Biodiversity value of agricultural drainage ditches; a comparative analysis of the aquatic invertebrate fauna of ditches and small lakes.

1. Drainage ditches are a common aquatic habitat in the lowland agricultural landscape of north-western Europe. The invertebrate fauna of these waters is poorly known compared with that of the semi-natural wetland fragments found in this region. While most wetlands are designated as nature reserves, drainage ditches are generally viewed purely as hydrological infrastructure in support of agriculture. 2. To assess the conservation value for aquatic invertebrates of these man-made habitats compared with that of wetland fragments, the taxonomic composition and life-history characteristics of invertebrate assemblages inhabiting nine small lakes and nine ditches in peatlands in the Netherlands were compared, as well as the environmental characteristics potentially influencing assemblage structure. 3. Although ditches comprised a smaller water volume, contained less diverse vegetation, displayed larger fluctuations in nutrient concentrations and were regularly managed, the total number of invertebrate taxa recorded per taxonomic group was comparable with that in small lakes, as was the number of nationally uncommon to very rare taxa. 4. Similarity in life-history characteristics between the two water-body types was high, except that a higher proportion of atmospheric air breathers was found in ditches, and more plant miners and collector–filterers in small lakes. 5. On a regional scale, a relatively high inter-ditch taxon dissimilarity was observed, resulting in total diversity estimates for ditches exceeding those of small lakes. 6. This study showed that drainage ditches can be a significant habitat type for aquatic invertebrates. In the Netherlands, water bodies in peatland agricultural areas can contain a diverse invertebrate fauna, similar to that of water bodies in nearby nature reserves