PARASITES BENEFIT THEIR HOSTS — AT THE SPECIES LEVEL
Donald A. Windsor
PO Box 604, Norwich NY 13815 USA
Why do hosts keep getting parasitized?
That big question bothers me because, surely, throughout all the eons of time that life has evolved on this planet, some host species would have managed to break free from the perpetual shackle of parasites. But no. Every free-living species has some other species parasitizing it. Why? It seems as if hosts are addicted to parasites.
The standard answer is the “arms race”. Whenever the host develops a better defense, the parasite develops a better offense. This arms race goes on in perpetuity. While accepting this answer, I suspect that there is more to it.
Perhaps parasites confer some benefit to their hosts. I have been trying to figure out what that benefit could possibly be. Finally, I think I have found it. Parasites help their hosts survive — but at the species level — not at the individual level.
What is good for a population may not be good for its individual members. For example, it is good for our society that each of us pay taxes. However, it is certainly not good for us as individuals. But we taxpayers do expect that our taxes will benefit us. Taxes are, in effect, the expense we pay for government.
Parasites, by definition, are symbionts that benefit themselves at the expense of their hosts. So, what are the hosts buying by paying these expenses? Survival. That is my claim based on my observations of host-parasite interrelationships. The reason is that parasites can harm their hosts, especially when their populations increase. But, it is the individual hosts that are harmed. The population, as a whole, can sometimes benefit from the loss of individuals. To be sustainable, ecosystems must have a way to regulate the populations of their members. When monocultures get too large, and competition and predation are not adequate to limit them, diseases break out.
A good example of how a deadly parasite can help its host species survive is anthrax. Caused by the bacterium, Bacillus anthracis, this disease plays an important ecological role in nature. B. anthracis spores lie dormant in the soil. When a grazer, such as a bison, uproots a tuft of grass, it disturbs the spores and contaminates itself. The spores may be touched, ingested, or inhaled. The infected animal is able to wander around symptom free for a few days before the disease disables it. Once it does, the animal dies. Meanwhile, the B. anthracis multiply rapidly, building up a high population. When the nutrients run out, the waste products build up, and the carcass condition becomes uninhabitable, the B. anthracis sporulate. As the carcass decomposes, the spores gravitate into the enriched soil. Plants move in and cover the site. From the perspective of the individual grazer, B. anthracis is indeed a deadly parasite. However, when viewed from the perspective of the grassland ecosystem, B. anthracis is the guardian of the prairie.
Grazers that nip grass blades without pulling up the roots have a much better chance of avoiding anthrax. Likewise, land that is not overgrazed will have more lush foliage for grazers to nibble and more decaying plant remains to bury the spores even deeper. That is, with B. anthracis spores in the ground, overgrazed land is deadlier to grazers. A field that is overgrazed will have patches of bare ground between the surviving tufts of grasses and forbs. During dry periods, dust containing anthrax spores and can be ingested and/or inhaled by the grazer. As the grazers die off, the field can recover. Later, when it is eventually overgrazed again, the latent anthrax spores will once more defend it. Perhaps this is a natural way in which grassland ecosystems regulate themselves. They punish overgrazers by imposing a death penalty.
There are several other ways that parasite species can benefit host species. I am currently preparing a full article, with references, on this subject, but some of the more elusive concepts still need clarification and more examples are being researched.