Sonja C. Pfister

Effects of hedgerows and riparian margins on aerial web-building spiders in cereal fields

Abstract Spiders (Araneae) are dominant predators in agro-ecosystems. Terrestrial seminatural habitats, such as hedgerows and grassy field margins, can enhance the abundance and diversity of spiders in adjoining fields, whereas the potential of riparian margins has rarely been studied. We compared the effects of hedgerows and riparian margins on aerial web-building spiders in adjacent cereal fields. While species richness and overall abundance did not significantly respond to distance from or type of field margin, each of the four dominant species responded differently. The abundance of Tetragnatha cf. montana Simon 1874 increased towards both hedgerows and riparian margins. Tetragnatha extensa (Linnaeus 1758) differentiated between field margin types and abundances increased only towards riparian margins. By contrast, Phylloneta impressa (L. Koch 1881) abundances decreased from field centers towards the field margins irrespective of the type. Type of field margin and distance showed an interactive effect on Mangora acalypha (Walckenaer 1802) abundances, which decreased from field centers towards hedgerows but changed only little towards riparian margins. Increasing spider densities towards field margins can be explained by the preference of spiders for adjoining seminatural habitats (overwintering, food availability, microclimate, vegetation structure), whereas increases towards field centers might be caused by interspecific competition and enhanced predation pressure near seminatural habitats and high prey numbers in crop fields. Overall, our study demonstrates that aerial web-building spider species respond differently to hedgerows and riparian margins.

Sensitivity of commercial pumpkin yield to potential decline among different groups of pollinating bees

The yield of animal-pollinated crops is threatened by bee declines, but its precise sensitivity is poorly known. We therefore determined the yield dependence of Hokkaido pumpkin in Germany on insect pollination by quantifying: (i) the relationship between pollen receipt and fruit set and (ii) the cumulative pollen deposition of each pollinator group. We found that approximately 2500 pollen grains per flower were needed to maximize fruit set. At the measured rates of flower visitation, we estimated that bumblebees (21 visits/flower lifetime, 864 grains/visit) or honeybees (123 visits, 260 grains) could individually achieve maximum crop yield, whereas halictid bees are ineffective (11 visits, 16 grains). The pollinator fauna was capable of delivering 20 times the necessary amount of pollen. We therefore estimate that pumpkin yield was not pollination-limited in our study region and that it is currently fairly resilient to single declines of honeybees or wild bumblebees.

Dominance of cropland reduces the pollen deposition from bumble bees

Intensive agricultural landscapes can be hostile for bees due to a lack of floral and nesting resources, and due to management-related stress such as pesticide use and soil tillage. This threatens the pollination services that bees deliver to insect-pollinated crops. We studied the effects of farming intensity (organic vs. conventional, number of insecticide applications) and availability of semi-natural habitats at the field and landscape scale on pollinator visits and pollen delivery to pumpkin in Germany. We found that wild bumble bees were the key pollinators of pumpkin in terms of pollen delivery, despite fivefold higher visitation frequency of honey bees. Critically, we observed that the area of cropland had stronger effects on bees’ pollen deposition than the area of seminatural habitats. Specifically, a 10% increase of the proportion of cropland reduced pollen delivery by 7%. Pumpkin provides a striking example for a key role of wild pollinators in crop pollination even at high numerical dominance of honey bees. In addition, our findings suggest that habitat conversion to agricultural land is a driver of deteriorating pollination. This underlines the importance to maintain sufficient areas of non-crop habitats in agricultural landscapes.

Interactive effects of local and landscape factors on farmland carabids: Local and landscape effects on farmland carabids

As a result of migration and spillover from noncrop habitats, predacious insects in agricultural fields respond locally to adjacent field margins and on a larger spatial scale to habitats in the surrounding landscape. However, the relative importance of local and landscape effects and their possible interactions have rarely been studied. We studied carabids in 18 pumpkin fields bordered by different field margins, along a gradient from simple to structural rich landscapes. Carabids were caught in pitfall traps along transects from the adjoining semi‐natural habitat towards the field centre. Although estimated species richness generally increased towards the field edges, carabid abundance increased towards adjacent woody margins and other crop fields. By contrast, abundance decreased towards adjacent herbaceous margins. Estimated carabid species richness in organically managed fields increased with increasing landscape heterogeneity, whereas this effect was absent in conventional fields. However, estimated species richness did not differ between organic and conventional fields. The differential response of carabid abundance to distance from the field edge in accordance with the type of field margin may explain the variability of patterns reported in the literature. The results of the present study show that both local and landscape parameters must be taken into account to promote carabids as natural enemies of crop pests.