The quality of pollen a plant produces is closely tied to its sexual habits, ecologists have discovered. As well as helping explain the evolution of such intimate relationships between plants and pollinators, the study - one of the first of its kind and published online in the British Ecological Society's journal Functional Ecology - also helps explain the recent dramatic decline in certain bumblebee species found in the shrinking areas of species-rich chalk grasslands and hay meadows across Northern Europe.
Relationships between plants and pollinators have fascinated ecologists since Darwin's day. While ecologists have long known that pollinators such as honeybees and bumblebees are often faithful to certain flowers, and have done much work on the role of nectar as a food source, very little is known about how pollen quality affects these relationships.
Working on Salisbury Plain, the largest area of unimproved chalk grassland in north west Europe, ecologists from the universities of Plymouth, Stirling and Poitiers in France collected pollen from 23 different flowering plant species, 13 of which are only pollinated by insects while the other 10 species can either pollinate themselves or be insect pollinated. They analysed the pollen for protein content and, in the second part of the study, recorded bumblebee foraging behaviour.
They found that without exception, plants that rely solely on insects for pollination produce the highest quality pollen, packing 65% more protein into their pollen than plant species that do not have to rely on insect pollinators. They also discovered that bumblebees prefer to visit plants with the most protein-rich pollen. According to the lead author of the study, Dr Mick Hanley of the University of Plymouth: "Bumblebees appear to fine-tune their foraging behaviour to select plants offering the most rewarding pollen. Although there is some debate about how they can tell the difference, it is possible they are using volatile compounds."
By helping understand the advantages and disadvantages of plant-pollinator relationships where particular plants rely on particular insects to reproduce, and those insects rely on the same plants for food, the results could help ecologists conserve certain bumblebee species and the species-rich chalk grassland and hay meadow communities in which they live, all of which are becoming increasingly rare.
"For the plant, relying on a small group of insects such as bumblebees as pollinators is very beneficial because it ensures efficient pollen transfer. Bumblebees quickly learn to visit the most rewarding flowers, so providing protein-rich pollen is one way plants can encourage bumblebees to be faithful. But this close relationship has many potential pitfalls, because if the pollinators are lost, the flowers may not be able to reproduce, and this may be what we are seeing in the hay meadows, chalk grasslands and bumblebees species throughout Northern Europe," Hanley says.
Michael Hanley et al (2008). Breeding system, pollinator choice and variation in pollen quality in British herbaceous plants, Functional Ecology, doi: 10.1111/j.1365-2435.2008.01415.x, is published online on 6 May 2008.