The kelp forest-urchin barrens mosaic: how sea urchin patch dynamics underpin ecosystem stability
From decades of stability to uncertainty
A large-scale ecological shift is occurring along the central coast of California. Seemingly uncontrolled populations of purple sea urchin (Strongylocentrotus purpuratus) grazers have shifted a once continuous kelp forest landscape to a patchwork mosaic of urchin barrens and kelp forests. Until now, mosaic patchiness in the distribution of barren areas within a kelp forest that is attributed to sea urchin grazing has not been recorded along the central coast of California.
My dissertation research capitalizes on the current ecological state to test several hypotheses regarding how sea urchin barren patch formation, persistence, or demise may contribute to the underlying stability of kelp forest ecosystems, including how these patch dynamics might serve as indicators of an impending phase shift.
Images on the right: (top) a photo of a kelp forest at Lovers Point, Pacific Grove, CA haphazardly taken in June, 2012. (bottom) That same location in June 2015.
Where has all the kelp gone? Why are sea otters not controlling sea urchin barrens? What will ultimately lead to the demise of sea urchins and enhance the recovery of kelp forests? These are a few overarching questions motivating my research. I'm specifically addressing the following:
What is the scale of patchiness in the distribution of sea urchin barrens and kelp forests around the Monterey Peninsula?
How are kelp forest and urchin barren patches changing through time?
Will predators (e.g., sea otters, sea stars, crabs) be able to reduce the abundance of sea urchins and promote the recovery of forests?
What other control mechanisms (e.g., disease, disturbance) regulate patch dynamics?
How do patch-level processes scale-up to influence the stability and resiliency of kelp forest ecosystems?
In 2017 and 2018 I conducted over 250 subtidal surveys at random points around the Monterey Peninsula designed to evaluate the extent of kelp forest and sea urchin barrens patches. The figure above illustrates the remarkable patchiness in both kelp and sea urchin distributions. The green circles highlight key areas where a distinct inverse relationship exists between kelp density, urchin density, and gonad index (as a measure of urchin condition, or health). For example, in areas where kelp density is high, sea urchin density is low. As a result of plenty of food availability and low urchin density, sea urchin gonad index is very high. However, there are other areas void of kelp where sea urchin density is high, and as a result gonad index is low. How might predators such as sea otters respond to this variation in prey density and quality?
Given that sea urchins in kelp forests are 'healthy' (as measured by their gonad index), and those in barrens are starved, perhaps sea otters might preferentially target sea urchins in kelp forests, and ignore those in barrens. If so, this would suggest that otters are contributing to the resistance of remaining forested patches, but not directly controlling urchin populations in barrens.
To answer the above question, I partnered with an otter-tracking team to observe patterns of sea otter foraging behavior. At each location where otters where observed eating urchins, I conducted extensive underwater surveys to determine (1) sea urchin focal patches (i.e., locations where otters made repetitive dives for urchins), (2) the density of urchins at each patch, and (3) the quality (or condition) of urchins at each focal patch. These sea otter-urchin focal patches were compared against the 250 random survey sites described above to determine whether or not sea otters preferentially target healthy urchins in kelp forests over starved sea urchins in barrens.
Sea urchin 'focal patches' were determined at areas where sea otters made three or more repetitive dives for sea urchins.
I then conducted underwater surveys at each of those focal patches to determine both landscape features and prey attributes facilitating sea otter foraging preferences.