I have a confession to make: I don’t like long walks on the beach. It’s a classic line in a singles’ ad, but I just can’t make myself fit in there.
The problem is that I get bored. So I start looking around for things to entertain myself, and usually wind up with the local wildlife.
While visiting my sister for New Year’s in Panama, where she is a Peace Corps volunteer (more on her community and tropical agriculture later), we spent a few days at a nearby beach on the Pacific ocean. I saw shorebirds like great white egrets and collared plovers and an unidentified species of sandpiper. I saw fiddler crabs abandoning their sand-rolling feeding as I approached to dash into their sandy burrows. I watched tiny green worms and snails inscribe mysterious and winding pathways as they bid farewell to each wave.
I could hypothesize that the birds eat the crabs or worms or snails based on their size difference and the behavior of the birds pecking at the sand, but it would take much more observation and experiments to prove it. In fact, a number of important ecological studies have been done on Pacific beaches.
In 1964, Robert Paine did just that: he spent a year observing and experimenting on the food webs on the Olympic Peninsula in Washington. He published an article in American Naturalist (a prestigious scientific, peer-reviewed journal) titled “Food Web Complexity and Species Diversity.” Paine described an experiment where he kept the purple ochre starfish (Pisaster ochraceus) out of an area for nearly a year. While the species of barnacles, tunicates, and shellfish it fed on remained the same in a neighboring plot, where the starfish was excluded things began to change. One of the barnacles, Balanus glandula, took over 60-80% of the space within a few months, leaving little room for less efficient competitors. But it did not stop there. By the next summer, two other barnacles (Mytilus californianus and Mitella polymerus), tiny and fast-growing, were displacing everything else, even the B. glandula. Sponges and their nudibranch predators, algae, tunicates, and limpets all disappeared as well.
Paine was hardly the first to notice that predators had real effects on their prey’s populations – Aldo Leopold had written decades before of the trophic cascade that resulted when eliminating wolves in Arizona caused deer populations to explode and completely denude their mountainous habitat. Paine’s point was that the entire ecosystem became much less diverse in species and simpler in food webs without that top predator. This was a big deal in ecological thinking: that predation could change the outcome of species’ competition for space!
Of course, now we know that predation goes further than just changing competition: it can have an equal and equally important impact on species diversity. So now I am left wondering which, if any, predators on the Panamanian beach are preserving the diversity I managed to capture in just one, long, boring walk on the beach.