The 2021 Heat Dome Killed Billions of Mussels but Let Sea Lettuce Thrive

Published in Nature Ecology and Evolution. Lead author Julia Baum, University of Victoria; co-authors at University of British Columbia.

Billions of mussels baked on shorelines. Insect larvae cooked inside scorched cherries. Baby birds fell to their deaths from overheating nests. The 2021 North American heat dome, which drove land temperatures past 50 degrees Celsius in some areas during a week in late June, left an ecological toll that scientists are still tallying.

A new study published in Nature Ecology and Evolution attempts the most comprehensive accounting yet of what that single event did to the ecosystems of British Columbia. The picture is overwhelmingly destructive -- but not uniformly so. Some species did not merely survive. They flourished.

The casualties by the numbers

The researchers, led by Julia Baum of the University of Victoria and Diane Srivastava of the University of British Columbia, combined remote sensing, field data meta-analysis, computer modeling, and media reports to assemble a broad picture of the heat wave's ecological impact.

More than half the thatched barnacle population died. Bay mussel mortality reached 92%. Before the heat wave, one in every two blueberry plants had aphids on them; afterward, fewer than one in 100. Counts of surf scoters, a native sea duck, dropped by 56%. Daily camera trap detections of caribou fell by half.

The pattern was stark: any animal that could not escape the heat was hard hit. This included mobile animals at vulnerable life stages -- baby birds trapped in heat-retaining nests, for instance, that could not yet fly to cooler locations.

Wildfire activity surged 37% during the heat wave itself and 395% the following week. Streamflow from snow and ice melt spiked 40% as the snowpack liquefied prematurely, then dropped below average later that summer when alpine systems needed water most -- a hydrological budget blown in a matter of days.

The winners

Sea lettuce, a heat-tolerant green alga, took advantage of the die-off among competing seaweeds. It expanded its beach area by 65% after the heat wave, filling ecological space vacated by less resilient species.

Moose, despite being known to be sensitive to high temperatures, bounced back to the same number of daily camera trap detections after the heat wave as before. Cole Burton, an associate professor at UBC, suggested they may have avoided the hottest areas by selecting cooler microclimates, such as sites with denser forest cover. The ability to move -- and the availability of somewhere cooler to move to -- appears to be a critical survival factor.

Cooler, wetter areas of the province also showed an unexpected response: they absorbed 30% more carbon than usual during the heat wave, as warmer temperatures boosted plant metabolism in regions where moisture was not limiting. This challenges the common assumption that heat waves are uniformly harmful to vegetation. In warmer, more arid areas, by contrast, carbon absorption dropped by 75%.

What determined survival

The study found that the effects on species were nuanced, depending on several factors: whether physical cover was available, a species' inherent heat tolerance, and behavioral flexibility -- particularly the ability to relocate to shade or cooler microclimates.

Sessile organisms -- those physically attached to surfaces, like barnacles and mussels -- had no option to flee. Their survival depended entirely on their thermal limits, which the heat wave exceeded dramatically. Mobile species had a better chance, but only if they had the capacity to move and suitable habitat to move into.

The interaction between heat and other environmental factors also mattered. The carbon absorption data show that heat alone is not the full story -- moisture availability, elevation, and local vegetation type all mediated the response.

A study limited by surprise

Baum was candid about the study's constraints. The heat dome arrived with little warning, and the research team had to rely on whatever studies were already under way or observations made during the event. She suspected the analysis has barely scratched the surface of the heat dome's full ecological effects.

The data are heavily weighted toward species and ecosystems with existing monitoring programs. Many organisms -- soil invertebrates, deep-water fish, small mammals in unmonitored habitats -- were simply not being tracked when the event hit. The true scope of ecological damage is almost certainly larger than what the study captures.

The reliance on media reports for some data points introduces additional uncertainty. News coverage tends to focus on the most dramatic impacts, potentially skewing the picture toward catastrophic outcomes and underrepresenting species that experienced moderate effects.

Long-term recovery trajectories also remain largely unknown. The study captures the immediate aftermath and short-term responses, but whether populations like thatched barnacles and Bay mussels can recover -- and how long that might take -- requires ongoing monitoring that in many cases does not exist.

Preparing for the next one

With 2026 projected to be the hottest year on record and heat waves expected to grow more frequent and intense under continued climate change, the findings carry urgent practical implications. Srivastava warned that repeated heat waves would not only melt snow too quickly but also begin drawing on long-term sources such as glaciers, accelerating their loss.

She proposed a coordinated monitoring network of species and ecosystems across British Columbia and Canada -- a system that would allow scientists to capture ecological data during the next extreme event, rather than scrambling to reconstruct impacts after the fact.

The study's central lesson is that heat waves do not affect ecosystems as a single, uniform force. They create ecological winners and losers based on physiology, behavior, geography, and chance. Understanding that complexity -- before the next event arrives -- is the difference between preparation and triage.