What Is a Pollinator? The Transport System Most Plants Depend On
By the BrainSnail editorial team. How these articles are written and checked, and how to tell us when one is wrong.
Most flowering plants cannot move pollen themselves and rely on animals to carry it between flowers. That arrangement is a trade in which the plant pays with food, and the whole structure of a flower is built around making the trade work.
Why plants need help
A flowering plant must get pollen from one individual to the receptive part of another, and since it cannot move, something else must do the carrying. Wind serves for grasses, many trees and a substantial minority of species, and it is wasteful, requiring enormous quantities of pollen because most of it lands nowhere useful. Animal transport is far more efficient per grain, since an animal moving between flowers of the same species delivers pollen to a place where it can work, and that efficiency lets the plant produce less pollen and invest instead in attracting and rewarding the carrier. The reward is usually nectar, a sugar solution produced for no other purpose, or surplus pollen itself, which is protein-rich. The arrangement is mutual, since neither party is doing the other a favour, and it breaks down when either side can cheat.
Who does the work
The carriers are more varied than the familiar image suggests:
- •Bees, of which the great majority are solitary species rather than the familiar social ones, and which are the most important group by a wide margin
- •Flies, including hoverflies and many others, which are substantially underrated and dominate in cold and high-altitude places
- •Butterflies and moths, with moths doing a large and largely unnoticed share of the work at night
- •Beetles, which were among the earliest pollinators and still serve for some ancient plant lineages
- •Birds, particularly hummingbirds and sunbirds, which visit flowers shaped and coloured for them
- •Bats, which pollinate many tropical plants including several of agricultural importance
How flowers advertise
Flower structure is an advertisement and a delivery mechanism aimed at particular visitors, and the correspondences are close enough that the likely pollinator can frequently be predicted from the flower. Colour is tuned to the visitor's vision, so bird-pollinated flowers are often red, which birds see well and bees see poorly, while bee-pollinated flowers frequently carry ultraviolet patterns invisible to people that guide the insect to the nectar. Shape controls access, with long tubes excluding all but long-tongued visitors and landing platforms suiting insects that need to perch. Scent is emitted at the time the target visitor is active, so night-pollinated flowers are strongly scented after dark and frequently white. Timing of opening follows the same logic. Some flowers deceive rather than reward, mimicking the appearance and scent of a female insect or of rotting meat and supplying nothing.
What actually helps
Advice on supporting pollinators is plentiful and uneven, and the measures with the best evidence are specific. Providing flowers across the whole season matters more than providing many at once, since the difficult periods are early spring and late summer when little is in bloom, and a garden full of flowers in June and bare in April supports less than it appears to. Native plants generally support more species than ornamental ones, particularly for specialists that can use only certain plants. Leaving bare ground, dead stems and undisturbed patches provides nesting sites, which most solitary bees need and which tidy management removes. Reducing pesticide use, particularly systemic products that persist in plant tissue, has direct effect. Keeping honeybee hives is frequently presented as helping and is closer to livestock keeping, since managed colonies compete with wild species for forage and can spread disease to them.
The decline and what it means
Pollinator populations have declined measurably in several regions, with evidence strongest for managed honeybees, for wild bees in Europe and North America and for insect biomass generally. The causes are multiple and interacting, including habitat loss and the removal of flowering plants from farmed landscapes, pesticide exposure, disease and parasites spread partly through managed colonies, and climate change disrupting the timing between flowering and emergence. The agricultural consequence is real and frequently misstated, since staple cereals are wind-pollinated and the food supply does not depend on insects for calories, while a large proportion of fruit, vegetables, nuts and oilseeds do depend on them, which means the effect falls on nutritional quality and variety rather than on quantity. Wild plant communities are affected more severely, since they have no substitute for the service.
The takeaway
Animal transport delivers pollen where it can work, so the plant produces less of it and invests in nectar and advertising instead. Solitary bees, flies, moths, beetles, birds and bats all carry it, and flower colour, shape, scent and timing are tuned to particular visitors. Cereals are wind-pollinated, so decline affects fruit, vegetables and nuts rather than the calorie supply.