Tuesday, 8 November 2011

Gardens offer bees an oasis in the middle of an agricultural desert

Researchers have known for some time that gardens provide a useful source of habitat in the midst of urban areas. The cultivated plants they contain encourage the growth of communities that can have a positive impact not only on the gardens themselves, but also on nearby habitats. Bees, for instance, may forage hundreds of meters away from the gardens in which they live; during forays outside of garden walls, they often pollinate native flowering plants, thus perpetuating populations of endemic species.

Few scientists have investigated whether gardens might also be beneficial in agricultural areas. Because farms contain plants, they may seem more "natural" than urban areas. However, like impervious surface cover and anthropogenic structures, agricultural fields can reduce habitat and decrease biodiversity. Agricultural intensification has been linked with declines in pollinator populations; this may be driving the simultaneous reductions observed in native out-crossing plant species. Thus, it would appear that flowering plants in and near farm areas are poised to benefit from bee-friendly gardens.

(Peach-leaved bellflower, Campanula persicifolia)

A group of researchers from Lund University recently investigated this possibility in an agriculture-dominated area in the south of Sweden. Using pan-traps (to capture bees) and phytometers (peach-leaved bellflower plants, Campanula persicifolia, deliberately set out to measure pollination), the researchers measured whether bee richness and abundance were higher close to gardens (at a distance of 15 m) than farther away (at a distance of 140 m). Because different types of bees have different pollination habits (for instance, because of variations in sociality and body size), the researchers also examined whether abundances of different groups of bees varied according to distance from gardens. Finally, the scientists harvested the seeds that developed from the phytometer bellflowers. By measuring seed weight, the researchers could evaluate pollination quality--since heavier seeds would indicate a higher number of successfully pollinated ovules.

(Buff-tailed bumblebee, Bombus terrestris)

Across all pan-traps, the researchers sampled 244 bees of 28 species and 8 genera. The most abundant social bee was the buff-tailed bumblebee (Bombus terrestris)--one of the commonest bumblebees in Europe--while the most abundant solitary species was a mining bee (Andrena nigroaenea). Bees were present in much higher numbers nearer gardens: On average, 24 bees were sampled per 14-m trap, while only ~7 were sampled per 140-m trap. These patterns were true for both social and solitary bees; however, the effect was stronger for the latter than for the former--likely related to the fact that larger social bees are stronger fliers and can more easily reach distant pollination sites. Capsules near gardens contained heavier seeds, indicating that more of the plant's ovules had been fertilized--a result of increased pollination.

(Farmland in southern Sweden)

The high richness and abundance of pollinators recorded here suggests that many bee species persist even in highly fragmented farmland. The authors suggest that the addition of even small "pockets" of beneficial habitat may have a major impact by building on remnant populations. Clearly, gardens are an important source of pollinating bees in agricultural, as well as urban, environments. They may be able to mitigate losses of both pollinators and native plants in areas dominated by farmland. Of course, it is unlikely that many farmers will agree to plant gardens in the middle of existing crop fields. However, as agricultural areas expand--something that is inevitable as the global human population continues to expand--landscape planners may be able to find ways of incorporating gardens into newly-developed farms. Additionally, it may be possible to improve or expand existing gardens near agricultural areas, or to develop field margins into long, narrow garden strips.

SamnegÄrd, U., Persson, A.S., Smith, H.G. 2011. Gardens benefit bees and enhance pollination in intensively managed farmland. Biological Conservation 144:2602-2606.

Thanks to the following websites for providing the images used in this post:

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