by Lia Metzger
The expansion of mass-flowering crops has been linked to the loss of biodiversity of farmlands because they escape into natural and semi-natural habitats. However, these mass-flowering crops have a higher density of flowers than non-crop species, and thus produce more food resources with more access to nectar and pollen, so they may enhance the abundance of wild foraging bees. Holzschuh et al. (2013) investigated how oilseed rape, a mass-flowering crop, affects the abundance of the solitary and polylectic Red Mason Bee Osmia bicornis, a generalist bee species that nests in both natural and semi-natural habitats. Using data from 67 sites in Germany, they compared the abundance of Osmia bicornic in grasslands adjacent to oilseed rape fields and isolated from oilseed rape fields and vice versa. Artificial nests were assessed for number of brood cells and for the percentage of oilseed rape pollen in larval food, and then compared between brood cells and the percentage of oilseed rape pollen. The authors found that Osmia bicornis colonized artificial nests in grasslands and oilseed rape adjacent to each other significantly more than in grasslands that were isolated from oilseed rape, and not at all in isolated oilseed rape. In addition, in landscape scales, more or less oilseed rape had no effect on the number of brood cells. Oilseed rape pollen in larval food increased in adjacent fields compared to isolated fields.
While the effects of mass-flowering crops on the biodiversity of natural and semi-natural habitats has been well studied, the potential positive relationship of mass-flowering crops and wild bee abundance has hardly been investigated. Knowing this, Holzschuh et al. picked oilseed rape as a mass-flowering crop because of its high density of 350,000—700,000 plants per hectare and 100 flowers per plant. They studied 16 grasslands that were isolated by at least 230 m from oilseed rape fields, 17 grasslands and 17 oilseed rape fields that were 1—17 m of each other, and 17 oilseed rape fields that were isolated by at least 570 m from grasslands. The oilseed rape fields were all very similarly managed and the percentage of flower coverage did not differ significantly between isolated and adjacent grasslands, so those factors could not have affected the results. Fields with Osmia bicornis in traps were included for brood numbers and pollen analyses, but the isolated oilseed rape fields did not have significant numbers and thus were not analyzed. Using GIS software, the proportions of oilseed rape fields and of grasslands in landscape circles with radii of 250, 500, 700, and 1000 m were calculated.
The authors set up trap nests just inside the edges and in the center of the fields to test for brood numbers of Osmia bicornis and for the percentage of oilseed pollen in larval food. The nests were set up in March and all the nodes from the nests were collected and examined for brood cells after the end of oilseed rape flowering in May. Females of Osmia bicornis can establish 30 brood cells and forage for pollen up to 600 m away from the nests to provide for the larvae. Thus, high numbers of brood cells could be due to numerous females in the species or a high preference for nesting in that place. The authors conducted pollen analyses from 36 sites where O. bicornis was found, and these numbers were summed within the traps for each site and then the percentage of oilseed rape pollen per brood cell was averaged over all brood cells in a site to find the percentage of pollen in the larval food.
A generalized linear model with quasibinomial errors and the predictor presence of adjacent grassland was used to rule out the effect of grasslands on the presence of O. bicornis in oilseed rape fields. ANCOVA’s were used to evaluate if the number of brood cells were higher in isolated or adjacent grasslands and oilseed rape fields. The authors used ANCOVA’s to analyze the effect of local and landscape-scale availability of oilseed rape on the percentage of pollen in larval food. Linear regression models with the dependent variable as the number of brood cells were used to assess whether the number of brood cells increased with increased percentage of pollen in larval food. All models and tests were reflected in bar graphs and linear regression graphs.
The presence of grasslands had a positive effect on the number of brood cells in nests in 59% of adjacent oilseed rape fields. Only 12% of the isolated oilseed rape fields had brood cells. Additionally, the mean number of brood cells was 59% more in adjacent grasslands than isolated grasslands, and this was not affected by the presence of oilseed rape in the landscape scale. The percentage of pollen in larval food was higher in oilseed rape fields and adjacent grasslands than isolate grasslands and it did not differ between oilseed rape fields and adjacent grasslands. The number of brood cells increased with an increasing percentage of oilseed pollen in pollen food and was not affected by the availability of other food sources in the landscape-scale. These results suggest that mass-flowering crops positively affect the abundance of wild bees because they increase the access to food resources and nesting habitats. However, more research must be conducted because the increase in brood cells could be due to an increase in numbers of females of O. bicornis. The positive correlation may not reflect a positive effect on the biodiversity of the habitats around mass-flowering crops because O. bicornis is a generalist species, and thus may outcompete other species for nesting areas.
Holzshuh, A. Dormann, C.F., Tscharntke, T., Steffan-Dewenter, I., 2013. Mass-flowering crops enhance wild bee abundance. Oecologia, 172:2, 477—494. http://bit.ly/1ANLX1U