By Athayde Tonhasca
The recent record-breaking temperatures have been welcomed by many of us, deprived for so long of sunny beaches and bright skies abroad. Canadians and Americans, though, have been less thrilled by summer’s temperatures, which have reached life-threatening levels. And it’s not only the sizzling heat: wild fires, power failures, transport disruption and water shortages for people, livestock and crops have added to the summer misery of many in the northern hemisphere. Heat waves are one of the extreme climatic events that seem to have become more frequent, as predicted by those pesky climate scientists.
Heat waves are a growing risk to people, and to bumble bees too. For the last 10 years or so, researchers have noticed accentuated slumps in the numbers of bumble bees in several European countries following a heat wave – which happens when the daily maximum temperature is 5 °C higher than the average maximum for more than five consecutive days, as defined by the World Meteorological Organisation.
Bumble bees are cold-adapted animals, and some species live comfortably in harsh alpine and arctic environments. This cold-hardiness is partially explained by their ability to generate internal heat: bumble bees cannot take off until their flight muscles reach 30oC: once they are airborne, their body temperature can go up to 40o C. This energy output may give the impression that bumble bees are heat resistant, but that is not the case. Internal heat is good, external heat is not. High air temperatures interfere with their metabolism, quickly leading to overheating and neuromuscular collapse (heat stupor).
Unsurprisingly, species from colder habitats seem to be more sensitive to hot spells. And species that reproduce later in the season are also more affected by heat waves because these events tend to occur towards the end of summer. What are the implications of these observations for our bumble bees? Certainly a sombre outlook for species such as the great yellow bumble bee (Bombus distinguendus), Britain’s most endangered bumble bee. Once widespread, this species has been pushed to the northern fringes of Scotland, where it’s active between May and September. As a cold-adapted, late-nesting bumble bee, it could see heat waves added to its woes.
Most studies about the consequences of climate change have focused on the gradual increase in average temperatures, and the overall conclusion is that many plants and animals in the northern hemisphere are responding to a hotter world by expanding their ranges northwards and upwards (to higher elevations). However, bumble bees have not been very successful in tracking a changing climate; they tend to stay put, thus becoming increasingly exposed to the risks of heat stress and food shortages. The creation, protection and expansion of suitable habitats can improve species’ endurance and facilitate their escape to better areas, but these mitigation strategies will not be as effective against heat waves and other extreme climatic events. Intense and frequent temperature variations are immediate threats: species may not have enough time to move towards better climatic conditions.
People in Britain will have to adapt to a higher frequency of sizzling summers, destructive floods and paralysing snowstorms. And very possibly, become used to a different bumble bee fauna: some species may be squeezed out of their current area, heat-tolerant species such as the buff-tailed bumble bee (Bombus terrestris) may become much more abundant. The consequences of these changes to pollination services are impossible to predict.
Although the damage of climate change to people and nature cannot be reversed, it can be contained. But time is running out.