Humlor på ekologiska och konventionella gårdar - Jordbruksverket

Humlor på ekologiska och konventionella gårdar - Jens O. Risberg
Abstract
Bumblebees (Bombus) on organic and conventional farms
– the effect of farming practice and landscape on keystone pollinators
During the last 50 years the agricultural landscape in Sweden has undergone large changes
with negative consequences for diversity. Reports of biodiversity losses have caused great
concern as many species contributes to important ecosystem services. One alternative to conventional
farming practices that has been proposed is organic farming.
Many crops are largely dependent on insect pollination. Therefore reports of declines of many
insect pollinators are worrying. In Sweden, bumblebees (Bombus spp.) are the most important of
the wild pollinators. Hence, this study concerns species richness and abundance of bumblebees
on organic and conventional farms.
Five organic and five conventional farms were paired in order to compare the two farming
systems. Landscape heterogeneity was calculated for each farm, both on a large scale (5×5 km)
and on a small scale (400×400 m). The study was performed during the summer of 2002 in the
province of Uppland, central Sweden. Leys with clover (Trifolium sp.) were the floral resource
growing on all the farms. Hence, leys together with a neighboring area with natural floral
resource were chosen as study objects. Bumblebees were surveyed through bee walk transects.
The abundance of bumblebees was significantly larger on organic farms, mainly because of the
abundance of white clover (T. repens) in organically managed leys. The flowering period of white
clover lasts longer than that of red clover (T. pratense). These findings suggest that to get strong
populations of bumblebees continuous flower resources during the entire summer are important.
There were no differences in species richness between the farming systems but the number of
species increased with increasing large-scale landscape heterogeneity. All species occurring in the
agricultural landscape of Uppland were observed in this study including three species on the
Swedish red list, B. subterraneus, two individuals of B. distinguendus and B. muscorum, which was the
first observation in Uppland since the 1950s, while B. subterraneus is among the more frequent
species in this study.
Most of the flower visits in the natural foraging resources were to leguminous flowers,
especially red and white clover. This shows that leys with clover are good foraging resources for
bumblebees. Long-tongued bumblebee species such as the threatened B. subterraneus and B.
distinguendus paid more visits to broad bean (Vicia faba).
Actions that are recommended as beneficial for bumblebees, and hence pollination as an
ecosystem service, include: 1) Increase the amount of ley with clover in the crop rotation. 2) Include
white clover in the leys, which will result in earlier and longer flowering period. 3) Increase
the diversity of flower resources through flower-rich field margins. 4) When harvesting the ley,
leave smaller parts as flower resources for bumblebees. 5) Use broad bean as fodder instead of
peas to favor especially long-tongued bumblebee species.
Keywords: Bumblebees, Bombus, organic farming, landscape ecology, biodiversity, diversiy,
abundance, ecosystem services, farming practice, ley, clover, Trifolium, red clover, Trifolium pratense,
white clover, Trifolium repens
Corresponding author: Jens O. Risberg, e-mail: jens.risberg@telia.com
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