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Question: Are humans parasites?
A parasite is kind of living thing that lives in, or on, another living thing, which It may or may not injure. In order for your question to make sense, there must be some living thing larger than a human, or larger than a human population, for people to parasitize. In order to answer your question, it will help to think in terms of 'living systems' instead of particular living things. Examples of living systems would be plants and animals, it is true, but also ecosystems like forests, beaches, prairies, and some folks would add the whole earth itself as well, since many of its changes are strongly affected by living things, including the people who live on it.
So, people could be thought of as a harmful kind of parasite, for example, in a forest, when they cut down the trees for lumber or firewood, and then prevent forest re-growth by planting crops or grazing cattle where the forest had been. A different kind of relationship that could be thought of as parasitic would be our relations with farm animals, such as chickens. Most of these are now domesticated, with only a few of the original red jungle fowl still living in southeast Asia. Since people raise these birds in order to eat them or to take their eggs, or sometimes to breed fighting champions to bet upon, it could possibly be considered that the human species (Homo sapiens) is a parasite on the chicken species (Gallus gallus). We do damage to individual chickens, but we also promote their species by fostering their breeding. There are now more chickens in the world than there were hundreds of years ago. So, we could be said to be 'good for' chickens as a species, while at the same time we are bad for individual birds by, for example, treating them as prisoners in industrial agribusiness farms. This example shows that the answer to your question depends upon a person's viewpoint.
Consider fisheries. Most of the ocean's fishes that people like to eat, like salmon, codfish and tunas, have been so heavily fished that these species have been much decreased, and their populations have been changed so that there are no longer any large individuals left. As well, it seems that fishes like these, as a result of our fishing, now begin reproducing at earlier ages and smaller sizes. So the human species has certainly had a strong effect on these fishes' lives, but how can we decide if that effect was good for them, or bad? This could only be decided in the future. Modern people's industrial activities have unintentionally injured the lives of many kinds of animals and plants as a result of pollution of various kinds. But these cases, even though the bad effects are caused by our economic activities, would not be examples of parasitism as it is defined. These effects could, however, be discussed as parasitism in connection with the earth itself.
Some scientists have proposed that the earth itself is a giant living thing, which they have named 'Gaia'. This is based on various feedback tendencies that the earth shows in its climate and weather patterns and geological activity, which result in it not making really big unexpected changes very often. This idea is based on the fact that living things show these kinds of regulation. For example, if you start to get cold, your body will automatically begin to shiver, which produces warming heat through the friction generated as muscles rub against each other. Another example is how you feel hungry when your body needs more energy. If we accept the idea that Earth is a giant living thing, then we ourselves are parts of it, not unlike the way that cells in our body are parts of us. It might make sense to think of the human species as having become a harmful parasite on the earth if our economic activities begin to affect the ability of living things to continue to exist here. For example, in the 1980's we learned that our use of certain chemicals was destroying the protective layer of the atmosphere that prevents harmful radiation from the sun penetrating through to the earth's surface. This could result in nothing being able to live except in the ocean or under ground. Fortunately, we were able to change that technology so that this problem is no longer very great.
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