Effects of Farming Practice on Pollinators and Pollination across Space and Time

Sammanfattning: Several studies have found that organic farms support higher biodiversity, but results vary between studies such that some also show no or even negative effects. There can be many reasons why studies comparing organic and conventional farming show conflicting results. The efficacy of organic farming can depend on focal taxa, landscape context, study scale and the proportion of organically managed farms in a region. I also considered a fourth possible source: heterogeneity in result from not considering historical processes, e.g. how long the farm has been managed organically. The main aim of this thesis was to evaluate if farming practices modify pollinator diversity and the ecosystem functions they provide while controlling for landscape heterogeneity and the time since the farm converted to organic practices. I found that Swedish organic farming practices can enhance diversity and abundances of pollinators such as hoverflies, solitary bees, and bumblebees, as well as that of butterflies and plants. Not all organism groups responded equally to organic farming, which may depend on trait differences between the pollinator groups. Thus the effect of organic farming may not only depend on at which scale it is applied in the landscape, but on how mobile organisms are. More mobile organisms are then able to utilize resources generated by organic farming at different spatial scales. Hoverflies and butterflies are not central place foragers, such as bees, and are relatively mobile and may therefore more easily trace changes in local habitat quality. Landscape homogenisation may be an important reason for loss of pollinator diversity. I found landscape heterogeneity, rather than local farming practice, to be more important for diversity of e.g. solitary bees. Landscape could be important, because solitary bees and bumblebees are central place foragers and therefore need habitats for survival and reproduction within a restricted area. The abundance of pollinators also increases with the time since conversion to organic farming, especially hoverflies and butterflies. Hoverflies may respond to a gradual increase in a local resource that I did not measure, such as plants. I found organic farming in homogenous landscapes to improve pollination success in strawberries. Strawberries need visits from pollinators of different sizes and organic farms maintained taxonomically broader communities even in homogenous landscapes. Other plants need pollinators more sensitive to landscape homogenisation, like long-tongued bumblebees. I found that the pollination of such a plant, field beans, increased with landscape heterogeneity but only on organic farms. It is important to improve agriculture for enhancing ecosystem services, like pollination, and organic farming could be one step in this development.