A long-running study of wild bees in Spain has found the insects have been losing weight for the last three decades, most likely due to global warming.
Since 1990, at the Spanish National Research Council and his colleagues have hand-netted over 1700 individual bees belonging to 137 species in Sierras de Cazorla, Segura y Las Villas Natural Park, the largest protected wilderness area in Spain.
Weighing them in the lab, the team has found that the average bee’s body mass dropped by 0.7 per cent per year across the study period. Larger species of bee were most affected, losing about 0.9 per cent of their body mass per year. For example, bee species that weighed around 120 milligrams in the 1990s now weigh around 90 milligrams, says Herrera.
Advertisement
The bees’ habitat is situated far from urban and agricultural activity and hasn’t been affected by pesticides, wildfires or invasive species. However, data from local weather stations shows that annual average daily maximum temperatures in the area have increased by 1.4°C since 2000, in line with global warming.
This temperature jump may explain the bees’ diminishing size, says Herrera. Previous studies have found that cold-blooded organisms like bees and fish tend to develop faster in warmer conditions, meaning they reach smaller sizes, a phenomenon known as the “temperature-size rule”.
The bees may also be facing a food shortage, says at the University of Adelaide in Australia. This is because they feed on pollen and nectar from flowers, which plants may struggle to produce when under heat stress, she says.
“During the heatwaves that Australia experienced in 2018 and 2019, many beekeepers were complaining they couldn’t make honey anymore because Eucalyptus trees weren’t producing any nectar or they weren’t flowering at all,” she says.
Other studies have found that bees are also getting smaller in and the . Bee shrinkage is concerning because it may reduce how much pollen they can carry and how many offspring they can produce, says Herrera. This, in turn, may cause their populations to dwindle and impair their ability to pollinate plants, he says.
His team is currently studying the pollination success of bee-pollinated plants in Sierras de Cazorla, Segura y Las Villas Natural Park to investigate if it has been affected.
The researchers are also surveying bee populations in the area and have found that several of the larger species are becoming harder to find.
At current rates of size reduction, the bees could “suddenly reach a tipping point beyond which size won’t matter anymore and it will be population survival that will be at stake”, says Herrera. “Preliminary bee population data in the area suggest that we are on the verge, or perhaps past, the tipping point for some species.”
Ecology
