Control of the Parasite Population Explosion

When there is no vertical subsystem, both the frequency of parasitism, and the injury from parasitism, are controlled exclusively by horizontal resistance. This horizontal resistance will be at a fairly high level. However, during a rare, abnormal season which greatly favours the parasite, this level of horizontal resistance could be inadequate. Both the frequency and the injury of parasitism will then be high, and the host population might be devastated.

A rare but devastating population explosion of an r-strategist parasite is a serious loss of pathosystem balance that threatens the survival of both the host and the parasite. In other words, the survival of the pathosystem itself is threatened, because of its reduced competitive ability compared with other pathosystems. The vertical subsystem effectively prevents such a loss of balance, and such a threat to survival. It is probable, therefore, that the main function of the gene-for-gene relationship is to stabilise the pathosystem by dampening the population explosions of r-strategist parasites.

When there is pathosystem continuity, and no vertical subsystem, the host species must have alternative mechanisms of recovery from an occasional devastation. Perennial evergreen trees can usually survive the loss of much of a single growing season, just as they would survive an occasional drought. This growth loss would be revealed as a narrow growth ring in the timber. A devastated annual species usually recovers from the devastation by a stock of dormant seeds in the soil. Biennial and perennial herbs have underground tubers and rhizomes, which serve a similar recovery function. Many of these survival mechanisms are also necessary for recovery from other kinds of population devastation, such as fire or drought. Nevertheless, discontinuous pathosystems appear to have a powerful evolutionary advantage over continuous pathosystems.

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