While vanishing bees have raised concerns for crops in the US, German researchers say some crops may suffer even if there are plenty of bees around. What matters is the different types, but bee biodiversity is also declining.
About a third of global food production, and possibly two-thirds of major crops, depends on pollination by animals, mainly bees.
AdvertisementPat Hoehn of the University of Göttingen, Germany and his colleagues realized the key to future while watching pumpkin cultivation on the Indonesian island of Sulawesi.
The team found that 25 different species of bee pollinated the pumpkins, with different sets of species visiting different pumpkin patches depending on the local temperature, humidity and height.
Because pumpkins have male and female flowers, they must be cross-pollinated to develop seeds. If insufficient pollen reaches the female flower it will develop fewer seeds, and smaller pumpkins, writes Debora MacKenzie in New Scientist.
The team watched which bees visited different pumpkin patches, and found that the more species of bees visited pumpkin flowers, the more seeds the pumpkins had.
The researchers then classified the bees into eight "guilds" - groups of species that all pollinated at the same time of day and height off the ground.
Pumpkin patches growing in different conditions were visited by different numbers of guilds.
The plants visited by only three guilds averaged fewer than 200 seeds per pumpkin. Those serviced by six or seven had 400 or more seeds - as many as when the researchers themselves smeared ample pollen on the female flowers.
The data showed it was not the number of bees, or the number of species, but the number of guilds that made the most seeds. This means, the team concluded, that the plants need different kinds of bees visiting at different times and behaving in different ways to get maximum pollination.
Jacobus Biesmeijer of the University of Leeds in the UK, points out that plants have different pollination strategies. Some have specialised flowers and only one or two pollinator species - the pollinators are efficient, but the plant is in trouble if those species are harmed.
Others have wide open flowers that can be pollinated by many species. For these, he says, "diversity provides insurance against pollination failure.
A group of pollinators is likely to be active during a broader range of climatic conditions, so together they provide better pollination service to certain crops." On the other hand, no one species is very efficient, so the plant needs the diversity.
Human-induced habitat change is hurting many pollinators, says Biesmeijer. "Crops with specialist pollinators will be hit hard if their visitor is affected. The new study seems to show that even crops with 'back-up plans' - a number of pollinators - can be adversely affected by habitat fragmentation."
In a world where food production needs to increase, the team hopes these results will lead to more efforts to conserve bee species.