Ecologists have long been fascinated by what makes ecological systems stable and resilient (May, 2001; Ives, 2005; Begon et al., 2006). In general there is a supposition that the key factor is diversity— measured in its simplest form as species richness. Many theoretical and empirical studies have attempted to illuminate the relationships between diversity, on the one hand, and stability and resilience, on the other. The results have often been apparently contradictory, sometimes even reversing the cause-and-effect relationships. For example, diverse systems may be relatively fragile and only in stable environments can they be maintained. In unpredictable environments the communities may be more resilient yet simpler. What is clear from these studies is that it is not the sheer variety of the species present that affects stability and resilience, but their nature, their function in the systems and the relationships they have with one another.
It is also clear that 'natural communities represent not random assemblages of species but rather collections of species that can coexist' (Ives, 2005). In many respects this is even more true of agroe-cosystems. The diversity of crop and livestock species and of their varieties and breeds are present because human beings have recognized that they can coexist. Humans have also recognized that there are strong benefits in their coexistence. People do not live only on the calories provided by staple crops; they need sources of proteins, vitamins, minerals, and other nutrients. A diverse agroecosystem provides for a diverse and healthier diet. Farmers also recognize that different species can benefit each other. Trees and shrubs can provide shade for herbs, legumes can provide nitrogen, and livestock furnish manure. Mixtures of crops can also deter pests, and when a disaster strikes the farm—a drought or cyclonic storm—the more diverse the farm the more likely that something will survive.
The home garden is a good example. Part of the reason for the minimal trade-off in the home garden is the deliberately inbuilt diversity that helps stabilize production, buffers against stress and shock and contributes to a more valued level of production. But equally important is the intimate nature of the home garden. The close attention that is possible from family labour ensures a high degree of stability and resilience and the link between the garden and the traditional culture leads to an equitable distribution of the diverse products.
A Doubly Green Revolution seeks to exploit these relationships, through a variety of ecologically based approaches to the central processes of agriculture, for example in pest control and nutrient management, as I will now discuss.
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