Insects’ conservation in agricultural landscapes: trends, assessment, and management implications

The objective of this dissertation is to address the challenges and opportunities associated with the conservation of insect diversity in agricultural landscapes. By investigating long-term trends and underlying environmental drivers, assessing the effectiveness of agri-environmental measures, and exploring timely assessment methods, this research seeks to contribute to the development of evidence-based strategies for promoting insect diversity. In agricultural landscapes, where intensive land use practices prevail, a long-term decline of insects has been proven in many studies, underscoring the need of effective conservation strategies to maintain ecosystem functions such as trophic interactions. Despite the growing recognition of this issue, there remains a lack of comprehensive data on long-term trends in several insect taxa within agricultural landscapes, especially for the megadiverse taxon Diptera. A promising attempt to promote insects in agricultural landscapes is the creation of flower strips, which serve as substitute habitats, enhancing food and nesting resource availability. However, the effectiveness of flower strips strongly depends on their design and the selection of the plant species sown. Flower strips, for which farmers in the EU receive subsidies as part of agri-environmental schemes, must meet certain requirements, which in turn do not necessarily meet the ecological requirements of insects worthy of protection. Thus, there is a need for evidence-based concepts for the design of flower strips to improve the conservation success of insects in agricultural landscapes. The ineffectiveness of previous agri-environmental measures to achieve their objectives is partly attributable to the fact that there are no standardized procedures for assessing their effectiveness. With regard to flower visiting insects, approaches solely quantifying species diversity are not always useful, because they do not allow to draw conclusions about the mechanisms underlying the success or failure of a particular measure. In contrast, network-based analyses that characterize interaction patterns between plants and flower visitors provide a deeper understanding of interspecific interactions within flower strips and therefore offer insights into the ecological processes that are crucial for conservation initiatives. Long-term monitoring data reveal significant shifts in the phenology and abundance of flies, with a notable shift in peak flight timing and a substantial decrease in fly abundance over four decades. This development might have negative effects on interaction partners, including higher trophic levels such as insectivorous birds. In addition, ecosystem processes commonly provided by flies, e.g., pollination, nutrient cycling, and pest predation might also be negatively affected. Flower strips sown with species rich seed mixtures of native, site-adapted plant species appear to be a particularly good way to counteract this negative trend. In addition to the composition of seed mixture, the location of the flower strips also plays an important role. Especially for insects with low dispersal ability or small foraging radius, the creation of flower strips at the boundary between the field and a semi-open forest edge is promising. The synergy effect between flower strips and semi-open forest edges highlights the importance of landscape-level perspectives in conservation efforts. Since network analyses detect ecological signals more comprehensively, they can also be used to functionally assess and to improve the effectiveness of flower strips, which in turn supports the long-term protection of site-specific insect assemblages. Therefore, network analyses should be used as the standard method for evaluating and improving pollinator-friendly agri-environmental measures. Overall, this dissertation provides a holistic view of insect conservation in agricultural landscapes, underscoring the need for tailored management strategies that consider local ecological conditions and species interactions. By integrating long-term monitoring approaches, habitat enhancement, and network analyses, the scientific findings offer valuable guidance for developing effective conservation practices to support insect diversity conservation in agricultural landscapes.