Impact of landscape characteristics on flower-visiting insects in agricultural environments across Europe

Abstract

In the last few decades, there has been growing concern about the decline in insect communities in Europe, including pollinators. One-third of syrphids and butterflies are considered to be in decline, and nearly one out of ten species of wild bees are threatened with extinction. Because flower-visiting insects provide a pollination service, declines might have consequences for ecosystems, human health and food production. One of the major drivers of insect decline is habitat loss, fragmentation and degradation. However, the responses of insects to different landscape composition and configuration heterogeneity vary, depending on their taxa and their ecological requirements, as well as according to the spatial and temporal scales considered. Data collection with standard methods on a wide geographic range can promote deeper knowledge of the current status of insects in Europe and their responses to landscape context, and enables comparisons across regions characterised by different environments. However, the use of standardised methods to study the impact of landscape composition and configuration across different biogeographic ranges is rare. It is in this context that my study takes place. Thanks to participation in a collaborative project (PoshBee project; Brown et al., 2021), my studies contribute knowledge, on a large-scale (across eight European countries), regarding the impacts of landscape features on five insect groups (honey bees, bumble bees, solitary bees, hover flies and butterflies), in two agricultural crops (apple and oilseed rape). Field-collected data were processed by research partners across a network of fourteen countries, covering different areas of investigation, so as to define, at a deeper level, which risks represent threats to the health of pollinators across Europe. In this thesis, my focus was gathering more data on the abundance of pollinator groups commonly present in cultivated fields in Europe and investigating whether they were threatened by landscape degradation and homogenization. I investigated the impact of the agricultural landscape and less-intensively managed/non-crop habitats on these communities, to provide data to implement or support agricultural policies already in force (Agri-Environmental Scheme ? AES) and to suggest new ad hoc projects for different groups of insects in different agricultural habitats. To investigate the impact that landscape has on the abundance of insect groups, I carried out three studies across different biogeographic areas in Europe. For the first, I used information about the composition and configuration heterogeneity of the landscape surrounding 128 sites at a 1 km radius. The sites were oilseed rape crops and apple orchards in Estonia, Spain, Germany, Great Britain, Ireland, Italy, Sweden, Switzerland (eight oilseed rape fields and eight apple orchards per country). The PoshBee team sampled the abundance of five groups of flower-visiting insects (honey bees, bumble bees, solitary bees, hover flies and butterflies) with 50 m long transects, during the flowering period of the target crops. I focussed on data collection in the 16 Irish sites. Using mixed effect models, I investigated the impact of landscape features on the abundance of each of the insects groups in the two crop types. The models included country as a random factor and some climate variables (annual temperature and precipitation and precipitation seasonality) as independent variables. Our results showed a positive impact of less-intensively managed and diverse habitats on the abundance of different groups of insects, even though the effect was taxon and crop specific. In the second study, I focused on insect groups in oilseed rape crops and apple orchards, located into a specific agricultural context, namely intensively managed grassland in Ireland. Across eleven sites, I used 100 m long transects to sample insects present along the flowering margins and in the centre of the cultivated crops, at three different periods of the year. At the same time, I surveyed the plants present along the margins of the field, using quadrats (1 m x 1 m). My goal was to investigate differences in the insect groups between the margins and the centres of crops and whether the changes in insects and floral communities through time were related. The results of the linear models (REML) showed that some groups of insects (hover flies and butterflies) were more abundant along the margins of the crops, rather than in their centres, across all three periods. Moreover, I found that although both insects and flower communities changed through the time (REML and NMDS models), this change was not correlated, except for a moderate correlation emerging between plant diversity and bumble bee abundance. In my third study, I investigated the role of the landscape on the fitness of Osmia bicornis, a solitary bee species, across Europe. Three O. bicornis nests were established in 96 apple and oilseed rape crops, in Estonia, Germany, Italy, Spain, Sweden, Switzerland and information on the pollen stored in the nests, and about different developmental stages of O. bicornis (occupation of the nests, number of pupae, number of adult hatched and their weight and sex ratio), was collected. We used the 1 km radius maps to check, with the use of mixed effect models, the impact of the non-crop landscape on the composition of the pollen (pollen from crop and non-crop species of plants, and pollen diversity) and on the occupation of the nests, in the two crops. We analysed the relationship of non-crop pollen and the different developmental stages of the bees. From this study, it emerged that there was a positive impact of the non-crop habitats on the non-crop pollen present in the nest. Moreover, we found that more adults hatched from those nests where the percentage of non-crop pollen was higher, suggesting an indirect impact of non-cultivated habitats of O. bicornis fitness. Overall, my results show that, independent of the biogeographic area considered, semi-natural habitats, non-cultivated habitats and less-intensively managed habitats support a high abundance of flower visiting insects in agricultural crops, both at a crop- and at a landscape-scale. However, this effect is taxon- and crop-specific. This suggests a positive role for natural elements on biodiversity. Guaranteeing the presence of insects in cultivated crops can both support the production of the crops themselves, given the role of insects as pollinators, and increase the biodiversity of the landscape, with positive repercussion on the ecosystem and on human health.