Extreme heat has extreme effects–but some like it hot

A sweeping new study of the 2021 heat wave reveals major ecological losses—but also surprising species that thrived, offering crucial insight into how climate extremes reshape ecosystems.

Mussels baked by the billions. Insect larvae cooked inside scorched cherries. Baby birds plummeted to their deaths from their overheating nests.

But some species did just fine during the 2021 North American heat wave, according to a new study published today in Nature Ecology and Evolution. With such events projected to become more frequent and intense due to climate change—and 2026 on track to be the hottest year ever—understanding these differing effects is vitally important, the researchers say.

“The heat wave had widespread ecological effects, including an almost 400-per-cent increase in wildfire activity and negatively affecting more than three-quarters of the species studied,” said co-author Dr. Diane Srivastava, professor in the UBC department of zoology and Biodiversity Research Centre.

“With little forewarning, we relied on whatever studies were already under way or observations made during the event,” said first author Dr. Julia Baum, a biology professor at the University of Victoria. “Thus, despite the breadth of this study, I suspect we have barely scratched the surface of the heat dome’s ecological effects.”

Lots of losers, some winners

The 2021 event, which lasted from June 25 to July 2, was one of the most intense climate change-fueled heat waves on record, with land temperatures reaching over 50 C in some areas. The researchers used remote sensing, a meta-analysis of field data, computer modelling and media reports to paint a broad picture of its effects on plant and animal species, as well as ecosystems around B.C.

They found the effects on species were largely detrimental, but nuanced, changing depending on factors such as whether cover was available, a species’ inherent ability to deal with heat, and behaviour including the ability to move to shade or not.

More than half the thatched barnacle population died, as did 92 per cent of Bay mussels. Before the heat wave, one in every two blueberry plants had aphids on them. Afterwards, this dropped to less than one in every 100 plants.

Counts of surf scoters, a native sea duck, dropped by 56 per cent post-heat wave, and daily camera trap detections of caribou dropped by half.

“Basically, any animal that couldn’t escape the heat was hard hit by it,” said Dr. Baum. “This included mobile animals at vulnerable stages of life, such as baby birds that couldn’t yet fly and were trapped in their heat-retaining nests.”

Sea lettuce was among the winners. Better able to handle the heat than its seaweed counterparts, it took advantage of their die-off and proliferated, increasing its beach area by 65 per cent after the heat wave.

Moose bounced back to the same number of daily camera trap detections after the heatwave as before. “We know from previous studies that moose are sensitive to high temperatures, so it’s possible they were able to avoid the hottest areas by selecting cooler microclimates, such as sites with denser forest cover,” said co-author Dr. Cole Burton, an associate professor in the UBC faculty of forestry and environmental stewardship.

Blowing the hydrological budget

The researchers also found that cooler, wetter areas of the province were able to absorb 30 per cent more carbon than usual, while warmer, more arid areas absorbed 75 per cent less than usual. “This challenges the common assumption that heat waves are uniformly harmful to vegetation,” said co-author Dr. Sean Michaletz, associate professor in the UBC department of botany.

Streamflow from snow and ice melt increased 40 per cent during the heat wave as the snowpack melted, before dropping below average later that summer, while wildfire activity surged 37 per cent during the heatwave and 395 per cent the following week.
In August, when alpine systems were baking in the sun and really needed water, there was a deficit—everything had melted already by then,” said Dr. Srivastava.

She warned that repeated heat waves would not only melt snow too quickly, but also start to draw on long-term sources such as glaciers, accelerating their melting.

Monitoring network

One solution could be a coordinated monitoring network of species and ecosystems across the province and Canada itself, said Dr. Srivastava. “We would be more prepared for the next time we have a heat wave to even better understand its effects and therefore, plan for the future.”