Calling All Commensals!

I have just posted a video in which Darwin explains that there are three rough categories of ecological interaction between any two species: parasitism, commensalism, and mutualism. Of course, whenever a scientist says “there are three categories” this is almost always an oversimplification—even if a useful one.

In parasitism, the parasite gets most or all of its resources from the host. Obviously, the parasite benefits, and the host is harmed, at least over the long term. A botanical example that is abundant in the forests of Alsace, from which Darwin reports, is mistletoe growing on a tree. The mistletoe roots actually grow into the wood and extract water and minerals from it, thus harming the tree.

In mutualism, both species benefit from the interaction. A botanical example from Alsace is lichens, in which single-celled algae live inside a sandwich of fungus tissue. Both species benefit from this: the algae make food when the weather is moist, and the fungi protect the algae from damage when the weather is dry.

Both parasitism and mutualism are fairly clear-cut. But commensalism? This is when the commensal benefits and the host is unaffected. The example Darwin uses is vines growing on a tree trunk. The vines do not actually put their roots into the tree tissue; the vines just climb on the tree like a trellis. This would seem to be commensalism: the vine benefits from getting its leaves up into the sunshine without having to make its own trunk, while the tree just stands there.

But many times the “host” is negatively affected by the commensal. In the case of the vine and the tree, the host has to bear the extra weight of the vine. A little vine might not have much impact, but a large vine can weigh a lot (see the photo). And while a vigorous tree might be able to bear the extra weight in normal times, the tree could collapse in times of high wind—or even without high wind. In fact, the vine might even shade the tree leaves, reducing the growth of the tree.

When you think about it this way, maybe there is no such thing as commensalism. The best example is the microbes that live in our guts. Some of them are mutualists—they produce waste products that enhance our nutrition by digesting molecules that the human gut cannot—and some of them are parasites under certain conditions. But there may be none of them that are commensals all of the time. We seldom notice the little mites that live in our hair follicles—that is, unless there are too many of them, or we have an allergic response to them.

Vine to tree: Thanks for letting me grow on you.

Tree to vine: I’m not dead yet!