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At this point, it might be useful to make a distinction between the frequency of parasitism, which is the proportion of host individuals that are parasitised, and the injury from parasitism, which is the damage suffered by those parasitised host individuals. Injury is usually expressed as the average of all the parasitised individuals.

An example will illustrate the point. A pride of lions may be said to parasitise a herd of zebras. The lions may kill one zebra, which they then consume almost entirely. This represents the minimum frequency of parasitism, but the maximum injury from parasitism. In ecological terms, the parasitism has a patchy distribution, and this extreme is often called the predator-prey relationship.

At the other extreme, every zebra is parasitised with ticks, but the injury caused by these ticks is negligible. This opposite extreme represents a maximum frequency of parasitism, but a minimum injury from parasitism. In ecological terms, the parasitism now has a uniform distribution, and this extreme is often called the host-parasite relationship.

The combination of frequency and injury represents the total parasite damage to the host population. In wild plants, this total damage never exceeds a rather low, permissible level. This permissible level is governed by the fact that the parasite must not impair the ability of its host to compete, either ecologically or evolutionarily. This is axiomatic, because any parasite that impaired its host's ability to survive would also threaten its own survival. For this reason, the frequency of parasitism, and the injury from parasitism, are inversely correlated in wild plants. A high frequency always results in a low injury, while a high injury always occurs with a low frequency.

In wild plants, frequency and injury are directly related to vertical resistance and horizontal resistance respectively. Vertical resistance provides a system of locking, which obviously reduces the frequency of parasitism. Horizontal resistance provides a second line of defence which, equally obviously, reduces the injury from parasitism. In a continuous epidemic, which has horizontal resistance only, there will be a high frequency of parasitism, but a low rate of injury. In a discontinuous epidemic which has vertical resistance as well as horizontal resistance, the frequency of parasitism will be low, particularly in the early part of the epidemic. But the individual injury from parasitism will be correspondingly higher in those individuals that were matched early in the epidemic.

In modern crops, on the other hand, we often have both a high frequency of parasitism, and a high injury from parasitism. The total damage is then high. This is because the vertical subsystem no longer operates as a system of locking, and the level of horizontal resistance is low.

Because we cannot employ a system of locking in our crops, it follows that we should aim at artificially high levels of horizontal resistance. We should domesticate horizontal resistance in the same way that our ancestors domesticated other continuous variables such as the yield and quality of wheat, rice, and maize. This would result in high frequencies of parasitism which, however, would not matter because the level of injury would be negligible.

Document(s) 13 of 33