Anyone who has added ladybugs to their garden to control the aphids that are destroying their roses has been playing with a food chain. The cascades of effects in which predators at the top influence plants at the bottom are fundamental components of the ‘balance of nature’. Yet, despite extensive work on food chains, especially the agriculturally important links between insects and plants, there are still plenty of unknowns. In fact, researchers based at UCDavis have recently uncovered a whole new world of previously hidden connections.

Ecologist Donald Strong, working at the Bodega Marine Laboratory, was intrigued by observations of regular die-offs of bush lupine in patches along the dunes and headlands of the Bodega Marine Reserve. Could this patchy failure of plants be due to a missing link in a food chain? Knowing the importance of root damage to plant survival, Strong and his many collaborators from UCD went underground for the answer.

Bush lupines, it turned out, were being killed by ghost moth caterpillars, voracious eaters of lupine roots. The more caterpillars per plant, the greater the damage.

But if caterpillars were controlling the numbers of plants, what was controlling the number of caterpillars? In those places where bush lupines were thriving, was something eating the root-munchers and thereby protecting the plants?

A great deal of searching for likely predators finally turned up the next link in the underground chain - a carnivorous nematode. This roundworm, only .5 mm long - that’s about 1/50th of an inch - turns out to be as monstrous as it is minute.

The nematodes live underground, and as juveniles, wiggle through the soil in search of prey such as a ghost moth caterpillar. Once a nematode has found its victim, it enters it through a breathing hole or some other orifice. Stay with me here, it gets better. Inside its "host", the nematode vomits up a special type of bacterium with which it has a sort of partnership. Within 48 hours the bacterium has not only killed the caterpillar, but is producing antibiotics that preserve its corpse as well as softening up the insides so that the nematode can feed on it.

In these ideal dining conditions, the nematode matures and reproduces. Being hermaphroditic, it doesn’t have to worry about finding mates, so procreation is a cinch. Whereas one to a few nematodes might originally enter a caterpillar, 6 weeks later, out comes a squirming throng of juveniles, sometimes numbering over 400,000.

How did Strong and his team confirm the nematode’s role in the subterranean food chain? They put ghost moth caterpillars near lupine seedlings with and without nematodes in the soil. The caterpillars burrowed down to the roots and the dirty work began. The only things visible to the scientists were the lupine plants which, two months later, showed dramatic differences.

About half of the seedlings in nematode-free soil were dead, whereas almost 90% of the plants growing with nematodes survived. So the ghost moth caterpillars eat the plants and the nematodes eat the caterpillars. Does the chain get any longer, ecologist now wonder? Does anything prey on nematodes?

Yes. The most likely suspects are among 13 species of nematode-trapping fungi living in the soil. Just how important the fungi ultimately are to the growth of bush lupines is not clear. While they definitely kill nematodes, the fungi also feed on rotting vegetation, which brings a whole different food channel into the picture, one based on the dead, not the living.

As Donald Strong points out, the nematode - lupine connection is just one set of links in a larger network. Above ground, for example, lupine plants contend with seed-eating rodents and flowers-eating insects, all of which have their own predators, and so on, and on. Simple, linear food chains are only parts of more complex webs.

Studies of this sort underline the fascinating complexity of nature, and emphasize its myriad connections. We have to know that if we meddle with one set of links, we can affect things far removed and out of sight. The findings also remind us that for truly deep ecological understanding, we must look below as well as above ground.

© Kelly Stewart

Dept. of Anthropology, University of California, Davis, CA 95616
e-mail: kjstewart@ucdavis.edu

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