Study: Parasites Sometimes Essential

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Parasites get little attention. But a new study shows that parasites can sometimes be an impressive presence in an ecosystem. A study of three Pacific estuaries found that parasites there actually outweighed larger animals, such as birds.

MELISSA BLOCK, host:

Parasites get no respect, and not much attention, either. But a new study shows these pests can be an impressive presence in an ecosystem, and that may teach us more about how these ecosystems work.

NPR's Richard Harris has the story.

RICHARD HARRIS: Parasites are well-known in the world of medicine. Think of intestinal worms or malaria protozoans, for example. But in the world of ecology, parasites barely get a second thought.

Armand Kuris says there is a reason for that.

Dr. ARMAND KURIS (Biologist, University of California, Santa Barbara): Parasites are perceived, I think, as small. If you were to do a word-association game with the word parasite, small would be one of the words that would come up.

HARRIS: But Kuris, at the University of California at Santa Barbara, says ecologists have been too quick to write them off.

Dr. KURIS: Parasites often have very big effects.

HARRIS: Kuris thinks of them as stealth predators - not eating their prey all at once but slowly and inexorably sucking energy out of them. He and his lab set out to get a sense of how big an effect parasites would have on an entire ecosystem. They studied three estuaries in California and Mexico. First, they counted up all the obvious plants and animals and found more than 200 species.

Dr. KURIS: We also sampled every one of those species for all the parasitic things inside them that we could find and quantified those with equal care.

HARRIS: And they found 150 species of parasites. When they then estimated the total weight of all the organisms, they were startled to see that parasites outweighed all the top predators in the ecosystems, that is, the birds. They're reporting that result now in Nature magazine. And it's a surprise, in part because ecologists have long assumed that parasites face some pretty strict natural limits.

Dr. KURIS: All parasites, as they take a bite out of their host, degrade their own habitat a bit. And therefore, there's a sort of tradeoff that most parasites have to deal with: don't take too much or you'll die, too.

HARRIS: In these estuaries, though, the most abundant parasites may have managed to skirt that rule. The parasites are (unintelligible) worms that mostly invade snails. Their trick is they effectively castrate the snails and then hijack all the energy the snails would have put into their own reproduction.

Dr. KURIS: Instead of the snail making snail babies, it now, for the rest of its life, makes worm babies.

HARRIS: Kuris suspects that parasites in other ecosystems will also turn out to outweigh the top predators there as well. And weight matters to ecologists. They measure weight to track the energy through a food web. It's an easier way to track energy that starts as sunlight for plants and then ends up as calories in, say, a heron's diet.

But that's also why Kuris' conclusions are drawing some skepticism. Michael Sukhdeo at Rutgers University says he doesn't believe that parasites play a bigger role in this energy flow than top predators.

Dr. MICHAEL SUKHDEO (Rutgers University): What they're arguing is that there is another predator in the system that is sucking all this energy. And that is just untenable. It might be true because, you know, we have to sort of respect their data. But I think what they have done is baffle-gabbed us.

HARRIS: As in talked gobbledygook. Sukhdeo says if you calculate the total mass of these parasites using a different method, you'd come up with a much smaller number. In fact, his study of another ecosystem found that parasites are only one-tenth the mass of the top predators.

Dr. SUKHDEO: I am not at all arguing that parasites are not important. They're extremely important. But it's how they affect their host, how they cause disease, for example, how they extract the energy.

HARRIS: And both these scientists are on the vanguard of trying to figure all that out.

Richard Harris, NPR News.

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