The Properties of Agroecosystems 123there is no obvious ‘goal’ for a population, community or ecosystem and these
properties are simply the outcomes of co-evolution.
However, for agroecosystems a clear goal, in the form of increased social value,
is once again apparent. Social value, defined here in terms consistent with classical
welfare economics (see, for example, Layard and Walters, 1978), is a function of
the amounts of goods and services produced by the agroecosystem, their relation-
ship to human needs (or happiness) and their allocation among the human popu-
lation. Like fitness, it has a time dimension, humans seeking not only increased
benefits in the immediate future but also a degree of security over the longer term.
Social value thus has several measureable components: the present production, its
likely level over a future time horizon and its distribution among the human pop-
ulation. Each agroecosystem, at each level in the hierarchy, has a social value and
it also follows that one form of agroecosystem may have a greater social value than
another (in much the same way that one organism is fitter than another) and
hence may be selected for by a human population.
While welfare economics provides a good theoretical basis for defining social
value, the concepts involved are of limited practical value. Production frontiers,
utility and welfare functions are difficult, if not impossible, to measure. In prac-
tice, therefore, an assessment of an agroecosystem’s performance has to be made
not in terms of the theoretical goal but in relation to those key system properties
that contribute most directly to realizing the goal. I have suggested there are four
such primary agroecosystem properties – productivity, stability, sustainability and
equitability (Conway, 1982a, 1982b). The first three approximately correspond to
the properties of natural ecological systems; the principal distinction is that each is
defined in terms of the valued output of the system and hence may be measured in
both biological and socioeconomic units (Altieri and Anderson, 1986). The fourth
property, equitability, has no direct counterpart in natural ecological systems.
Productivity
Productivity is defined here as the output of valued product per unit of resource
input. Common measures of productivity are yield or income per hectare, or total
production of goods and services per household or nation, but a large number of dif-
ferent measures are possible, depending on the nature of the product and of the
resources being considered. Yield may be in terms of kilograms of grain, tubers, leaves
or of meat or fish or any other consumable or marketable product. Alternatively, it
may be converted to value in calories, proteins or vitamins or to its monetary value at
the market. Frequently, the valued product may not be yield in conventional agricul-
tural terms. It may be employment generation, or an item of amenity or aesthetic
value or one of a wide range of products that contribute, in ways that are difficult to
measure, to social, psychological and spiritual well-being (Chambers, 1986).
The three basic resource inputs are land, labour and capital. Strictly speaking,
energy is subsumed under land (solar energy), labour (human energy) and capital
(fossil fuel energy). Similarly, technological inputs, such as fertilizers and pesticides,