Efforts to curb the loss of biodiversity have intensified in recent years, but they remain modest and have not kept pace with the rate of human-induced change. Furthermore, their application has been primarily focused on preserving a small number of species of large plants and animals, while neglecting the small organisms. However, the numerous small organisms that inhabit the soil, such as fungi, nematodes, insects and bacteria, dominate the structure and the basic functions of natural ecosystems. Holistic strategies are needed to protect whole ecosystems to conserve total biological diversity.

Reduction in the use of biodiversity in agriculture is driven by the increased pressures and demands of urban and rural populations and by the global development paradigm, which is favouring specialisation and intensification. Given that terrestrial ecosystems provide roughly 99% of the world's food supply and the population is estimated to reach 8 billion by 2020 (FAO, 1996) the question is - Will be possible to have a sustainable agriculture, able to feed these numbers and meet increasing consumption patterns in an ecologically compatible way? . The scientific database on these issues has not yet provided indications that are unanimously accepted. Assessment of the sustainability of agriculture, and thereby food security predictions, requires a prior understanding of the intricate relationship that exists between below and above-ground biodiversity and agricultural systems.

Arguments for conserving and managing soil biodiversity, including ecological, agronomic, socio-economic and ethical or moral reasons, are discussed below (noting that there are interdependencies among these considerations).

As human interference and the use of external inputs decreases, soil biodiversity and the role of soil biological processes in maintaining soil fertility and productivity increase, and the opportunities for soil biological management become more feasible (Swift, 1996). For example, management interventions might reduce the populations of soil-borne pathogens, pests and parasites, and enhance the populations and activities of beneficial organisms such as symbiotic rhizobacteria and mycorrhizae, organic matter decomposers, mineralizers, and ecosystem engineers. The outcome will be increased food and agriculture production, improved resilience of agricultural ecosystems and increased capacity to sustain production in the short and long term.

On an economic basis, soil biodiversity has both direct (the organisms themselves and/or their metabolic products) and indirect (the long-term outcome of their activities) uses. It is estimated that the value of ecosystem services provided each year by soil biota in agricultural systems worldwide (e.g., organic waste disposal, soil formation, N₂ fixation, bioremediation and biocontrol) may exceed US$ 1,542 billion as illustrated in Table 1.

a) Recycling of organic waste
Each year, human, livestock and crops produce approximately 38 billions metric tons of organic waste worldwide. These wastes are recycled by a variety of soil decomposer organisms. A succession of micro-organisms occurs in the detritus, involving mainly bacteria and fungi as well as detritus-feeding invertebrates, decomposing organic matter until it is finally reduced to elemental nutrients that are incorporated into the system. Assuming a conservative value of $ 0.02/kg for all organic wastes that are recycled by decomposers, the contribution made by decomposer organisms is worth more than US$ 760 billion per year worldwide. This calculation does not take into account the benefits of decreased environmental pollution, the recycling of nutrients, the decrease in the need for landfills and the significant reduction in human diseases.

b) Soil formation
More than 99% of the total worldwide human food supply is produced on land, whereas only 0.6% comes from oceans and other aquatic ecosystems (FAO 1991). Diverse soil biota facilitate soil formation and improve it for crop production. For example, earthworms bring between 10 and 500 tons/ha/year of soil to the surface, whereas insects often bring between 1 to 10 tons/ha/year of soil to the surface (Pimentel et al., 1995). The combined activity of a substantial amount of soil invertebrates contribute to redistribution of nutrients, aerate the soil, facilitate top soil formation, and increase rates of water infiltration, thereby enhancing plant productivity (Pimentel et al., 1995).

c) Nitrogen fixation
Nitrogen is essential for plant growth, and an insufficient quantity of it frequently limits biomass production in both natural and agricultural ecosystems. Biological nitrogen fixation by obligate endophytic diazotroph bacteria (e.g. Rhizobium, Azotobacter, Azospirillum, etc) is a process in which atmospheric nitrogen is converted into substrates of nitrogen that plant can use. Worldwide, 140-170x10⁶ tons/year of nitrogen, valued at approximately $90 billion, is fixed by many microorganisms in both agricultural and natural ecosystems (Bezdicek and Kennedy 1988, Peoples and Craswell, 1992). Nitrogen fixation only by leguminous plants produces an average of 80 kg/ha/year of nitrogen worldwide. In addition, recent discoveries indicate that obligate endophytic diazotroph bacteria add as much as 150 kg/ha/year of nitrogen to agricultural and natural ecosystems (Dobereiner, 1995). Therefore biological nitrogen fixation is the major alternative to the use of commercial nitrogen fertilizer in agriculture (Dobereiner and Pedrosa, 1987).

d) Bioremediation of chemical pollution
Removing chemicals from the environmental (remediation) can be achieved by biological methods. Biological treatments that use microbes (bacteria and fungi) and plants to degrade chemical materials, can both decontaminate polluted sites (bioremediation) and purify hazardous wastes in water (biotreatments). Overall biological methods are more effective than physical, chemical, and thermal methods, because the latter methods often simply transfer the pollutant to a different medium instead of converting it to a less toxic substrate, as biological methods often do. The ability of bioremediation to provide continuous cleanup of contaminated sites, such as agricultural ecosystems with toxic pesticide residues, is a significant advantage of this method. Furthermore, a significant degree of self-regulation is present in such biological systems because the added microbes survive by consuming and degrading chemicals but die when the nutrient source, that is the pollutant, is reduced or eliminated.

Bioremediation is effective in cleaning up highly polluted soils. For example, in one case the application of a nitrogen nutrient and bacterial mixture reduced the various oil-tar pollutants in a soil by 40-90% after just 70-90 days of treatment (Warith et al., 1992). The presence of large numbers of microorganismal species expands both the variety of and extent to which chemical pollution in the environment can be degraded.

e) Biotechnology
The present economic benefits of biotechnology products are significant, conservatively estimated to be between $6.2 billion per year (Kathuri et al., 1993) and are projected to increase to $9 billion per year by the turn of the century (USDC, 1984). Nearly half of the current economic benefits of biotechnology relate to agriculture, with significant benefits in the pharmaceutical industry. Many micro-organisms are extracted from the soil and used in industrial production of various kinds (e.g. food processing and production, development of biocides, biocontrol agents, medicines and other natural products), and pharmaceutical companies spend millions of dollars annually screening soil and litter for useful microorganisms (RAFI, 1995). The high value of many of these soil-derived industrial products and the resulting bio-prospecting activities are of particular concern because many countries still lack or have poorly-defined national policies to protect and benefit accordingly from their indigenous soil resources (and prevent ‘bio-piracy’).

f) Biological pest control
Worldwide, pests reduce the yields of major crops by approximately 42% each year, despite the application of pesticides. The total cost of losses to pest is estimated to be $244 billion per year.

Approximately 99% of pest are controlled by natural enemy species and host plant resistance. Each insect pest has an average of 10-15 natural enemies that help to control it (van den Bosch and Messenger, 1973) and many of them have an edaphic phase in their life-cycle. However, the value of these natural enemies to pest control is often overlooked.

g) Pollination
As much as one-third of the world's food production relies either directly or indirectly on insect pollination (Richards, 1993). Although many major crops are self-or wind pollinated, others require and benefit from insect pollination to increase quality or increase yields (Richards, 1993). Assuming conservatively that the economic value of animal pollinators worldwide is at least five times that in the United States, the contribution of animal pollination to world agriculture is estimated to be $200 billion per year.

h) Wild animals and ecotourism
Agro-tourism is fast becoming an especially lucrative industry for some developing nations, therefore the maintenance of a clean environmental and an enjoyable rural-landscape is very important. A world value for foods harvested from the wild can be estimated in developed countries, as it is rather specialised and the populations who exploit wildlife are rather limited social groups. However it is difficult to estimate in developing countries because rural communities depend far more extensively on gathering and hunting wild biota for their food, including, mushrooms, earthworms, small arthropods, etc.

Ecologically, soil biota are responsible for regulating several critical functions in soil and the stability, resilience and resistance of the ecosystem to perturbation can be significantly affected by their activities. Soil biota also play an important role in regulating decomposition and nutrient cycles, soil structure, gas exchange, soil hydrological processes, control of pests and diseases, soil detoxification and plant production. The presence of a range of species and organisms capable of supporting these critical soil processes, is essential for the maintenance of healthy productive soils in the face of changing environmental conditions, which subject the system to different degrees of stress and magnitudes of shock. When certain critical functions are largely undertaken by only one keystone species or ecosystem engineer, its removal may lead to catastrophic effects on the ecosystem. Similarly, the introduction of a keystone organism may also have catastrophic or beneficial effects, depending on the situation

Thus, excessive reduction in soil biodiversity, especially the loss of keystone species or species with unique functions (e.g., symbiotic microorganisms) may have catastrophic ecological effects (Barros, 1999), leading to the long-term deterioration of soil fertility and the loss of agricultural productive capacity.

Ecological health is often considered to be embodied in ecological function. Measurements of ecological function involve basic ecosystem functions such as nutrient cycling or energy transfer which result from the interaction of many components. Because the function of soil sub-system may be the key to understanding the health of agroecosystems from an ecological perspective, soil biodiversity and soil health can also be seen as one measure of environmental quality.

On the ethical or moral stance, the intrinsic value (i.e., the value in and of itself), regardless of its potential or actual use, of biodiversity has been stressed by various authors (Johnson, 1991; Kellert and Wilson, 1993; Hågvar, 1994; McNeely et al., 1995). It is also well recognized that, to varying degrees, most of the world’s religions give intrinsic worth to the natural world, and it is unlikely that this deep-seated notion will disappear, even despite the force of the economic use values placed on biodiversity (Gaston and Spicer, 1998). However, the world view that denies any sacred value or self-worth to nature is being rapidly spread throughout the world by globalization and modern industrial societies who view the world as a warehouse of commodities for human enjoyment (Barbier et al., 1995). The danger of this world view to biodiversity conservation cannot and should not be underestimated, although the possibility of using biodiversity for enjoyment (e.g., ecotourism) and other benefits may serve to counteract the negative forces of ecologically ignorant consumer societies.

References