Janzen (1968) pointed out that we could usefully think of hosts as islands that are colonized by parasites. By using the same vocabulary, this brought host-parasite relationships into the same arena as MacArthur and Wilson's (1967) study of island biogeography (see Section 21.5). A human colonized by the malarial parasite is in a sense an inhabited island or patch. The chances of a mosquito vector carrying the parasite from one host to another correspond to the varying distances between different islands. Populations of parasites are thus maintained by the continual colonization of new host patches as old infected patches (hosts) die or become immune to new infection. The whole parasite population is then a 'metapopulation' (see Section 6.9), with each host supporting a subpopulation of the whole.

Different species of parasite are, of course, transmitted in different ways between hosts. The most fundamental distinction, perhaps, is that between parasites that are transmitted directly from host to host and those that require a vector or intermediate host for transmission. Amongst the former, we should also distinguish between those where infection is by physical contact between hosts or by a very short-lived infective agent (borne, for example, in coughs and sneezes), and those where hosts are infected by long-lived infective agents (e.g. dormant and persistent spores).

We are largely familiar, through our own experience, with the nature of these distinctions amongst animal pathogens; but essentially the same patterns apply in plants. For example, many soil-borne fungal diseases are spread from one host plant to another by root contacts, or by the growth of the fungus through the soil from a base established on one plant, which gives it the resources from which to attack another. The honey fungus Armillaria mellea spreads through the soil as a bootlacelike 'rhizomorph' and can infect another host (usually a woody tree or shrub) where it meets their roots. In naturally diverse communities, such spread is relatively slow, but when plants occur as 'continents' of continuous interplant contacts, there are greatly increased opportunities for infection to spread. For diseases that are spread by wind, the foci of infection may become established at great distances from the origin; but the rate at which an epidemic develops locally is strongly dependent on the distance between individuals. It is characteristic of wind-dispersed pro-pagules (spores, but also pollen and seeds) that the distribution achieved by dispersal is usually strongly 'leptokurtic': a few propagules go a very long way but the majority are deposited close to the origin.

hosts as islands direct and indirect transmission; short-and long-lived agents

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