Introduction

The numbers of plant and animal species on Earth represents only 0.1% of those that have existed since life appeared on this planet, the other 99.9% being already extinct as a result of five episodes of mass extinction over geological time (Leakey and Lewin, 1996) and the current period of extinctions (30,000/year against a background of 0.25/year) arising from our population growth and lifestyle. Land-use changes associated with colonization have been the major cause of these species losses over the last few centuries. Agriculture, which has been described as the "engine of economic growth" because of its powerful role in facilitating and stimulating growth of other sectors of the economy (Mellor et al., 1987), started as a subsistence activity 8000 years ago. The early subsistence systems are generally considered to have been sustainable, while large-scale, capital-intensive, modern agriculture has traded innate sustainability for chemical and other inputs and is characterized by deforestation and a decrease in the overall numbers of associated wild plants and animals to favor the growth of the planted crop. Typically, agriculture systems and forestry plantations are monocultures of staple food or tree crops with the almost total disappearance of the biodiversity and spatial complexity of natural ecosystems. Characteristically, these monocultures are also based on the few plant species that have been domesticated (Leakey and Tomich, 1998) and which also have a narrow genetic base. In recent years, these developments have given rise to concerns about deforestation, the loss of biodiversity, and the sustainability of our lifestyle and, particularly, the crucial food production systems that are essential to prevent famine and malnutrition in the tropics.

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