A lethal combination
It is well established that predators are essential for the structuring and maintenance of biotic communities. One of the first demonstrations of this importance came from studies of the importance of sea otters to the maintenance of kelp forests. Rasher et al. now show that the effects caused by the absence of this predator can be further exacerbated by climate warming. In North Pacific kelp forests, otter absence led to a decline of slow-growing calcareous algae through sea urchin herbivory, and this pattern was amplified by warming temperatures. Keystone predators are thus essential not only for trophic structure but also for mitigating the impacts of climate change.
Science, this issue p. 1351
Predator loss and climate change are hallmarks of the Anthropocene yet their interactive effects are largely unknown. Here, we show that massive calcareous reefs, built slowly by the alga Clathromorphum nereostratum over centuries to millennia, are now declining because of the emerging interplay between these two processes. Such reefs, the structural base of Aleutian kelp forests, are rapidly eroding because of overgrazing by herbivores. Historical reconstructions and experiments reveal that overgrazing was initiated by the loss of sea otters, Enhydra lutris (which gave rise to herbivores capable of causing bioerosion), and then accelerated with ocean warming and acidification (which increased per capita lethal grazing by 34 to 60% compared with preindustrial times). Thus, keystone predators can mediate the ways in which climate effects emerge in nature and the pace with which they alter ecosystems.