What if bees disappeared?

Bees fly from flower to flower. They drink sweet nectar.

As they go, they carry pollen, a yellow dust. Pollen helps flowers turn into fruit. What if no bees came?

When a bee visits a flower for nectar, tiny grains of pollen stick to its hairy body. At the next flower, some of that pollen rubs off. Moving pollen from flower to flower like this is called pollination, and for most plants it is needed to make seeds and well-shaped fruits.

Bees are not the only pollinators: butterflies, moths, flies, beetles, birds and bats help too. So what would happen to our food if bees and other pollinators disappeared? In this lab you can find out on a model farm.

Pollination is the transfer of pollen from the male part of a flower (the anther) to the female part (the stigma) of the same species. It can be biotic, carried by animals, or abiotic, carried by wind, water or gravity; some plants, such as wheat, are self-fertile and pollinate inside the closed flower.

Of more than 250,000 flowering plant species, over 75 percent rely on animals for pollination. This lab asks what a loss of animal pollinators would mean for crop harvests, and why the answer is very different for different crops.

On Olly's farm, there are lots of bees. They visit every flower.

The watermelons, apples, strawberries and tomatoes all grow well. The wheat and corn grow well too.

Our model farm grows 15 crops, from watermelons and apples to tomatoes, wheat and corn. With a healthy number of pollinators, every flower gets enough visits, so every crop gives its full harvest.

The crops are sorted into groups by how much they need animal pollinators: some can hardly fruit without them, some need them a lot, some a little, and some, like wheat, rice and corn, not at all.

The model sorts 15 example crops into five dependence classes from Klein et al. (2007): essential (yield falls 90 percent or more without animal pollinators), great (40–90), modest (10–40), little (under 10) and none. Three crops represent each class: watermelon, pumpkin and cocoa; apple, almond and blueberry; strawberry, sunflower and soybean; tomato, pepper and bean; wheat, rice and corn.

At 100 percent pollinator visits every class yields 100 percent, and all 15 crops count as being at full harvest.

In our pretend farm, Olly turns the bees down to none.

Now the watermelons and pumpkins make hardly any fruit. Apples make fewer. But the wheat and corn still grow just fine!

Turn the pollinators down to zero in the model. The essential crops, like watermelon, pumpkin and cocoa, drop to just 5 percent of their harvest. Apples, almonds and blueberries fall to about 35 percent, and strawberries to 75 percent.

Tomatoes, peppers and beans still give about 95 percent, and wheat, rice and corn give their full harvest, because the wind or the plant itself does the job. Only 6 of the 15 crops stay near their full harvest, and the average falls to 62 percent.

The model's yield is 1 − D × (1 − a), with class midpoints D = 0.95, 0.65, 0.25, 0.05 and 0, and a the pollinator level as a share of healthy. At a = 0 the classes yield 5, 35, 75, 95 and 100 percent. Six crops (the little and none classes) stay at or above the 90 percent full-harvest line, and the unweighted average of the 15 falls to 62 percent.

At half the pollinators, the same rule gives 52.5, 67.5, 87.5, 97.5 and 100 percent, with an average of 81. The losses are concentrated in the fruit, vegetable, nut and seed crops that depend on animals.

Some plants need bees to carry their pollen. Without bees, their flowers do not turn into fruit.

Other plants, like wheat and corn, use the wind instead. They do not need bees.

Plants such as watermelons and pumpkins need pollen carried from flower to flower, and bees and other animals do the carrying. Without visitors, few flowers turn into fruit. Grains like wheat, rice and corn are pollinated by the wind or by themselves, so they don't need animals at all.

The model keeps things simple. Each group uses one middle number for how much it depends on pollinators, the harvest falls in a straight line as visits drop, and the 15 crops all count equally, so the average is not a share of the world's food.

The yield response is driven entirely by each crop's dependence on animal vectors. Wind- and self-pollinated grains (wheat, barley, oats, sorghum, rice and corn are examples) set seed without animals, so they are unaffected.

The model's assumptions: class midpoints rather than each crop's own dependence; a linear yield loss, although a few visits can pollinate many flowers, so real losses may be smaller at first and larger later; equal weights, so the average is not a global food figure; and only yield, not fruit quality, wild plants or the animals that depend on them. Real harvests also depend on weather, soil, variety, which pollinators visit, and whether farmers bring in hives.

Bees have not disappeared. But in some places there are fewer than before.

Lots of people are helping! They plant flower gardens and use fewer bug sprays, so bees and butterflies have food and safe homes.

More than three-quarters of the world's food crops rely at least partly on pollination by insects and other animals, according to the United Nations' food agency. Of 115 kinds of food people eat, 87 need animal pollinators.

In many places, wild bees and butterflies are declining, because of changes in land use, farming, pesticides, invasive species, diseases and climate change. People are helping by creating more flower-rich habitats on farms and in towns, using fewer pesticides, and planting native pollinator gardens. The U.S. Fish and Wildlife Service says any action like this helps.

The 2016 IPBES global assessment, reported by FAO, found that more than three-quarters of the world's food crops rely at least in part on animal pollination, and that US$235–577 billion of annual food production depends directly on pollinators. About 16 percent of vertebrate pollinators are threatened with global extinction, and regional assessments often find more than 40 percent of invertebrate pollinator species, especially bees and butterflies, threatened locally.

The drivers named are land-use change, intensive agriculture and pesticides, invasive species, diseases and pests, and climate change. Options include more diverse pollinator habitat in farms and cities, less pesticide exposure, and better disease management for managed bees. The U.S. Fish and Wildlife Service reports declining pollinator populations across the United States and encourages native pollinator gardens.