Sea otters prey on urchins and keep their population in check. When otters disappear, urchin populations explode, leading to overgrazing on kelp and a decline in kelp forests. In the cold waters of the North Pacific, a long-lived calcifying alga is a key component of these forests. Douglas Rasher and colleagues show that with the continued decline of the sea otter population in Alaska's Aleutian Islands, urchins have heavily overgrazed this alga and that ocean warming and acidification have amplified the negative impacts of grazing by reducing the alga's resistance. The lethal combination of otter loss and warming shows how the healthy presence of keystone predators in a community can be important for mitigating the impacts of climate change. To quantify the impact of urchin overgrazing, the researchers looked at the urchin bite scars left on the reefs between 2014 and 2017. Urchins could bite deeper into the climate-weakened reefs, sometimes removing up to seven years' worth of prior algal growth in one bite and reducing the alga's capacity to generate new growth. Among the six Aleutian islands examined in the study, reefs lost on average 24% and up to 64% of their total calcified reef framework over the three years. Rasher et al. suggest that the reef erosion began with the otter loss and resulting urchin explosion.