Climate change leaves urchins on a rock in a hard place to live

Abstract

The kelp forests off the West Coast of North America are often described as underwater rainforests – dense, beautiful and teeming with life. Unfortunately, climate change is altering seawater temperature, acidity and oxygen content in the forests, which could be highly stressful for the sea creatures that inhabit them. One of the most valuable players in the kelp forests are red sea urchins, which function as engineers, controlling how dense the forests are through their consumption of kelp. While urchins may be able to cope with changes in one of these stressors at a time, they might not be able to handle changes in all three simultaneously. Ultimately, urchin populations may collapse if they cannot endure this triple threat, leaving the future of the kelp forests uncertain. Emily Donham at the University of California, Santa Cruz, USA, worked with a team of researchers at Reef Check Foundation, USA, Monterey Bay Aquarium Research Institute, USA, and the University of California, Santa Cruz, to find out how resilient red urchins in California are to the environmental changes they face because of climate change.First, the researchers needed to find out what ocean conditions are like in California's urchin habitat. But California is a gigantic state, so the urchins living off the coast of northern California may be accustomed to different ocean conditions when compared with urchins from the south and, thus, have different vulnerabilities to climate change. So, the team placed sensors that measured seawater temperature, oxygen and water acidity in kelp forests along the length of the state. After determining the seawater conditions of the different populations, the team collected northern and southern urchins and brought them back to the lab. Then they held the urchins in seawater that mimicked the two regions and measured how much the urchins grew, ate and expended energy. They predicted that the urchins would thrive more in ocean conditions that mimicked their home environments.As expected, the sea urchins performed better in the water that mirrored their own natural conditions, such that when the northern urchins were held in the southern-type water, they had lower survival rates and grew less than the southern urchins. The same thing could be said for the southern urchins, which outperformed the northern urchins across most tests, but did so to a much greater extent when held in the waters mimicking their own home southern water conditions.The team also discovered that urchins in the north are used to experiencing simultaneous fluctuations in seawater temperature, acidity and oxygen levels, which makes these changes more predictable and therefore potentially easier for the urchins to react to. In contrast, the southern urchins appear to be accustomed to a more unpredictable environment. As climate change will make seawater conditions more erratic and variable, the team expected that the northern urchins would struggle more than the southern urchins when exposed to a climate change scenario.To test this, Donham and the team conducted a second study in which they exposed urchins from each population to conditions that simulated how climate change will affect their local environments. They found that climate change will harm urchins across California, as both populations grew and ate less in the future climate change conditions. But, surprisingly, the urchins in the north had a greater chance of surviving those troubling times. This is concerning news for urchins and kelp forests in the south but, hopefully, the populations will persist if enough urchins are able to withstand the coming changes. Ultimately, the best thing we can do for urchins is fight to keep climate change predictions from becoming reality.