Equal Rights for Parasites

EQUAL RIGHTS FOR PARASITES DONALD A. WINDSOR" "Equal rights for parasites" is a slogan which I champion [I]. From a medical and veterinary aspect, parasites are unwanted invaders that must be eliminated. Of course, they are; I certainly do not want any in or on me. However, from a strictly biological perspective, encompassing the entire biosphere, parasites play an essential role. In a period in which every minority group is demanding equal rights, my campaign is notjust another plea for recognition. For one thing, parasites are definitely not a minority; they are a majority. Every species of freeliving plant or animal has parasites, so balancing species specificity with multiple hosts and multiple parasites, the number of parasites probably exceeds the number of free-livers. Anything so pervasive in nature must play a role in the mechanics of the biosphere. Parasites do just that. In fact, my theory is that parasites, by exerting a cohesive drag on natural selection, act like a "glue" which holds the biosphere together. That is, parasites do not hamper life on earth; they make it possible. Parasites are a burden to their hosts. They present an obstacle which has to be overcome, the same way that weather, competitors, and predators do. In fact, all of these components of the habitat immerse the host in a "struggle matrix." Weather can be endured. Competitors and predators can be eluded. But the insidious nature of parasitism is hard to avoid. Perhaps a business world analogy would be enlightening. Parasites drag down their hosts the way taxes and interest rates drag down an economy . But taxes and interest rates also prevent an economy from "overheating ," from growing too fast and then crashing. Slow steady growth is what our Federal Reserve Bank tries to achieve, and taxpayers constantly badger legislators to lower taxes. So too with parasites. The parasitic burden is resisted by the hosts, through their immune systems and evasive actions, the so-called "arms race." One side gets an advantage and the other side overcomes it and gets its own advantage. When it comes, the other side *P.O. Box 604, Norwich, NY 13815.© 1996 by The University of Chicago. All rights reserved. 0031-5982/96/3904-0975$01.00 222 Donald A. Windsor ¦ Rights for Parasites retaliates and on it goes. The overall effect is an entanglement ofall players in a cohesive matrix. Our economy is such a cohesive matrix, as is our biosphere.Just as taxes and interest rates hold our economy together, parasites hold our biosphere together. It has long been an axiom of physics that "nature abhors a vacuum." An equivalent motto for biology might be that "nature abhors a monoculture ." Monocultures are always limited to finite areas or time spans. All human attempts at raising monocultures have met intense biological resistance . Consider the enormous amount of money and energy spent trying to maintain agricultural plantings. Parasites disrupt monocultures. If it were not for parasites, the Earth would be covered with just a few very successful species. Biodiversity could not exist. Sex has been acclaimed as making the world go around. Well, if it were notforparasites therewould be no sex. The main purpose ofsexis to produce newcombinations ofgenes, so thatthe resultingoffspringcan adaptto changing conditions and to escape parasites. When a strong, handsome male struts around showing off to impress females, he is actually proclaiming his health, which includes freedom from parasites. Health and vigor go hand in hand. Brightly colored birds may have evolved so that their parasite burdens can be more accurately assessed by females [2] . While male plants cannot strut, flowers are advertisements to pollinators and good blooms are found on healthy plants. Even plants with wind- or water-borne pollination compete for the bestvantage points for dispersal. The same is true for sessile animals. Sex, of course, also serves nature in other ways, but let us here give credit to parasites for their role. Parasites play positive roles in nature which are not normally associated with the negative connotations of morbidity and mortality. It is these roles that cement relationships within the biosphere, the cohesive "glue" aspect of parasitism. A slogan in ecology is "everything is connected." These connections are usually thought...