The Parasitic Optimum

In order to ensure its own survival, the parasite must produce a minimum biomass each season. If it is an obligate parasite, it can do this only at the expense of the biomass of its living host. But any damage to the ecological or evolutionary competitive ability of its host impairs the survival of that host. If the survival of the host is impaired, the survival of the parasite is also impaired. Consequently, any parasite that damages its host, beyond its own minimum survival requirements, jeopardises it own survival.

It follows that, in a balanced pathosystem, there must be a limit to the production of parasite biomass. Any excess of parasite growth, beyond the minimum required to guarantee its own survival, will threaten the parasite's own survival, simply by threatening its host's survival. Consequently, any mechanism that prevents an excessive parasitic drain on the host biomass, is an evolutionary survival value for the parasite. This argument applies to any mechanism that limits parasitism, whether it occurs in the host, the parasite, or both. And this must surely be the ultimate function of the gene-for-gene relationship, when it functions as a system of locking.

The parasitic optimum may be defined as a level of parasitism such that the parasite does not exploit the host biomass beyond its own survival requirements.

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