The ecological and biodiversity conservation values of farm dams: A systematic review

in agricultural landscapes is often associated with the presence of features such as farm dams, shelterbelts, rocky outcrops, and large old paddock trees [10–12]. These elements, though fre- quently of artificial origin, can contribute to biodiversity enhancement in the landscape by offering habitat, promoting heterogeneous landscape structuring, and providing shelter [11]. Farm dams (sometimes also termed ‘ponds’) can serve as valuable assets for biodiversity in many agricultural landscapes [13–16]. We use the term “farm dams” hereafter to distinguish these smaller agricultural reservoirs (~102–105 m2) [17] from large-scale irrigation storages and the larger human-made structures used for flood control and/or hydroelectricity. Farm dams often constitute the only wetlands in heavily modified landscapes where natural water bodies have been drained, in-filled and disappeared [17]. Hence, these artificial systems pres- ent an opportunity to partly conserve and enhance biodiversity while also contributing to agri- cultural production [17–20]. While natural wetlands have diminished significantly in number, up to 51% since 1970 in many parts of the world [1], farm dams are abundant with a consider- able number of new farm dams being constructed each year [21–23]. For example, there was a 2% per annum increase in the number of farm dams in Australia between 1988 and 2000 and, as of 2019, the continent supported ~ 1.765 million farm dams [17]. In the United Kingdom, approximately 2000 farm dams are created each year [24,25] with the numbers increasing

from 425,000 in 1998 to 478,000 in 2007 [24]. The importance of farm dams for conservation is underscored by their capacity to offer distinctive habitat for various taxa. For instance, farm dams support waterbirds through diet provision, enhance the habitat for aquatic macroinver- tebrates, and provide breeding habitat for amphibians [26–28]. Recognizing the links between aquatic and terrestrial ecosystems, farm dams have the potential to positively influence not only alpha diversity but also contribute to beta and gamma diversity, further emphasizing their contribution to biodiversity conservation. The characteristics of farm dams and how they are managed may have important impacts on their value for biodiversity [10,24], with regionally varying benefits [17]. An understand- ing of the cultural, social, and economic significance of dams in different regions is impor- tant to understand the motivation behind, and application of, management practices and help guide conservation plans and outcomes. For example, it is common for dams in North America to be stocked with large numbers of fish for recreational fishing purposes, and for dams in Australia to be used as the primary water source for livestock and irrigation, much like their European counterparts [29,30]. Additionally, in countries with developing agricul- ture, such as South Africa, farm dams are relied upon as a dependable water source during periods of drought or water scarcity [31]. In each case, these dual purpose/usages could neg-

atively impact biodiversity values. Furthermore, while farm dam configuration varies, they are often designed and constructed to have steep banks and deep basins which promote water retention [17,32] and make it harder for cattle to become bogged in muddy areas [16,32]. Whilst farm dams constructed in this manner provide a reliable source of water for stock and irrigation [21,33,34], they lack shallow edges and littoral zones with vegetation, both characteristics of many natural water bodies [35]. Littoral zones are particularly impor- tant for biodiversity as the vegetation provides habitat for foraging, refuge, and egg-laying for organisms such as amphibians, fish, and macroinvertebrates [16,36]. Gentler slopes along the edges of artificial farm dams serve to mitigate the risk of these environments becoming ecological traps, where animals entering the dam are at risk of being unable to exit [37]. Despite being recognized as important for biodiversity, there are still key knowledge gaps about the ecological value of farm dams in agricultural areas. These gaps may limit the devel- opment of effective policies, programs, and practical management prescriptions to improve the conservation value of these small water bodies. To date, there has been no global review PLOS ONE The ecological and biodiversity conservation values of farm dams PLOS ONE | https://doi.org/10.1371/journal.pone.0303504 May 13, 2024 2 / 18