A house of cards: Patterns of aquatic invertebrate diversity in agricultural ditches

This thesis provides an overview of the interrelationships between different fractions of the ecosystem in North Holland’s intensively managed agricultural drainage ditches. It has been demonstrated that both submerged and emergent vegetation strongly influence invertebrate assemblages. Moreover, specific abiotic factors driving invertebrate diversity in these waters are associated with the inlet of mineral rich, river derived water and include concentrations of nutrients, bicarbonates and sulphate. Nutrients and macro-ions cause degradation of peat soils, leading to the accumulation of amorphous layers of mud in remnant peatland environments. In addition, temporal abiotic variability is structuring macroinvertebrate taxonomic and functional composition in the landscape.
Agricultural intensification is placing increased pressures on aquatic ecosystems, via inputs of nutrients, suspended sediments and water abstraction. In the province of North Holland the demand for water is leading to greater influence of mineral rich waters which is ultimately degrading the aquatic environment. Moreover, the annual removal of vegetation is weakening the plant community which is already stressed by eutrophication and turbidity and this has a knock-on effect in causing a decline in invertebrate diversity. Despite the implementation of habitat improvement schemes, such as nature friendly banks in the Netherlands, there appears to be a lack of evidence supporting their effectiveness in promoting biodiversity. The isolation and size of these habitat creation measures are likely to be partially responsible. Small patches of suitable habitat, surrounded by a matrix of intensively managed agricultural land limit the ability of species to colonize these isolated habitats. For management strategies to successfully increase biodiversity in agricultural landscapes the requirements of habitat size and quality for biota must be met. Fortunately, with the large number of ditches and canals in North Holland’s landscape there is good potential to provide the habitat necessary to support a diverse range of aquatic biota, granted management seeks to do so.