Pathosystem Stability

Wild plant pathosystems cannot afford wide fluctuations in the level of parasitism. Such fluctuations would severely impair the competitive ability of the host species, both ecologically and evolutionarily. Impairment of the host survival is an impairment to the parasite survival also. It follows that wild pathosystems must have both resilience and stability. The resilience would cope with such fluctuations in parasitism that do occur, and the stability would ensure that these fluctuations remained small.

The most obvious mechanism of stability is the system of locking of the gene-for-gene relationship. It can be argued (see 10.16) that the primary function of the gene-for-gene relationship is to reduce the positive feedback in the reproduction of the parasite. This positive feedback occurs with the very rapid asexual reproduction of an r-strategist parasite, and it results in a population explosion. This asexual reproduction increases exponentially and, given an abnormal summer that favours the parasite, it could lead to an enormous population increase that would be extremely destructive if it remained unstabilised.

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