The feared demise of bumblebees could bring the evolution of the plants they pollinate grinding to a halt – leaving them vulnerable to new diseases and other threats – a new study suggests.
Researchers in Switzerland tested what happened when field mustard plants were pollinated solely by bumblebees or hoverflies over nine generations.
The results were dramatic: the bee-pollinated plants grew taller and produced twice as much scent.
But perhaps the most alarming finding was that the plants left to the hoverflies showed a 15-fold increase in the ability to self-pollinate.
This means the plant reproduces itself, without any genetic input from another plant.
If this asexual reproduction becomes commonplace, the plants would struggle to evolve to adapt to new threats, for example from pests moving into an area as the climate warms.
The decline in bee populations has been causing alarm, partly because of their key role as the pollinators of food crops.
The European Union is currently considering whether to licence the weedkiller glyphosate, the main ingredient of Roundup, which has been linked, controversially, to cancer in humans and sub-lethal effects on bees.
Other chemicals used by farmers are thought to be have had a devastating effect on bee populations.
Professor Florian Schiestl, of Zurich University, told The Independent: “Bumblebees and hoverflies have different preferences when the visit the flowers. Those preferences decide which plants are being visited by the pollinator and that means which plants mate with each other.
“We found the bumblebee-pollinated plants were a lot more fragrant. They had about double the scent compared with the hoverfly-pollinated plants.
“The bumblebee-pollinated plants were taller and the hoverfly plants had more self-pollination.”
A paper in the journal Nature Communications about the research said: “Hoverfly-pollinated plants, however, showed a 15-fold increase in the ability to produce seeds without pollinators (that is, autonomous selfing).”
But it added: “Notably, components of inbreeding depression (seed weight and germinationrate) did not differ between pollination groups.”
The researchers cautioned that the study had been carried out in a lab, rather than the real world, and had only involved a single species of bumblebee, hoverfly and plant.
But Professor Schiestl said: “I think there are many reasons why we should not destroy our bees. I think we have added one more to them.”
If plants change to become less attractive to bees, this could increase the pressure on them.
Animals in decline
Animals in decline
1/8 Harbour seal (Phoca vitulina)
Where: Orkney Islands. What: Between 2001-2006, numbers in Orkney declined by 40 per cent. Why: epidemics of the phocine distemper virus are thought to have caused major declines, but the killing of seals in the Moray Firth to protect salmon farms may have an impact.
2/8 African lion (Panthera leo)
Where: Ghana. What: In Ghana’s Mole National Park, lion numbers have declined by more than 90 per cent in 40 years. Why: local conflicts are thought to have contributed to the slaughter of lions and are a worrying example of the status of the animal in Western and Central Africa.
3/8 Leatherback turtle (Dermochelys coriacea)
Where: Indonesia, Malaysia, Mexico, Costa Rica. What: Numbers are down in both the Atlantic and Pacific. It declined by 95 per cent between 1989-2002 in Costa Rica. Why: mainly due to them being caught as bycatch, but they’ve also been affected by local developments.
4/8 Wandering albatross (Diomedea exulans)
Where: South Atlantic. What: A rapid decline. One population, from Bird Island, South Georgia, declined by 50 per cent between 1972-2010, according to the British Antarctic Survey. Why: being caught in various commercial longline fisheries.
5/8 Saiga Antelope (Saiga tatarica)
Where: Kazakhstan, Mongolia, Russia, Turkmenistan, Uzbekistan. What: fall in populations has been dramatic. In the early 1990s numbers were over a million, but are now estimated to be around 50,000. Why: the break up of the former USSR led to uncontrolled hunting. Increased rural poverty means the species is hunted for its meat
6/8 Swordfish (Xiphias gladius)
Where: found worldwide in tropical, subtropical and temperate seas. Why: at risk from overfishing and as a target in recreational fishing. A significant number of swordfish are also caught by illegal driftnet fisheries in the Mediterranean
7/8 Argali Sheep (Ovis mammon)
Where: Central and Southern Asian mountains,usually at 3,000-5,000 metres altitude. Why: domesticated herds of sheep competing for grazing grounds. Over-hunting and poaching.
8/8 Humphead Wrasse (Cheilinus undulatus)
Where: the Indo-Pacific, from the Red Sea to South Africa and to the Tuamoto Islands (Polynesia), north to the Ryukyu Islands (south-west Japan), and south to New Caledonia. Why: Illegal, Unregulated and Unreported (IUU) fishing and trading of the species
“This may have a long-term effect that may not be reversible. Nobody knows if this can be reverted to a trait that is attractive to bees. We may be going down a one-way road,” Professor Schiestl said.
He made clear the dangers of self-pollination over a long period.
“Self-pollination reduces genetic variability in the population. The same individual mates with itself. The idea of sexual reproduction is to increase variation,” he said.
“If that stops, it’s basically a kind of vicious circle. You have less genetic variation so you have less potential to respond to different selection pressure.
“You can reach a stage where you have no genetic variation. Such a population cannot evolve anymore, [for example] to develop resistance against a pathogen.”
Friends of the Earth bee campaigner Paul de Zylva said the study underlined the importance of bees as pollinators.
“Bees aren’t the only pollinator, but many plants will not thrive if they are only visited by other insects, as this new research shows," he said.
“But Britain’s bees are under threat, and we can all do more to help them – such as by growing pollinator-friendly plants, avoiding pesticides and turning gardens and other spaces into bee-friendly habitats.
“And you can check out the bees in your garden, park or neighbourhood by taking part in the Great British Bee Count later this spring.”Reuse content