Enhancing the Biodiversity of Ditches in Intensively Managed UK Farmland

Drainage ditches, either seasonally flooded or permanent, are commonly found on intensively managed lowland farmland in the UK. They are potentially important for wetland biodiversity but, despite their ubiquity, information on their biodiversity and management in the wider countryside is scarce. We surveyed 175 ditches for their physical and chemical characteristics, spatial connectivity, plant communities and aquatic invertebrates in an area of intensively managed farmland in Oxfordshire, UK and collected information on ditch management from farmer interviews. Water depth and shade had a small impact on the diversity of plant and invertebrate communities in ditches. Increased shade over the ditch channel resulted in reduced taxonomic richness of both channel vegetation and aquatic invertebrates and channel vegetation cover was lower at shaded sites. Invertebrate taxonomic richness was higher when water was deeper. Spatial connectivity had no detectable impact on the aquatic invertebrate or plant communities found in ditches. The number of families within the orders Ephemeroptera, Plecoptera and Trichoptera (EPT), which contain many pollution-sensitive species, declined with decreasing pH of ditch water. As time since dredging increased, the number of EPT families increased in permanent ditches but decreased in temporary ditches. Whether or not a ditch was in an agri-environment scheme had little impact on the reported management regime or biodiversity value of the ditch. Measures for increasing the amount of water in ditches, by increasing the water depth or promoting retention of water in ditches, could increase the biodiversity value of ditches in agricultural land. Some temporary ditches for specialised species should be retained. Reducing the amount of shade over narrow ditches by managing adjacent hedgerows is also likely to increase the species diversity of plant and invertebrate communities within the ditch. We recommend that to preserve or enhance the biodiversity value of ditches, and improve their ecosystem service delivery, management prescriptions for hedgerows adjacent to ditches should differ from those aimed at hedgerows only.英国集约化管理的低地农田中广泛分布着季节性淹水或永久性农田排水渠（drainage ditches）。这类渠系对湿地生物多样性具有潜在重要意义，但尽管其分布极为普遍，目前关于乡村区域内这类渠系的生物多样性与管理状况的相关研究信息仍较为匮乏。本研究针对英国牛津郡一片集约化管理的农田区域内的175条排水渠开展了系统调查，记录了其理化特征、空间连通性、植物群落与水生无脊椎动物（aquatic invertebrates）群落组成，并通过农户访谈收集了排水渠的管理相关信息。研究结果显示，水深与遮阴程度对渠内植物与无脊椎动物群落的多样性存在小幅影响：渠槽上方遮阴程度升高会导致渠内植被与水生无脊椎动物的分类群丰富度降低，且遮阴位点的渠内植被盖度更低；当渠内水深增加时，无脊椎动物的分类群丰富度会随之升高。空间连通性对渠内水生无脊椎动物与植物群落未表现出可检测到的影响。包含诸多对污染敏感物种的蜉蝣目（Ephemeroptera）、襀翅目（Plecoptera）与毛翅目（Trichoptera）（简称EPT）的科类数量，会随渠水pH值降低而减少。随着清淤（dredging）作业完成后的时间推移，永久性排水渠内的EPT科类数量会逐步上升，而暂时性排水渠内的该类群科数则会出现下降。排水渠是否纳入农业环境计划（agri-environment scheme），对农户报告的排水渠管理模式或其生物多样性价值均未产生显著影响。通过提升渠内水深或促进水体滞留来增加排水渠蓄水量的措施，可有效提升农田排水渠的生物多样性价值。针对依赖特定生境的物种，应保留部分暂时性排水渠。通过优化管理相邻农田树篱（hedgerows）来降低窄幅渠槽上方的遮阴程度，同样可提升渠内植物与无脊椎动物群落的物种多样性。本研究建议，若要保护或提升排水渠的生物多样性价值并改善其生态系统服务供给能力，毗邻排水渠的树篱管理规范应与仅针对树篱的专属管理规范有所区别。