124 Agroecology and Sustainability
are components of capital, but both energy and technology can be treated, for
many purposes, as separate inputs.
Each possible combination of output and input can be regarded also as meas-
ures of efficiency of production when two or more agroecosystems are compared
(NAS, 1975; Spedding, 1979). Assessments may be made of productivity at differ-
ent levels in the hierarchy of agroecosystems, of the field, farm, village, watershed,
region or nation. Also comparisons may be made between agroecosystems of dif-
ferent types (e.g. between a cornfield and a cottonfield, or a lowland and an upland
village). Over time productivity may rise, fall or remain static.
Stability
Stability may be defined as the constancy of productivity in the face of small dis-
turbing forces arising from the normal fluctuations and cycles in the surrounding
environment. Included in the environment are those physical, biological, social
and economic variables that lie outside the agroecosystem under consideration.
The fluctuations, for example, may be in the climate or in the market demand for
agricultural products. Productivity may be defined in any of the ways described
above and its stability measured by, say, the coefficient of variation in productivity,
determined from a time series of productivity measurements. Since productivity
may be level, rising or falling, stability will refer to the variability about a trend.
Sustainability
Sustainability is defined as the ability of an agroecosystem to maintain productiv-
ity when subject to a major disturbing force. The actual or potential disturbance
may be caused by an intensive stress, where stress is defined as a frequent, some-
times continuous, relatively small and predictable disturbing force which has a
large cumulative effect. Salinity, toxicity, erosion, indebtedness or declining mar-
ket demand are examples of such forces. Alternatively, the disturbance may be
caused by a shock, defined here as an infrequent, relatively large and unpredictable
disturbing force which has the potential of creating an immediate, large disturbance
or perturbation. Examples of shocks include a rare drought or flood, or a new pest or
the sudden rise in an input price, such as that of oil in the mid-1970s.
Following a stress or shock the productivity of the agroecosystem may be unaf-
fected, or may fall and then return to the previous level or trend, or settle to a new
lower, or sometimes higher, level or may disappear altogether. Various measures of
sustainability are available: they include the inertia (resistance), elasticity, ampli-
tude, hysteresis and malleability of the agroecosystem in response to a disturbing
force (Orians, 1975; Westman, 1978). Sustainability thus determines the persist-
ence or durability of an agroecosystem’s productivity under known or possible
conditions. It is a function of the intrinsic characteristics of the agroecosystem, of
the nature and strength of the stresses and shocks to which it is subject, and of the
human inputs that may be introduced to counter these stresses and shocks.