Free trade was the key to Adam Smith's (1723-90) economic theories, which he published in The Wealth of Nations in 1776. His ideas were closely similar to those of modern complexity theory, which recognises the importance of self-organisation in a non-linear system. Frequency of parasitism
The frequency of parasitism is the proportion of host individuals that are parasitised. The injury from parasitism is the actual amount of damage done to an individual host, or the average amount done to a host population, by the parasite. In a wild plant pathosystem, the injury from parasitism is inversely proportional to the frequency of parasitism. That is, the higher the frequency, the lower the injury, and, conversely, the higher the injury, the lower the frequency. In this way, the total damage from parasitism never exceeds a tolerable level that does not impair the host's ability to compete ecologically and evolutionarily. vertical resistance, with its system of locking, reduces the frequency of parasitism. horizontal resistance, as a second line of defence, reduces the injury from parasitism. Continuous plant pathosystems, that have horizontal resistance only, usually have a high frequency of parasitism, and a low injury from parasitism.
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