ABSTRACT
No agro-ecosystem can exist and function as an isolated self-contained selfsustaining unit. All are, by their very nature, more open than any unmanaged and than most other managed ecosystems. Even in the least technically developed type of subsistence agricultural system there has to be an input of energy in the form of human and/ or animal labour; and, of the food produced and consumed, some of the waste in the form of human or animal excreta will be evacuated outside the immediately cultivated areas. In addition, disturbance of pre-existing ‘natural’ ecosystems will tend to increase the loss of nutrients by increased leaching, surface run-off and accelerated soil erosion. The relative intensity of agriculture in terms of number and volume of inputs and outputs has increased rapidly with the application of technical and scientific knowledge to agriculture since the 1940s. Intensity of agriculture also varies spatially, depending on the severity of the environmental constraints under which it operates and with the stage of regional development.
