Pollination and pollinators

Flowering plants reproduce sexually. Pollen, produced by the male parts of a flower called anthers, contains the cells that must reach the female part, the stigma, to fertilise the plant's ovules, which then develop into seeds.

Because plants cannot move, they need something to carry pollen for them. Some rely on wind or water, but most of the world's flowering plants depend on animals.

Grasses, including wheat, rice and maize, and many trees such as oaks, birches and pines release enormous quantities of light pollen into the air. It is a wasteful strategy, since most pollen lands nowhere useful, so these plants produce it in vast amounts.

Wind-pollinated flowers are usually small and plain, with no scent or nectar, because they do not need to attract anyone. Their airborne pollen is also what causes hay fever.

• Bees are the most important pollinators; there are more than 20,000 known species, and most are solitary rather than living in hives.
• Butterflies and moths feed through long tongues, and many night-blooming pale flowers are moth-pollinated.
• Flies and beetles pollinate many plants, including some that smell of rotting meat to attract them.
• Hummingbirds in the Americas and sunbirds in Africa and Asia pollinate red and orange tubular flowers.
• Bats pollinate plants such as agave and many tropical fruits, visiting large, strong-smelling flowers at night.

Flowers offer rewards, usually sugary nectar or protein-rich pollen, and advertise them with colour, shape and scent. Bees see ultraviolet light, and many flowers have ultraviolet patterns, invisible to us, that guide bees toward the nectar.

Over millions of years some plants and pollinators have become closely matched. Charles Darwin examined a Madagascar orchid with a nectar tube about 30 centimetres long and predicted that a moth with an equally long tongue must exist. Such a moth was described decades after his prediction. Some plants cheat: certain orchids mimic the look and smell of female insects, and males pollinate them while trying to mate.

Most staple calories come from wind-pollinated grains, but a large share of crop types depend at least partly on animal pollinators. Estimates suggest around three quarters of leading global food crops benefit from animal pollination, including apples, almonds, blueberries, coffee, cocoa, tomatoes and many vegetables grown for seed.

These crops provide much of the vitamins and minerals in human diets. Farmers of some crops rent managed honeybee hives, and California's almond orchards alone require a large share of the honeybee colonies in the United States each spring.

Many wild bee, butterfly and other pollinator populations have declined. The main causes are habitat loss, as wildflower meadows and hedgerows disappear; pesticides, including neonicotinoids, which can harm bees even at low doses; diseases and parasites such as the varroa mite in honeybees; and climate change, which can shift flowering times out of step with pollinators.

Helpful measures include planting diverse native flowers, leaving patches of land unmown, reducing pesticide use, providing nesting sites for solitary bees, and restoring hedgerows and wild margins on farms.