Often the parasites themselves have parasites, and some of those parasites have parasites of their own. Parasites can take the form of animals, including insects, flatworms, and crustaceans, as well as protozoa, fungi, plants, and viruses and bacteria. By one estimate, parasites may outnumber free-living species four to one. Indeed, the study of life is, for the most part, parasitology. Parasites are the most numerous types of creature on Earth.
New studies indicate that parasites are also the most influential creatures on Earth.
The Discover article traces the reseach of a sceintist named Kevin Lafferty who set out to detemine how much influence a certain parasite has over an entire ecosystem. Lafferty traced the path of a species of the fluke Euhaplorchis californiensis. The eggs hatch and grow in the snails and render them infertile. There they wait until the fish is eaten by a bird where the cycle starts all over again. Interestingly, Lafferty found that fish infested with the fluke are more likely to wriggle, flash and surface and are 30 times more liklely to become lunch for a bird.