Urchin Stampedes!
“When you’re dealing with the serendipity of nature, you have to do it now, because you never know if it’s going to happen again.” - Alan Shanks
In January of 2006, I visited the tidepools at South Cove of Cape Arago for the first time since late summer. (The absence of daytime low tides below the mean lower low water in the fall makes it more difficult to observe sea urchins in those months.) When I reached the lowest tidepools, I was shocked to find them filled with sunflower sea stars (Pycnopodia helianthoides), and sea urchins were piled up outside of the water. The smell of sea stars, which were present in abnormally high densities, caused sea urchins to “stampede” out of the water so as to avoid being eaten. This adaptive behavior, however, led many sea urchins out of the frying pan and into the fire.Sunflower sea star with a small urchin stampede on the edge of a tidepool
In following months, black oystercatchers and raccoons began eating sea urchins in great numbers. Using behavioral observations
of each predator, I was able to estimate that at just one site, sea stars, oystercatchers, and raccoons ate about 10000, 8600, and 11200 sea urchins, respectively. Since this population of urchins contains well over 150,000 individuals, they are not in any danger of disappearing from South Cove. Still, the presence of high abundances of sea stars (73 in August 2006 and 140 in February 2007) seems to have altered predator-prey dynamics in this system. Sea stars mediate a behavior in sea urchins that make them more available to terrestrial predators. If it weren’t for urchin stampedes, oystercatchers and raccoons would not have eaten nearly as many sea urchins. It was only after urchins evacuated their pits and tidepools that these predators were able to utilize them as a food source.
of each predator, I was able to estimate that at just one site, sea stars, oystercatchers, and raccoons ate about 10000, 8600, and 11200 sea urchins, respectively. Since this population of urchins contains well over 150,000 individuals, they are not in any danger of disappearing from South Cove. Still, the presence of high abundances of sea stars (73 in August 2006 and 140 in February 2007) seems to have altered predator-prey dynamics in this system. Sea stars mediate a behavior in sea urchins that make them more available to terrestrial predators. If it weren’t for urchin stampedes, oystercatchers and raccoons would not have eaten nearly as many sea urchins. It was only after urchins evacuated their pits and tidepools that these predators were able to utilize them as a food source. I have to credit Alan Shanks, my advisor, with encouraging me to keep up my detailed observations through all of 2006, even though I was trying to complete my Master’s thesis on time. Like Alan, I am now a strong believer in serendipity and the importance of taking advantage of what is going on in nature right now. Unique events may only happen once, and since I was lucky enough to notice the Pycnopodia-urchin event near its beginning, I was able to piece together some fascinating natural history.
