What pollination is
Flowering plants reproduce by making seeds, and in most cases that requires pollen to travel from one part of a flower to another. Pollen grains are tiny packages that carry the plant’s male genetic material. They are produced in the anthers, at the tips of the stamens. To make a seed, a pollen grain must reach the stigma, the sticky receiving surface at the top of the female part of a flower, called the pistil. From there it grows a tube down toward the ovules, where fertilization takes place. The fertilized ovule becomes a seed, and the surrounding flower tissue often swells into a fruit.
Moving pollen from anther to stigma is pollination. Some plants can pollinate themselves. Many others need pollen from a different plant of the same species, which mixes genes and generally produces more varied offspring. Since plants cannot walk, they rely on outside help: wind, water in a few cases, and, for most flowering plants, animals.
The bargain between flower and visitor
Animal pollination is a trade. The plant offers a reward, and the animal moves pollen as a side effect of collecting it. The main rewards are:
- Nectar, a sugary liquid that gives energy.
- Pollen itself, which is rich in protein and fats. Many bees collect it to feed their young.
- Occasionally oils, resins or a place to shelter or lay eggs.
A bee landing on a flower brushes against the anthers and picks up pollen on its body hairs. At the next flower of the same kind, some of that pollen rubs off on the stigma. Bees also have specialized structures for packing pollen, such as pollen baskets or brushes on the legs or abdomen, depending on the species, which carry the food home but also leave plenty of loose grains on the body to be transferred.
Flowers advertise their rewards. Bright colors, scent and patterns act as signals. Bees see in a range that includes ultraviolet light, which humans cannot see, and many flowers have ultraviolet markings that likely guide insects to the center. Flowers pollinated at night, often by moths, tend to be pale and strongly scented.
Many kinds of pollinators
Honey bees are the best known, but they are only one of many pollinators. There are thousands of species of wild bees, including bumble bees and solitary bees that nest in soil or hollow stems. Other insects that pollinate include:
- Flies, especially hoverflies, which visit a wide range of flowers.
- Butterflies and moths, whose long tongues reach nectar in deep tubes.
- Beetles, among the earliest flower visitors in evolutionary history, which tend to favor large, bowl-shaped flowers.
- Wasps, including the fig wasps that have a close partnership with fig trees.
Some vertebrates also pollinate, including hummingbirds, certain bats and a few small mammals. The wind pollinates grasses, cereals and many trees, which is why those plants usually have small, plain flowers with lots of light pollen.
Some partnerships are very specialized. Certain orchids and yuccas depend on one or a few insect species. Such narrow relationships are fascinating but also vulnerable: if the partner disappears, the plant may struggle to reproduce.
Pollinators and food crops
A large share of the crops people eat benefit from animal pollination, including many fruits, vegetables, nuts and seeds. Apples, almonds, blueberries, squash and coffee are commonly cited examples. Some of them produce better or larger yields when well pollinated, while others will produce little without pollinators. Staple grain crops such as wheat, rice and maize are mostly wind-pollinated or self-pollinated, so the major sources of calories are less dependent on insects, though many sources of vitamins, minerals and variety in the diet do depend on them.
Estimates of how much of global food production depends on pollinators exist, but they vary depending on method, so it is safest to say that a meaningful share of crop output, and a larger share of crop types, benefit from them. In some regions, farmers rent managed honey bee hives to boost pollination in orchards.
Pressures on pollinators
Researchers have reported declines in some pollinator populations, especially of certain wild bees and managed honey bee colony losses in some regions. The causes appear to be several and interacting. Commonly discussed ones include:
- Loss and fragmentation of flower-rich habitat as land is converted to farming and development.
- Pesticides, including some insecticides that can harm bees at low doses. Their effects depend on the chemical, the dose and how it is applied.
- Parasites and diseases, such as the varroa mite in honey bees.
- Changing climate, which can cause flowers to bloom out of step with pollinators’ activity.
The picture differs widely among species and places. Not every pollinator is declining, and some species have done well. Scientists generally agree, though, that diverse, healthy pollinator communities are a good thing for natural ecosystems and agriculture.
What helps
Typical suggestions from conservation groups include planting a variety of flowers that bloom across the seasons, leaving some bare soil or undisturbed stems for nesting, reducing pesticide use, and preserving hedgerows and meadows. A mix of wild bees and other insects provides a kind of insurance: if one species has a bad year, others may fill the gap, which is the same logic behind the importance of species biodiversity in general.