Article: Rasher, D. B., Steneck, R. S., Halfar, J., Kroeker, K. J., Ries, J. B., Tinker, M. T., Chan, P.T.W., Fietzke, J., Kamenos, N.A., Konar, B.H., Lefcheck, J. S., Norley, C.J.D., Weitzman, B.P., Westfield, I.T., & Estes, J.A. (2020). Keystone predators govern the pathway and pace of climate impacts in a subarctic marine ecosystem. Science, 369(6509), 1351-1354. DOI: 10.1126/science.aav7515
Scientists have long been warning about the dangers of climate change and the loss of species on ecosystems. While we know that either one can be disastrous for ecosystems on its own, less is known about what can happen when both occur together. Given the complicated interactions that occur between plants, animals, and the physical environment, there is the potential that the combined effects of climate change and species loss can be much worse than the sum of its parts.
A team of scientists led by Douglas Rasher from the Bigelow Laboratory for Ocean Sciences looked at the combined impacts of climate change and species loss in the kelp forests off the coast of the Aleutian archipelago. The kelp forests grow on top of large limestone reefs, which are built by a species of algae. While we often hear about tropical reefs being impacted by rising ocean temperatures and rising carbon dioxide levels (see an example and explainer from another Envirobites post), these subarctic reefs can also be impacted by climate change. More specifically, the rising temperatures and higher carbon dioxide can weaken the structure of these slow-growing reefs.
In addition to the impacts of climate change, subarctic reef ecosystems have also been impacted by the disappearance of sea otters over the last 30 years. Otters nearly went extinct because of the fur trade in the 19th century. However, recent reductions in otter population numbers is thought to be driven by increased predation from killer whales, likely due to the disappearance of other killer whale food sources from industrial whaling. In ecosystems where sea otters live, they often act as a keystone species, which means that the ecosystem is particularly dependent on them, and without them there could be significant changes. One big way that sea otters historically have an impact in the Aleutian kelp forests is by eating a lot of sea urchins, one of the biggest predators of the reef-building algae.
Were otters’ paws putting a pause on impacts of climate change?
The scientists set out to determine the simultaneous impacts of climate change and a decreasing otter population to the reefs. The team hypothesized that the reefs would be experiencing greater damage as a result of these coinciding events. To see if this was true, they made observations across the Aleutian archipelago, estimating the extent of urchin damage to the reefs. The scientists were able to see how this impact from the urchins has changed over time by examining scars that are left behind on the reefs from the urchins. In addition, Rasher and colleagues did experiments to see how different water temperatures and carbon dioxide affect the reefs susceptibility to urchin bites.
Lack of otters leaves reefs in a lurch with too many urchins
Across the Aleutian archipelago, the scientists observed extensive destruction of reefs due to urchin grazing. The rate of damage from urchins increased in the 1990s, corresponding to the time that sea otter numbers started to decline. This suggests that sea otters were playing an important role in helping to protect the reefs by keeping the number of urchins under control. The experiments showed that with higher temperatures and higher CO2, the reef structure becomes weakened such that each bite from an urchin is bigger and more damaging than under cooler temperatures and lower CO2. On top of that, there was evidence that warmer temperatures also cause the urchins to eat more, further exacerbating the damage.
All of these combined effects mean serious damage for the reefs and the kelp forests they support. The results demonstrate how multiple stressors can be ecologically disastrous. It appears healthy otter population numbers can help to mitigate some of the impacts of climate change. This study is an important step in helping us to understand the simultaneous impacts of climate change and species loss, as well as a warning sign that we must keep working to prevent further damage on both fronts.