Urchins and crabs vying for dominance


I’ve been thinking a lot recently about this article from Bob Steneck and colleagues: Ecosystem Flips, Locks, and Feedbacks: the Lasting Effects of Fisheries on Maine’s Kelp Forest Ecosystem. In it, they describe two distinct states of shallow subtidal ecosystems on the east coast of the US: a macroalgae dominated system in which urchins are absent and an urchin dominated state where macroalgae is absent and the rocks are covered in crustose coralline algae. This idea isn’t necessarily a new one – Steneck and others have been documenting shifts between these two states for several years.  The general perception is that urchin harvesting reduced urchin abundance in many locations, which allowed fleshy macroalgae to take over. What is new in this recent paper by Steneck et al. is that switching a macroalgal system back to an urchin-dominated form is extremely difficult, and they attribute this to one culprit in particular – crabs.  In relocating thousands of urchins to macroalgal dominated habitats, they found that large predatory crabs reeked havoc on their study subjects, brutally inflicting 100% mortality on recent transplants. Steneck et al. suggest that the crabs are present in such abundance because macroalgae helps them settle as juveniles and survive to adulthood.  In other words, when urchins eat up all the macroalgae, they are reducing habitat for baby crabs, but once the urchins are gone, baby crabs have more macroalgae than they could ever dream of and they go wild, preventing urchins from reestablishing.

I have a number of questions about this paper and I’m not sure all the pieces are there yet to make their case.  But it is an interesting idea, particularly when I think about the patchy distribution of urchins in the Seattle area.  As might be expected, I too see differences in the benthic community in places where urchins are present versus absent (see previous post), with red macroalgae dominating at sites without urchins. So where exactly are the urchins and why are they there?

Here’s a paper from Daniel Cheney and colleagues from the early 1990s: Creation of rocky intertidal and shallow subtidal habitats to mitigate for construction of a large marina in Puget Sound, Washington.  They were writing about the construction of Elliott Bay Marina in Seattle.  Although there is no explicit mention of urchins in their paper, Elliott Bay Marina is one of the only breakwaters in the area where green urchins appear to be present.  All of the urchins I’ve seen there are large adults and the benthic community includes kelps, crustose coralline algae, and fewer red macroalgal species than one finds in similar habitats nearby.  Elliott Bay Marina breakwater may be one of the most recently constructed breakwaters in the area – is it possible urchins were able to settle there before the establishment of large crabs and that the same individuals have persisted all this time?  That might mean that the system is destined to become dominated by red macroalgae when the urchins finally die off from old age…  Hmmmm…