the US has protected the nation's navigable waters under the Clean Water Act since 1972, but these rules did not consider isolated and non-navigable wetlands as a“significant nexus”within the hydrological system before two heated debates in 2001 and 2006 (Leibowitz et al., 2008). China has recently spent millions of dollars (billions of RMB) to remedy widespread environmental and ecological deterioration through various actions including converting farmlands to forests and grasslands, establishing wastewater treatment plants, and removing algae in lakes and rivers (Huang et al., 2019). Although these programs, in conjunction with management strategies, such as the River/Lake Chief Mechanism, and examination criteria,such as the Environmental Quality Standards for Surface Water (GB 3838-2002),have obtained phased achievements, the protection of farm ponds in upstream areas has been entirely ignored.
The extensive use of synthetic fertilizers and manu re is a major characteristic of modern agriculture, and the associated discharge of nitrogen and phosphorus has become a major source of nonpoint pollution in rural areas. These pollutants can damage ecosystems, impair material and energy flows, and reduce or eliminate ecosystem services. For example, ammonia can increase the pH and temperature of ponded waters and can be toxic to fish and invertebrates (Zheng et al., 2017). At elevated concentrations, nitrogen and phosphorus are the main causes of algal blooms in surface water, while nitrate in drinking water sources is linked to birth defects, methemoglobinemia in infants, and high blood pressure in adults (Ongley et al., 2010; Chen et al.,2016a). In addition, it is noteworthy that farm ponds are more vulnerable to pollution than larger water bodies, due to their smaller capacity of diluting pollutants physically, chemically, or biologically, especially when the surrounding drainage network is modified (Huang et al., 2012; Céréghino et al., 2014).
Although intensive point source discharge, such as industrial wastewater,is rare in agricultural regions, mini-point pollution sources, including animal feeding operations and domestic sewage and wastes, pose nonnegligible threats to the ecosystems of farm ponds. According to the First National Census of Pollution Sources (Ministry of Agriculture of the PRC, 2009), poultry and livestock feeding produces 12.7 Tg (1 Tg = 10 12 g) total nitrogen and 0.2 Tg total phosphorus annually, of which approximately 43.6% originates from unfenced feeding operations. Scattered domestic animals live in riparian areas,discharging their manure and urine into the water. Domestic sewage in these areas is barely treated before being discharged into ponds and ditches owing to the lack of sewage collection and treatment facilities (Ongley et al., 2010). In addition, domestic wastes, including plastic bags, obsolete electrical appliances,and food residues are usually discarded into the water, resulting in persistent microplastic and heavy metal pollution as well as unpleasant colors and smells(Zhang et al., 2018).
Farm ponds, owing to their small catchment area and size, are vulnerable to changes in temperature and precipitation patterns. According to the Fifth Assessment Report of the Intergovernmental Panel on Climate Change (IPCC),the increase in temperature was the highest during the past century, while extreme precipitation and storm events have increased dramatically, and sea levels have risen 0.17 to 0.21 m in the Northern Hemisphere (Stocker et al.,2014). In addition, owing to the combined effects of El Niño and La Niña events,East Asia, especially the coastal provinces of southeastern China, usually suffers continuous precipitation and floods from early spring through the entire monsoon period (i.e., March to July, Karori et al., 2013; Luo et al., 2018a). These climatic extremes can cause an abnormal hydroperiod and negatively affect invertebrate diversity and plant species composition. For example, widespread droughts occurred almost every year in the southwestern regions during the past two decades, and many ponds dried up and were abandoned (Han et al.,2014). In the southeastern regions, however, soil erosion tends to be severe in the low mountains and hills, where frequent dredging operations are required to avoid siltation, eutrophication, and flooding.
Invasive species can pose a serious threat to freshwater biodiversity.China's humid weather conditions, widespread transportation network, and fragmented habitat of native species have provided favorable conditions for exotic and invasive species. Invasive species in farm ponds include plants,crayfishes, and amphibians (Yan et al., 2001; Wang et al., 2017). For example,the growth and propagation of water hyacinth can cover the entire water surface, blocking light, suppressing the growth of plankton, and causing severe degradation of other aquatic flora and fauna. These plants can even be harmful to human health, promoting the breeding of rats, flies, and other carriers of bacteria, if the waters simultaneously contain domestic wastes. To reduce such adverse effects, the Shanghai government has spent $2.18 million (15 million RMB) annually to remove water hyacinth in the suburbs, but some biological and chemical agents have resulted in secondary pollution (Lu et al., 2007).
The above natural and anthropogenic threats can be significant by their own, but the interactions of the changing climate, invasive species, and landscape degradation due to increasing demands for food, space, and water resources make the need for suitable management strategies clear if we are to sustain the multifarious benefits of these aquatic features.
Farm ponds act as biological, physicochemical, and ecological hotspots within an agricultural matrix and require management at both the within-pond and landscape scales (Chou et al., 2013; Mushet et al., 2014; Hill et al.,2018). Because the hydroperiod of farm ponds is mainly determined by the local climate, microtopography, and water consumption, they are extremely susceptible to changes in the surrounding land uses and crop types. Due to their small water area and volume, farm ponds are also very sensitive to alterations in chemistry of sediments and pollutants. In addition, farm ponds usually support life forms of amphibians from larva to adult and migratory birds from hundreds of miles away, making the adjacent terrestrial habitat an integral part of their ecosystem services. Direct losses or degeneration of these water bodies decreases the regional wetland density and increases travel distances for wildlife, particularly those relying on multiple aquatic resources. In the past,studies on farm ponds have been focused on their single ecosystem service(Chou et al., 2013; Gao et al., 2016). Integrated pond management is usually difficult to achieve due to predefined priorities, funding limitations, and narrow research objectives that lack the scientific understanding of those multi-scale processes (Spiteri and Nepalz, 2006; Raebel et al., 2012). Therefore, one major challenge is to manage, conserve, and restore ponds by integrating several spatial scales, while taking into account all fluxes of materials, organisms, and energy. Although it is very difficult, being aware of the implications of pond management across these aspects is important.
Limited public awareness and understanding of small, scattered wetlands complicates the management of farm ponds. Compared with agricultural irrigation and flood alleviation, the less visible ecosystem services (e.g., providing habitats for endangered species and contributing to groundwater recharge and discharge) are not widely recognized, which diminishes local support for public conservation actions (Hunter, 2005; Zhang, 2009; Golden et al., 2014). On the other hand, some distinguished natural landscapes usually have a flagship wildlife species to capture the hearts and minds of the public,such as pandas for the Chinese bamboo forests and penguins for the Antarctic ice sheets. Although migratory birds or amphibians may be good candidates if marketed well, farm ponds lack such charismatic species but can be regarded as breeding havens for some unpleasing animals, such as poisonous snakes,mosquitoes and leeches. Another challenge is therefore to improve the broader communities’understanding of the ecosystem services even though farm ponds seem only related to the rural households nearby.
The small size, large number, and wide distribution of farm ponds make developing conservation strategies confusing and costly. Ecosystem services of farm ponds must be better articulated to change the perceptions of the public and authorities. Some naturalist associations and scientific organizations, such as the European Pond Conservation Network, the Million Ponds Project in the UK, and the Taoyuan Pond and Waterway Conservation, are active in compiling documents, illustrations, and recommendations, but few provide numerical assessments to assist practical conservation actions. Widespread benefits and concentrated costs can challenge conservation strategies based on negotiations with individual landowners who have ponds on their property but envision a more profitable use of the land (Shogren et al., 2003). This problem is more difficult in China, as small water conservancies are formally state-owned, but paradoxically, the dredging and reinforcement of farm ponds are generally funded by local communities (Tan, 2005). While the ecological and social importance of farm ponds can be quite significant, only limited value is offered to small farmers who have ponds in their fields, which decreases their interest in conserving the projects. Therefore, management policies and approaches that quantify ecosystem services and recognize the full extent of conservation costs are more likely to succeed in navigating these challenges.