Parasites Outweigh Predators

Ecologists have left parasites out of their food web models, but now that they are adding them in, they are finding out just how important they are.

Ecologists are beginning to understand that the traditional picture of the food web is missing a vast menagerie of invisible creatures—creatures that might exercise far more control over an ecosystem than even the top predators. Every ecosystem is loaded with tiny parasites—viruses, bacteria, protozoans, fungi, and animals—invisibly feeding on just about every living thing. The Serengeti’s lions, for example, are the sole host to 31 species of worms and flukes, along with two species of bacteria, two arthropods, six protozoans, and ten viruses. In fact, a recent study in Nature found parasites to be so plentiful that, in terms of biomass, they actually outweigh predators in some ecosystems—sometimes by a factor of 20.

Parasites may be the most successful life form on Earth, but scientists know relatively little about them because they’re so hard to study.

Amazing that parasites outweigh predators in many ecosystems, and yet they have been ignored from the food web models up until now.

For example, take a look at the Carpinteria salt marsh ecosystem.

Among the parasites that dominate the Carpinteria food web are flukes, flatworm-like invertebrates related to tapeworms. Flukes start their lives in snails. In a diabolical twist, they castrate the snails, so that the resources the hosts might direct to reproduction can instead go to sustaining the parasite. Flukes can take up as much as one-third of the body mass of a snail, and 17 species of flukes in the salt marsh can live in a single species of snail.

After flukes develop, they escape into the water in a free-living stage as cercariae. Cercariae swim in search of their next host (in this case, fish). The marsh, Lafferty and his colleagues have shown, positively seethes with these free-swimming parasites. He estimates that the cercariae outweigh all the birds of the marsh. In other words, more of the sunlight that strikes the salt marsh ends up as parasite biomass than bird biomass.

More biomass in flukes than birds, amazing.

And this next part is really interesting.

At the Carpinteria salt marsh, for example, parasites help the predators find food. Many flukes in the wetlands need to get from fish into birds in order to mate and produce eggs. The birds, which do not get sick from the parasites, shed the fluke eggs with their droppings. Lafferty has found that when one species of fluke, Euhaplorchis californiensis, infects a killifish, the fish begins to swim jerkily near the surface of the water, flashing its silver scales. This behavior makes it easier for birds to spot the fish and catch them. Lafferty estimates that an infected fish is 10 to 30 times more likely to be eaten than a noninfected one.

Not only do they have more biomass, they affect how many predators there are by making their prey more easily accessible.

via Conservation Magazine