Agricultural drainage ditches, their biological importance and functioning

We reviewed studies on the biological state of agricultural drainage ditches in the temperate and boreal zones of the Northern Hemisphere. We looked at the relative importance of ditches for farmland biota as compared to that of other habitats, and assessed the degree to which biological communities of ditches contribute to the provisioning of ecosystem services. We evaluated impacts pertaining to replacement of open drains by subsurface drainage, removal of main ditches, rehabilitation of old drainage systems, and maintenance of ditches. Most ditches support species also common elsewhere. Whenever comprehensive surveys were conducted, ditches were shown to provide valuable wet vegetated non-cropped habitats to both aquatic and terrestrial taxa, supply food resources lacking in otherwise dry and intensively managed cropland, and perform connectivity functions within a wider landscape. Regionally ditches were shown to harbour rare species or species not found presently in other farmland habitats. Some functions of drainage ditches, such as regulating water flow and nutrient retention, are likely to depend on the composition and structure of biological communities of ditches, though the issue remains poorly explored. The biggest threat to the quality of ditch networks as ecosystems is presented by a severe runoff from the fields, management in disregard of a habitat value of ditches, and removal of ditches. (C) 2008 Elsevier Ltd. All rights reserved.