Native bees face heat in different ways
Bees build their homes in many different environments, and those choices may shape how they cope with climate change. While many people are familiar with European honeybee hives inside tree cavities, many native bees use very different nesting sites.
Australia has roughly 2,000 species of native bees, and many of them nest underground, inside plant stems, or in wood cavities. New research published in Nature Communications shows that the kind of nest a bee uses can strongly influence how much heat it can tolerate.
A surprising climate finding
The study found an unexpected pattern: the native Australian bees that can tolerate the highest temperatures may also be the most exposed to future warming.
This matters because scientists are trying to understand whether species can evolve greater heat tolerance as the climate continues to change. If that ability is limited, many animals may struggle to keep pace with rising temperatures.
Why microclimates matter
Most previous studies have compared heat tolerance with average temperatures across a species’ range, often using broad measurements at a 1 by 1 kilometre scale. Those studies usually find little connection between heat tolerance and average air temperatures for land-based species.
That has led to concerns that many animals may have only a limited ability to evolve stronger heat tolerance. But broad temperature averages do not capture the actual conditions animals experience in their daily lives.
The new study focused on microclimates, meaning the temperatures species experience because of their behaviour and where they live. For bees, the nest environment can be very different from the surrounding air temperature.
Researchers crossed mainland Australia
To study how native bees respond to heat, researchers completed a four-month field trip from northern to southern mainland Australia. They collected bees at flowering plants using butterfly nets and then moved them into collection tubes with a device called a pooter.
A pooter is a long, flexible tube that allows researchers to draw bees into a collection vial. A piece of fine gauze inside the device prevents the person using it from accidentally inhaling the insects.
The bees were then taken to a transportable laboratory, where the researchers tested the heat tolerance of more than 95 species of native bees.
How heat tolerance was tested
The team measured heat tolerance by slowly increasing the temperature around each bee until it lost coordination. This allowed the researchers to compare how different species respond when exposed to rising heat.
The results showed a clear link between the temperatures bees experience in their nests and their ability to tolerate heat.
Stem-nesting bees tolerate the most heat
Species that nest in plant stems, which are among the hottest nesting environments, showed the strongest heat tolerance. Bees that nest in cavities, where temperatures are somewhat buffered, had intermediate tolerance.
Ground-nesting bees had the lowest heat tolerance, but they also benefit from the ability to escape extreme surface heat by nesting underground.
Evolution and nesting behaviour are linked
The researchers also found that closely related bee species often shared similar nesting strategies and similar heat tolerances. That made it difficult to determine whether nesting behaviour shaped heat tolerance first, or whether heat tolerance influenced nesting choices.
However, because some closely related bees used different nesting strategies, the team was able to compare patterns across species. They found that both evolutionary history and nesting ecology influence how heat tolerance develops.
This suggests that bees can evolve greater heat tolerance over time, but the process may be slow. Future research will need to determine whether bees can adapt quickly enough to keep up with climate change.
Vulnerability changes when nests are considered
The researchers also assessed which bees are most vulnerable to warming by calculating a measure called the thermal safety margin. This is the difference, in degrees Celsius, between a species’ heat tolerance and the hottest environmental temperature it experiences.
Thermal safety margins are often calculated using air temperatures. But because different nesting strategies expose bees to different microclimates, the researchers compared vulnerability estimates based on air temperature with estimates based on nest temperature.
Heat risk predictions were reversed
When air temperatures were used, ground-nesting bees appeared to be the most vulnerable because they had the lowest heat tolerance.
But when nest microclimates were included, the result changed completely. Stem-nesting bees became the most vulnerable because they cannot escape extreme temperatures in the same way ground-nesting bees can.
Tropical bees face greater pressure
The study also found that vulnerability to climate change tends to increase toward tropical latitudes. This means tropical bees that nest above ground are likely to face some of the highest risks as temperatures rise.
Conservation depends on cooler habitats
The findings have important implications for protecting pollinators. Tropical Queensland still has one of the fastest land clearing rates on Earth, and the loss of native vegetation removes cooler microclimates that bees and other species depend on.
To protect native plant pollination, agricultural pollination, and future food security, Australia will need to conserve tropical forests and native vegetation. Reducing greenhouse gas emissions will also be essential to limiting the warming pressures faced by native bees.
