Summer photos 5: intertidal zonation

This post has actually been in my brain for a while. When I was on a collecting trip in May (yes, we're still in May in my picture review), I noticed this on a boulder:

Spotted in the Rhode Island intertidal. The light tan stuff on the tops of the rocks are barnacles. The black clumpy stuff on the bottoms are mussels.

 Looking more closely, you can see these two animals more clearly:

Barnacles (Semibalanus balanoides, probably) and mussels (Mytilus edulis) coexisting on a rock. Note that the barnacles are growing on some of the mussels. If you look carefully, you can spot several periwinkles (introduced in this post) crawling around this habitat.

This is one of the few times that I'm out in the field and really see a textbook example of an important ecological process. What's going on here is called zonation, and it is arguable one of the most important processes in marine communities. It's also a great example of critical experimental work and leads to two of the most influential marine ecologists of all time.

But to back it up a bit, we now have an observation: mussels live lower than barnacles on the shore, and the division between them is often very sharp. It happens that this observation is repeated all over the world on rocky shores.

Zonation on a rocky shore in Washington. The photo's from Wikipedia, and the zones are a little harder to see, but they're there. Also note that in this photo you can see a band of orange and purple sea stars.

Ok, so as scientists we want to know what is causing this pattern that we observe. And this brings me to Joseph Connell and Bob Paine, two ecologists who were crucial in bringing experimentation to the discipline of ecology and changed the way we as marine ecologists do science.

Working in Scotland in the 1950s, Connell designed a series of very elegant experiments with two species of barnacles to show why one lived higher in the intertidal zone than the other, and why they were limited at the lower end of the intertidal zone. To explain the experiment fully takes a whole class period for our marine ecology students, but here is a brief summary of the salient points for these photos:

1. Barnacles dry out if they are exposed to the air and sun for too long (i.e. out of the water too long at low tide). This limits how high they live on the shore.
2. Barnacle predators live lower in the intertidal because they are more sensitive to dessication and heat stress. This means that the barnacles are limited at the lower end of the intertidal zone by a high density of things that eat them.

Paine's experiments were done on a remote island in Washington (Tatoosh Island) in the 1960s. He was interested in the lower limit of the mussel band in the intertidal zone. By going out and removing all of the sea stars (the main predator of the mussels) in his experimental plots, he showed that mussels could grow lower in the intertidal zone in the absence of their predators. To flip that around, their distribution was limited by their survival in the face of predation. But removing the predators did more than extend the mussel zone: it also allowed the mussels to outcompete all other invertebrates in that zone (especially barnacles), leaving a monoculture of mussels. That is, the mussels outcompete the barnacles. BUT, the mussels have a lower dessication tolerance than the barnacles, so they can't get as high on the rocks.

To put all of this together, barnacles are most tolerant of being out of the water, which is why they can live on the tops of those boulders in the first photo. But the mussels can outcompete them lower in the intertidal zone, so there is a sharp division between mussels and barnacles that presumably corresponds to microclimatic conditions on the rocks. There's not many sea stars in the Rhode Island area I was visiting, but if there were they should exist even lower on the shore than the mussels.

So there you have it: one of the most important stories in marine ecology to explain one of the most pervasive patterns on rocky coasts around the world.

See how jagged the lines are here? That probably relates to either variation in microclimate (i.e. warmer / cooler places on the rocks) or potentially disturbance events. I'd have to take temperature measurements to know.