The effect of patch size and separation on bumblebee foraging in oilseed rape: implications for gene flow

1. Theory predicts that the susceptibility of patches of plants to gene flow mediated by incoming pollinators will depend on the length of pollinator residence (the number of flowers visited during a bout in a patch). We sought to determine whether pollinator residence is sufficiently sensitive to patch size and separation to influence levels of gene flow significantly. We studied oilseed rape Brassica napus as an exemplar of a conventional animal-pollinated plant and as a species where there is a risk of genetic escape from genetically modified (GM) varieties. 2. We determined pollinator residence in 36 patches created by mowing an agricultural field of flowering oilseed rape. Each square patch had an edge length of 1, 2, 3, 4, 5 or 10 m. Patches were separated by either 9 m or 18 m from the border of a large unmown area of crop. We used the residence of bumblebees (Bombus spp.) to solve a model of bumblebee-mediated gene flow. 3. Bumblebee residence increased significantly with patch area, but did not depend on the separation distance of the patch from the large areas of crop. Even in the largest patches, bees visited only c. 60 of the c. 1 x 10(5) flowers before leaving. 4. If bumblebees were the sole pollinators, the model predicted that 4-8% of the seed produced by the smaller patches (c. 80-320 plants) had fathers outside the patch, whereas in the larger patches (720-8000 plants) the prediction was 2-4%. Nevertheless, larger patches should produce the greatest number of seeds with extrinsic paternity. 5. Synthesis and applications. Our observations demonstrate that susceptibility to pollinator-mediated gene flow is liable to decrease systematically as the size of plant patches increases. These findings can inform the management of genetic diversity in and among small or fragmented populations. If the patch size-gene flow relationship that we observed applies to larger scales, the model could estimate gene flow between GM crops and volunteer or feral populations in agricultural landscapes.