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Institute
Grassland management has been increasingly intensified throughout centuries since mankind started to control and modify the landscape. Species communities were always shaped alongside management changes leading to huge alterations in species richness and diversity up to the point where land use intensity exceeded the threshold. Since then biodiversity became increasingly lost. Today, global biodiversity and especially grassland biodiversity is pushed beyond its boundaries. Policymakers and conservationists seek for management options which fulfill the requirements of agronomic interests as well as biodiversity conservation alongside with the maintenance of ecosystem processes. However, there is and will always be a trade-off.
Earlier in history, natural circumstances in a landscape mainly determined regionally adapted land use. These regional adaptions shaped islands for many specialist species, and thus diverse species communities, favoring the establishment of a high β-diversity. With the raising food demand, these regional and traditional management regimes became widely unprofitable, and the invention of mineral fertilizers ultimately led to a wide homogenization of grassland management and, as follows, the loss of biotic heterogeneity. In the course of the green revolution, this immediate coherence and the dependency between grassland biodiversity and traditional land use practices becomes increasingly noticed. Indeed, some traditional forms of management such as meadow irrigation have been preserved in a few regions and thus give us the opportunity to directly investigate their long-term relevance for the species communities and ecosystem processes. Traditional meadow irrigation was a common management practice to improve productivity in lowland, but also alpine hay meadows throughout Europe until the 20th century. Nowadays, meadow irrigation is only practiced as a relic in a few remnant areas. In parts of the Queichwiesen meadows flood irrigation goes back to the Middle Ages, which makes them a predestined as a model region to study the long- and short-term effects of lowland meadow irrigation on the biodiversity and ecosystem processes.
Our study pointed out the conservation value of traditional meadow irrigation for the preservation of local species communities as well as the plant diversity at the landscape scale. The structurally more complex irrigated meadows lead to the assumption of a higher arthropod diversity (Orthodoptera, Carabidae, Araneae), which could not be detected. However, irrigated meadows are a significant habitat for moisture dependent arthropod species. In the light of the agronomic potential, flood irrigation could be a way to at least reduce fertilizer costs to a certain degree and possibly prevent overfertilization pulses which are necessarily hazardous to non-target ecosystems. Still, the reestablishment of flood irrigation in formerly irrigated meadows, or even the establishment of new irrigation systems needs ecological and economic evaluation dependent on regional circumstances and specific species communities, at which this study could serve as a reference point.
Natural pest control and pollination are important ecosystem services for agriculture. They can be supported by organic farming and by seminatural habitats at the local and landscape scale.
The potential of seminatural habitats to support predatory flies (chapters 2 and 3) and bees(chapter 7) at the local and landscape scale was investigated in seminatural habitats. Predatory flies were more abundant in woody habitats and positively related to landscape complexity. The diversity and the abundance of honey and wild bees were positively related to the supply of flowers offered in the seminatural habitats.
The influence of organic farming, adjacent seminatural habitats and landscape complexity on pest control (chapter 4) and pollination (chapter 6) was investigated in 18 pumpkin fields. Organic farming lacked strong effects both on the pest control and on the pollination of pumpkin.
Pest control is best supported at the local scale by the flower abundance in the adjacent habitat. The flower supply positively affected the density of natural enemies and tended to reduce aphid densities in pumpkin fields.
Pumpkin provides a striking example for a dominant role of wild pollinators for pollination success, because bumble bees are the key pollinators of pumpkin in Germany, despite a higher visitation frequency of honey bees. Pollination is best supported by landscape complexity. Bumble bee visits and as a result pollen delivery in pumpkin were negatively related to the dominance of agricultural land in the surrounding landscape.
The influence of aphid density (chapter 8) and pollination (chapter 5) on pumpkin yield was evaluated. Pumpkin yields were not affected by aphid densities observed in the pumpkin fields and not limited by pollination at the current levels of bee visitation.
In conclusion, especially seminatural habitats, that provide diverse, continuous floral resources, are important for natural enemies and pollinators. A sufficient proportion of different seminatural habitat types in agricultural landscapes should be maintained and restored. Thereby natural enemies such as predatory flies, wild pollinators such as bumble bees, and the pest control and pollination provided by them can be supported.