Multiplicity of Agro Ecosystems

An agro-ecosystem is usually quite large, and is defined first by the crops that can be grown within it. For example, a relatively large area of a region may be suitable for cultivating wheat. But, within that crop area, there may be subsystems based on wheat types, such as spring and winter wheats, or bread and pasta wheats. Each of these subsystems might be further reduced in size by the epidemiological competence of various parasites.

For the purposes of plant breeding clubs, an agro-ecosystem may be defined as the area in which one horizontally resistant cultivar of a crop species can be satisfactorily cultivated (see 6.4). That is, all the horizontal resistances of the cultivar are in balance with the epidemiological competence of each of the locally important parasites. Unfortunately, we have very little experience of the behaviour of horizontally resistant cultivars, and the determination of most of these agro-ecosystem boundaries belongs in the future.

Many agro-ecosystems might approximate to the size of individual states within the United States of America although, obviously, there would be no correlation between State boundaries and agro-ecosystem boundaries. A couple of plant breeding clubs per agro-ecosystem, for each crop grown in that agro-ecosystem, would provide all the competition needed for an effective self-organising crop improvement. This would not be an exorbitant number of clubs for a country the size of the USA.

Terminological note: The definitions of an agro-ecosystem, and its supersystem and subsystems, are likely to vary with context. In the present book, the wheat belt of North America is a supersystem, and the agro-ecosystem is defined by the epidemiological competence of all the locally important wheat parasites. That is, it is the area in which one horizontally resistant cultivar can be successfully cultivated. Each of these agro-ecosystems has subsystems called pathosystems, and these pathosystems have subsystems called the vertical subsystems and horizontal subsystems.

An applied ecologist, however, might regard the wheat belt itself as the agro-ecosystem, and its parasitological subdivisions as subsystems. And a biosphere scientist might regard all the wheat lands of the world as a single agro-ecosystem. This is 'different usage' (as opposed to 'wrong usage') and it is entirely legitimate, provided that the intended meaning is made clear.

0 0

Post a comment