Agriculture is under pressure to reform towards a greater degree of sustainability (Oborn et al. 2003), which can be achieved by conversion from conventional to organic farming systems (Condron et al. 2000) that adopt approaches that stimulate the self-regulating capacity of the agroecosystem as much as possible (Lammerts-van-Bueren et al. 2002). Organically grown crops should have characteristics that fit and support those self-regulating capacities such as natural resistance, natural pest control and biotic regulation of soil fertility. Self-regulating ability of organic ecosystems can be defined as the capacity to resist the effects of small and large perturbations or as the presence of enough resilience to counter them without high external chemical inputs (Lammerts-van-Bueren et al. 2002). This self-regulating ability increases system stability and reduces risk of reduction in the agroecosystems' productivity. Organic agriculture bases its sustainable self-regulating production system on the concept of a farm as an agroecosystem. An agroecosystem is shaped by the strong interaction between the biotic and nonbiotic environment, the genetic composition of species involved and the management of resources available to the farmer (Swift and Anderson 1993; Almekinders et al. 1995; Lammerts-van-Bueren et al. 2002). The biotic diversity includes associated organisms (pests, diseases, antagonists, predators and beneficial organisms) that contribute to the self-regulating capacity through balancing or feedback mechanisms (Almekinders and Struik 2000). Organic farmers support all these interactions at different levels of the production system in such a way that the farm can utilize ecosystem functions provided by agro-biodiversity, such as nutrient cycling, water and soil conservation and applying biological control strategies (Altieri and Nicholls 1999). For example, application of integrated pest management (IPM) rather than individual control methods is highly emphasized by organic agriculture. However, many agroecologists believe that IPM is not just about management of pests alone; it is a sustainable crop production approach based on sound ecosystem analysis (Speiser et al. 2006).
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