Alaskan reefs face danger from predator loss and climate change

The Aleutian Islands are a string of 14 large volcanic islands and 55 smaller islands stretching from Alaska to Russia. If you happen to be an avid trivia player, you may know them as being both the most westward and eastward points of the United States by longitude. These islands are also home to a study in the journal of Science by Douglas Rasher titled “Keystone predators govern the pathway and pace of climate impacts in a subarctic marine ecosystem”.

Rasher starts off by pointing out how the Anthropocene (an age defined by human’s altering our climate) is marked with predator loss. The Aleutian Islands are home to dense kelp forests that are historically inhabited by healthy populations of sea otters, a key predator. Over the past 30 years these predators have seen dramatic declines. Like most ecosystems, the effects of predator loss do not stop there. One of the sea otter’s favorite foods are urchins, which are known for being veracious grazers on just about everything. Now urchin populations are skyrocketing, and the effects of that are widespread. The structure of the Aleutian kelp forests is actually built on a crustose coralline alga by the name of Clathromorphum nereostratum. Unlike the type of algae that probably pops into your mind these organisms are more akin to coral reefs. C. nereostratum grow over the course of centuries to millennia and like corals build large frameworks of calcium carbonate structure that create habitats for whole ecosystems. Urchins preferred meal is soft fleshy kelp, which they chomp down in alarming speeds leaving what is called urchin barrens which consists of nothing but the urchins and our friend C. nereostratum.

Historically C. nereostratum hasn’t had to worry about being eaten by urchins because the urchins have a preferred food source and they have a hard calcium carbonate structure that was designed to protect them from this very problem. Unfortunately, the urchins are running out of food alternatives and C. nereostratum might not have as strong a protection as it once did. Another thing the Anthropocene is marked by is raised CO2 emissions which the ocean absorbs and in doing so makes itself more acidic, a process coined ocean acidification. Rasher hypothesized that for C. nereostratum, this is a problem because in more acidic conditions it may not be able to build as strong of a calcium carbonate structure that would protect itself from hungry urchins. These compounding problems are what Rasher and his team are trying understand, they are trying to uncover what these Alaskan reefs will look like in the future.

To test Rashers hypothesis, he and his team were able to actually take cross-sections of C. nereostratum and, similar to tree growth rings, C. nereostratum grows in layers and within those layers they were able to discern marks of grazing. C. nereostratum was also collected and grown in tanks that reflected future ocean acidification conditions and then exposed to hungry urchins. Rasher and his team discovered that in 2014 alone 40% to 85% of every colony had lethal grazing defined by bite marks that go deeper than what C. nereostratum can recover from, a process called bioerosion. The other grim news was that C. nereostratum that grew in tanks that mimicked ocean acidification conditions had higher rates of bioerosion.

Rasher and his team went far beyond your average climate change study. If they had simply stopped at the discovery that C. nereostratum is being affected by ocean acidification we would not have been given the full picture. C. nereostratum is facing an attack from two sides; a population collapse causing a trophic cascade leading to increased urchin herbivory, and ocean acidification causing it to have weakened protection. Rashers research has broad impacts for understanding how the Aleutian kelp forest ecosystem will continue to change but is also a valuable example of the complexity of nature and how a problem is rarely isolated.

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.