Not enough time for recovery: Coral bleaching occurs when stressful conditions result in the expulsion of the algal partner from the coral. Before anthropogenic climate warming, such events were relatively rare, allowing for recovery of the reef between events. Hughes et al. looked at 100 reefs globally and found that the average interval between bleaching events is now less than half what it was before. Such narrow recovery windows do not allow for full recovery. Furthermore, warming events such as El Niño are warmer than previously, as are general ocean conditions. Such changes are likely to make it more and more difficult for reefs to recover between stressful events. Science , this issue p. 80

Significance: Most studies of the impact of global warming focus on the direct physiological impacts of climate change. However, global warming is shifting the distribution of many species and leading to novel interactions between previously separated species that have the potential to transform entire ecological communities. This study shows that an increase in the proportion of warmwater species (“tropicalization”) as oceans warm is increasing fish herbivory in kelp forests, contributing to their decline and subsequent persistence in alternate “kelp-free” states. These tropical and subtropical herbivores are increasingly impacting temperate algal communities worldwide, posing a significant threat to the long-term stability of these iconic ecosystems and the valuable services they provide.