The ecological role of biodiversity in agroecosystems

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Abstract

Increasingly research suggests that the level of internal regulation of function in agroecosystems is largely dependent on the level of plant and animal biodiversity present. In agroecosystems, biodiversity performs a variety of ecological services beyond the production of food, including recycling of nutrients, regulation of microclimate and local hydrological processes, suppression of undesirable organisms and detoxification of noxious chemicals. In this paper the role of biodiversity in securing crop protection and soil fertility is explored in detail. It is argued that because biodiversity mediated renewal processes and ecological services are largely biological, their persistence depends upon the maintenance of biological integrity and diversity in agroecosystems. Various options of agroecosystem management and design that enhance functional biodiversity in crop fields are described.

Introduction

Biodiversity refers to all species of plants, animals and micro-organisms existing and interacting within an ecosystem (Vandermeer and Perfecto, 1995). Natural biodiversity has provided the foundation for all agricultural plants and animals. The entire range of the domestic crops used in world agriculture is derived from wild species that have been modified through domestication, selective breeding and hybridization. Most remaining world centers of diversity contain populations of variable and adaptable landraces as well as wild and weedy relatives of crops, all of which provide valuable genetic resources for crop improvement (Harlan, 1975).

In addition to producing valuable plants and animals, biodiversity performs many ecological services. In natural ecosystems, the vegetative cover of a forest or grassland prevents soil erosion, replenishes ground water and controls flooding by enhancing infiltration and reducing water runoff (Perry, 1994). In agricultural systems, biodiversity performs ecosystem services beyond production of food, fiber, fuel, and income. Examples include recycling of nutrients, control of local microclimate, regulation of local hydrological processes, regulation of the abundance of undesirable organisms, and detoxification of noxious chemicals. These renewal processes and ecosystem services are largely biological, therefore their persistence depends upon maintenance of biological diversity (Altieri, 1994). When these natural services are lost due to biological simplification, the economic and environmental costs can be quite significant. Economically, in agriculture the burdens include the need to supply crops with costly external inputs, because agroecosystems deprived of basic regulating functional components lack the capacity to sponsor their own soil fertility and pest regulation. Often the costs involve a reduction in the quality of life due to decreased soil, water, and food quality when pesticide and/or nitrate contamination occurs.

The net result of biodiversity simplification for agricultural purposes is an artificial ecosystem that requires constant human intervention,whereas in natural ecosystems the internal regulation of function is a product of plant biodiversity through flows of energy and nutrients,and this form of control is progressively lost under agricultural intensification (Swift and Anderson, 1993). Thus commercial seed-bed preparation and mechanized planting replace natural methods of seed dispersal; chemical pesticides replace natural controls on populations of weeds, insects, and pathogens; and genetic manipulation replaces natural processes of plant evolution and selection. Even decomposition is altered because plant growth is harvested and soil fertility maintained, not through nutrient recycling, but with fertilizers (Cox and Atkins, 1979).

Thus modern agricultural systems have become productive but only by being highly dependent on external inputs. A growing number of scientists, farmers and the general public fear for the long-term sustainability of such highly input-dependent and ecologically simplified food production systems. Questions are being raised about the growing dependence of modern farming on non-renewable resources, the loss of biodiversity, the loss of land through soil erosion and the heavy reliance on chemical fertilizers and pesticides. Farm chemicals are questioned on grounds of cost but their widespread use also has implications for human and animal health, food quality and safety and environmental quality. The commercial agricultural sectors of developing countries suffer from similar problems but the greater challenge for them is to determine new ways to increase small farm productivity that not only benefit the rural poor under marginal agricultural conditions (hillsides, rainfed and marginal soils), but also conserve and regenerate the resource base (Altieri, 1995).

In both scenarios, the development of agroecological technologies and systems which emphasize the conservation-regeneration of biodiversity, soil, water and other resources is urgently needed to meet the growing array of socioeconomic and environmental challenges. Enhancing functional biodiversity in agroecosystems is a key ecological strategy to bring sustainability to production. As a way of illustrating this point, the role of biodiversity (predators, parasitoids, antagonists and soil microflora and microfauna) in securing crop protection and soil fertility is explored in this paper.

Section snippets

The nature of biodiversity in agroecosystems

Modern agriculture implies the simplification of the structure of the environment over vast areas, replacing nature’s diversity with a small number of cultivated plants and domesticated animals. In fact, the world’s agricultural landscapes are planted mostly with some 12 species of grain crops, 23 vegetable crop species, and about 35 fruit and nut crop species (Fowler and Mooney, 1990); i.e., no more than 70 plant species spread over approximately 1440 million ha of presently cultivated land in

Biodiversity and insect pest management

Nowhere are the consequences of biodiversity reduction more evident than in the realm of agricultural pest management. The instability of agroecosystems, which is manifested as the worsening of most insect pest problems, is increasingly linked to the expansion of crop monocultures at the expense of the natural vegetation, thereby decreasing local habitat diversity (Altieri and Letourneau, 1982). Plant communities that are modified to meet the special needs of humans become subject to heavy pest

Manipulating biodiversity at the landscape level

Most studies of the effects of biodiversity enhancement on insect populations have been conducted at the field level, rarely considering larger scales such as the landscape level. It is well known that spatial patterns of landscapes influence the biology of arthropods both directly and indirectly. One of the principal distinguishing characteristics of modern agricultural landscape is the large size and homogeneity of crop monocultures which fragment the natural landscape. This can directly

Biodiversity, soil fertility and plant health

A key feature of annual cropping systems is the nature and frequency of soil disturbance regimes. Periodic tillage and planting continually reverts the tilled area to an earlier stage of ecological succession. Physical disturbance of the soil caused by tillage and residue management is a crucial factor in determining soil biotic activity and species diversity in agroecosystems. Tillage usually disturbs at least 15–25 cm of the soil surface and replaces stratified surface soil horizons with a

Conclusion

The search for self-sustaining, low-input, diversified, and energy-efficient agricultural systems is now a major concern of many researchers, farmers, and policymakers worldwide. A key strategy in sustainable agriculture is to restore functional biodiversity of the agricultural landscape (Altieri, 1994). Biodiversity performs key ecological services and if correctly assembled in time and space can lead to agroecosystems capable of sponsoring their own soil fertility, crop protection and

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