The Quiet Work of Pollinators
When people picture pollination, they usually imagine a honeybee moving from flower to flower in a sunny garden. The image is not wrong, but it is incomplete. Pollination, the transfer of pollen from the male part of a flower to the female part of the same or another flower, is carried out by a remarkable range of animals, including moths, flies, beetles, birds and even some bats. Among all of these visitors, however, bees are generally the most important, partly because, unlike most other insects, they collect pollen deliberately in order to feed their young. A butterfly that lands on a flower picks up pollen by accident; a bee goes looking for it.
The relationship between bees and flowering plants is an ancient one. Fossil evidence suggests that the two groups developed alongside each other over many millions of years, each gradually shaping the other. Many flowers produce nectar, a sugary liquid that serves as fuel for adult bees, while pollen supplies the protein that developing larvae need in order to grow. In return for this food, a bee that brushes against one flower carries grains of pollen on to the next. Some plants have become so specialised that they can be pollinated by only one type of bee, an arrangement that works well until one of the partners declines, at which point the other is put at risk as well.
It is often assumed that honeybees do most of the work of pollinating crops. In reality, wild bees, of which there are thousands of species, are frequently more efficient. Bumblebees, for example, are capable of what is known as buzz pollination: they grip a flower and vibrate their flight muscles very rapidly, shaking loose pollen that would otherwise be difficult to release. Tomatoes and blueberries respond particularly well to this technique. Solitary bees, which do not live in colonies at all, are also highly effective. Rather than packing pollen tightly into special baskets on their legs as honeybees do, they tend to carry it loosely on the hairs of their bodies, where it falls easily onto the next flower they visit.
Farmers have long recognised the value of bees, and in some regions hives are now transported by lorry from one farm to another as different crops come into flower. This system allows growers to produce large harvests from a single crop, but it has clear disadvantages. Moving colonies over long distances places considerable stress on the insects, and it can spread diseases from one hive to another. Moreover, when a landscape is dominated by a single crop, bees have plenty to eat for a few weeks while that crop is in flower and then almost nothing for the rest of the season.
Over recent decades, scientists in many parts of the world have recorded falls in the numbers of a wide variety of bee species. No single explanation accounts for this decline. Loss of habitat is widely considered the most serious problem, as meadows, hedges and patches of rough ground have been replaced by large fields, roads and buildings. Certain pesticides have been shown to interfere with bees' ability to find their way back to the nest. Parasites add further pressure, and so do changes in climate, which can cause some flowers to open before the bees that depend on them have emerged from their winter shelter.
One encouraging finding is that relatively modest changes in the way land is managed can make a measurable difference. In a three-year study carried out on a group of farms in a lowland valley, researchers sowed strips of wildflowers along the edges of fields. By the third summer, the number of wild bees recorded on these farms had nearly doubled, and the yield of a neighbouring fruit crop had risen by around a tenth. Importantly, the strips occupied less than three per cent of the farmland, so very little productive land was lost. The farmers involved reported that the strips required little attention once they were established.
Towns and cities can contribute too. Parks departments in a number of cities have started leaving some grass verges uncut until late summer, which allows clover and other low-growing flowers to bloom. Gardeners are being encouraged to choose plants that flower at different times, from early spring to late autumn, so that food is available for as much of the year as possible. Even a bare patch of soil can be valuable, since most solitary bees nest in the ground rather than in hives or hollow trees. Such measures are cheap and simple, and they suggest that the future of pollinators depends not only on farmers and scientists but on ordinary householders as well.
