Many of the means to increased productivity and profitability are now perceived by society as carrying too high a cost in social disruption, human inequity, and environmental degradation. The problem in trying to address this is how to define and quantify sustainability. Izac and Swift (1994) developed an operational framework to assess sustainability based on the premise that "a cropping system is sustainable if it has an acceptable level of production of harvestable yield which shows a non-declining trend from cropping cycle to cropping cycle over the long term." Their framework is based on the assessment of key ecological and economic parameters at the field, farming system, and village/catchment scales and the concept that a sustainable system never reaches threshold levels of irreversibility and that it achieves a sufficient level of economic efficiency and social welfare. One of the requirements identified by Izac and Swift (1994) is that the by-products (soil and water quality, biological diversity, etc.) of agricultural activities must not disrupt the biological functions of the system to the extent that the capacity of the system to absorb these disruptions is surpassed.
Sustainability thus involves a symbiosis between the properties of the ecosystem and the management activities that results in nondeclining and relatively stable outcomes. Izac and Swift (1994) consider that the key to this symbiosis lies in the assumed positive relationship of agroecosystem function to biodiversity and complexity. Biodiversity therefore is a keystone in sustainability, and its loss has been one of the common outcomes of agricultural intensification (Figure 1).
Agroforestry, through the replenishment of soil fertility and the domestication of indigenous trees producing marketable forest products, has been proposed as one way of diversifying and intensifying agroecosystems in a way that is beneficial to the environment and can maintain and perhaps enhance biodiversity (Sanchez and Leakey, 1998; Sanchez et al., 1998). In its Medium Term Plan 1998-2000, the International Centre for Research in Agroforestry (ICRAF, 1997) foresees that agro-forestry can contribute to human welfare and environmental resilience, with improved systems providing:
1. Tree products that both increase food and nutritional security and generate cash income for poverty alleviation and
2. Services that support and enhance ecosystem function (Figure 2).
The relevant services of trees are those that increase the crop yields (nitrogen fixation, increased soil organic matter, nutrient cycling, soil conservation, etc.), create environmental resilience (niche diversification, food web complexity, carbon sequestration, reduced greenhouse gas emissions, etc.), and provide social benefits (boundary delineation, shade, etc.). Of these, the least is known about the ways in which trees enhance the environment, although the body of information is increasing (see Ingram, 1990; Swift et al., 1996).
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