Parasites

Though organisms that parasitize humans have been well understood, for obvious reasons, for a long time, many biologists have neglected to study the effect of parasites on ecosystems. In a recent article in Discover magazine (August 2000), Kevin Lafferty, a marine biologist at the University of California at Santa Barbara discusses the impact of parasites.

Parasites make up the majority of species on Earth. By one estimate, parasites may outnumber free-living species by four to one. Every living thing has at least one parasite that lives inside or on it, and many, including humans, have far more. Often the parasites themselves have parasites, and some of those parasites have parasites of their own.

Some parasites must journey through a series of animal species in order to survive and procreate. Such parasites can exert extraordinary control over their hosts, transforming them into seemingly different creatures. They can change a host’s looks or scent to appeal to a predator. They can even alter its behavior to force it into the next host’s path.

One example of the control parasites can have over their host is in the case of Euhaplorchis californieinsis, a species of fluke that who uses California killifish as a host. The fluke attaches to the fish’s gills and then migrates its way to the brain. There, the flukes are able to manipulate the fish’s behavior such that it will make the fish more likely to be eaten by birds. Once inside the bird, the flukes then make their way to the liver, mate, and the eggs are released into the environment in the birds’ feces.