Like Tansley's term 'ecosystem' (see 1.13), the term 'pathosystem' was intended to emphasise holistic studies at the higher systems levels (Robinson, 1976). Indeed, a pathosystem is a subsystem of an ecosystem, and it is defined by parasitism. A plant pathosystem is one in which the host species is a plant. The parasite may be any species in which an individual spends a significant proportion of its life cycle inhabiting, and obtaining nutrients from, one host individual. The parasite may thus be an insect, mite, nematode, parasitic Angiosperm, or any of the various categories of plant pathogens. However, herbivores, which graze a population of plants, are normally considered to be outside the conceptual boundaries of a pathosystem, and to belong to the higher systems level of the ecosystem.

The pathosystem concept involves an ecological approach to the study of plant parasitism. Being concerned primarily with the higher systems levels, it is holistic rather than merological. For this reason, both the host and the parasite are considered in terms of populations. Normally, a pathosystem consists of a population of one species of parasite interacting with a population of one species of host. However, two species of parasite may be involved, as with some virus and insect vector pathosystems, and two or more species of host may be involved, as with alternating pathosystems (see 8), and with parasites that have an optionally wide host range.

A plant pathosystem may exist physically, as a subsystem of an ecosystem. It is possible to walk into such a pathosystem, to take samples, and to study it in various ways. Alternatively, a plant pathosystem may be conceptual, and exist only in the form of a model, a computer simulation, or a theoretical discussion. However, use of the term 'pathosystem' does have clear parasitological and ecological implications, and its use in other contexts should be avoided. For example, a host-parasite association, considered taxonomically, is not a pathosystem. And a host population, described without any reference to a parasite, is a subsystem of an ecosystem but, on its own, it cannot be considered a subsystem of a pathosystem.

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