Frequency and Injury

The frequency of parasitism is the proportion of host individuals that are parasitised. The injury from parasitism is the damage suffered by those host individuals that are parasitised, usually expressed as an average. It is a fundamental feature of pathosystem balance that frequency and injury are inversely proportional. This is because the total theft of nutrients by the parasite from the host is fixed according to the parasitic and resistance optima. This total is consequently large enough to ensure the survival of the parasite, but small enough not to impair the survival of the host. A balanced pathosystem will have powers of recovery from the occasional, small transgression of these optima.

It follows that the greater the frequency of host individuals that are parasitised, the less the average parasite injury to each individual, and vice versa. The total host tissue consumed by the parasite is a constant, defined by the limit of acceptable loss to the host population. This loss of host tissue must not impair the survival of the host, by being too great, or the survival of the parasite, by being too small.

Obviously, frequency and injury are continuously variable. When comparing a wide range of different plant pathosystems, there is continuous variation between an extremely low frequency and an extremely high frequency of parasitism.

The extreme of low frequency and high injury is often called the predator-prey relationship. At this extreme, a small minority of host individuals are totally consumed by the parasite, while the majority of host individuals escape unscathed. For example, a pride of lion will entirely consume one zebra while leaving the remainder of the herd untouched. In ecological terms, this represents the extreme of a patchy distribution of parasitism.

The opposite extreme of high frequency and low injury is often called the host-parasite relationship. At this extreme, all host individuals are injured equally, but they are injured only slightly. For example, ticks will parasitise every zebra individual in the herd but will cause negligible injury. In ecological terms, this represents the extreme of a uniform distribution of parasitism.

In plant pathosystems, the extreme of low frequency and high injury possibly occurs with downy mildew (Sclerospora gramininicola) of pearl millet (Pennisetum typhoides). Here, a complex vertical subsystem apparently reduces frequency to the minimum. But, when a matching allo-infection does occur, the host is invaded systemically, and all its living tissue is utilised by the parasite, which produces spores on every aerial surface of the host. Similar patchy distributions can occur with insect parasites that are gregarious, such as tent-caterpillars. Such parasites are likely to damage a few host individuals severely, even to the point of total destruction, while leaving most of the host population untouched.

The extreme of high frequency but low injury occurs in continuous plant pathosystems that have no vertical subsystem.

These pathosystems are likely to have very high levels of horizontal resistance.

This inverse correlation between frequency and injury is also true with a low frequency in time. A low frequency in time occurs typically with swarming locusts, such as the desert locust (Schistocera gregaria) which swarms only once in several years. Between swarms, the host populations are unharmed. But, at the time of a swarm, the host populations are totally consumed over quite large areas, and they survive only by virtue of buried, dormant seeds, or other underground organs of regeneration. The affected area is likely to be different during each infestation, and a particular host population is likely to be devastated rather rarely. One effect of this patchy distribution in both time and space is that the locust does not exert any selection pressure for resistance on its many species of host.

In the converse situation, when there is a very high frequency of the parasite, every host individual is injured equally but the injury is very slight. This is seen typically with savannah grasses in which every individual in a huge host population is likely to carry one or two rust pustules, but no more.

In other words, the total damage (i.e., frequency multiplied by injury) caused by parasitism, in a wild plant pathosystem, is a constant, within the limits of a modest seasonal fluctuation. It is also a level of damage that is controlled by both the resistance optimum and the parasitic optimum, which jointly ensure that the survival of neither the host nor the parasite is impaired by this strictly limited damage. Under these circumstances, the frequency of parasitism can increase only at the expense of the average injury, and the average injury can increase only at the expense of the frequency. Frequent parasitism is not injurious; and injurious parasitism is not frequent.

More complex situations are possible. For example, the level of injury may vary widely within a host population, depending on when the vertical resistance of the host individual in question was matched during the period of the epidemic. Here the inverse relationship between frequency and injury is maintained, but only at particular time intervals during the epidemic. The frequency of parasitism in individuals that are matched at the start of the epidemic will be minimal, but their injury will be maximal. The frequency of parasitism in individuals that are matched at the end of the epidemic will be maximal, but their injury will be minimal.

The total number of vertical genes in the subsystem governs the complexity of the vertical subsystem. We may assume the mathematically most efficient system of n/2 genes per individual.

The complexity then determines the probability of an allo-infection being a matching infection. High complexity means both a low frequency of an early matching allo-infection, and a correspondingly high injury of parasitism.

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