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By Dr Connal Eardley
The Bee’s Perspective
Flowers provide food for bees. To plants they are sex organs. The origins of flowering plants and insect pollinators began simultaneously about 100 million years ago. Cross-pollinated plants use vector, such as wind, water or an animal, mostly an insect, to deliver their pollen to a receptive stigma of another flower. Animal pollination requires that flowers attract pollinators, mostly for food (pollen, nectar and oils), but also through scent and/or visual cues. This co-evolution resulted in plants producing rewards for the service they receive, namely pollination, although the bees are blissfully unaware they’re performing a service.
Bees as Pollinators
Bees are among the most important pollinators. Commonly, beetles, moths, bats and birds pollinate, and a host of other organisms may pollinate as well. Pollination precedes fertilisation in plants, but does not guarantee it. Therefore it also precedes plant reproduction, seed set and fruit production. Consequently, it is an essential ecosystem service, meaning that in the wild it directly contributes to the production of food for seed- and fruit-eating animals, and ensures that there will be future generations of plants. The same applies to agriculture. Flowers depend either on a specific pollinator, on a suite of closely related pollinators, or on many different pollinators. Similarly, some bees are adapted to forage on specific plants while others visit many different types of flowers. Specialisation reduces competition between plant species for a pollinator, and/or between bee species for the food they obtain from a specific plant species. However, specialisation is risky because if one goes extinct the other will follow. Most pollinator/plant relations are webs of activities, and these webs are poorly understood. Not all flower visitors pollinate – some steal their nectar and pollen, and contribute no benefit to the plant. They are known as robbers.
A Leaf Cutter Bee
The Flower’s Perspective
To a flower, a bee is a pollen vector, and the balance between reward and pollination service must be finely tuned. The reward must be sufficient to make the pollinator’s visit worthwhile, but insufficient to completely meet the food requirements so that the pollinator is forced to visit another flower. Flowers also develop their structure in such a way that the receptive stigma touches the bee’s body in precisely the place where the pollen was lodged, either the pollen basket or another part of the bee’s body. These differences explain why we have such a beautiful array of flowers, and with them fruit and nuts – and bees. Plants have excelled beyond all other groups of organisms in producing beautiful, intricate structures to ensure sexual reproduction. Their beauty in our gardens and the wonderful variety of foods that come from these flowers is testimony to the most miraculous process in nature – biodiversity. This is nature’s way of coping in a complex, changing world.
The Bee’s Perspective
Bees don’t see flowers as we see them. They see them as food. Pollination mechanisms are survival strategies for plants competing with one another for pollinators, and bees competing with one another for food. Daisies attract many different pollinators, but the anthers and stigmas are close together. Cross-pollination requires stigmas not being receptive to their own pollen. Aloes have tubular corollas suited to small and long-tongued insects. Orchids have their pollen in pollinia that they usually place on a part of the bee’s body that is inaccessible to the bee, but not to the stigma of another orchid that the bee visits. Orchids that don’t provide a reward are decoy pollinated, i.e. bees are lured to flowers by their structure and scent, but don’t receive a reward. Buzz pollination occurs in the tomato family (Solanaceae) – these plants require that a bee places it forelegs onto the anthers and then buzzes. The vibrations of its wings resonate through the forelegs and burst the anthers releasing the pollen. This is why one hears a distinct buzz every time a bee visits a tomato flower – listen next time you’re in your veggie garden!
A Yellow-Banded Carder Bee
Dr C. Eardley is from the Agricultural Research Council, Plant Protection Research Institute, and has worked on the taxonomy of African bees and the conservation of their biodiversity for 40 years. His email address is firstname.lastname@example.org.