Biodiversity In Soils And Its Impacts On Terrestrial Ecosystem Function

There is increasing concern among biologists in the fates of the very diverse array of organisms in all ecosystems of the world. What do we know of the full species richness, particularly in soils, to make even educated guesses about the total extent of the organisms, or how many of them may be in an endangered status (Hawksworth, 1991a, 2001; Coleman et al., 1994b; Coleman, 2001)? Soil biodiversity is best considered by focusing on the groups of soil organisms that play major roles in ecosystem functioning. Spheres of influence of soil biota are recognized; these include the root biota, the shredders of organic matter, and the soil bioturbators. These organisms influence or control ecosystem processes and have further influence via their interactions with key soil biota (e.g., plants) (Coleman, 2001; Wardle, 2002). Some organisms, such as the fungus and litter-consuming microarthropods, are very spe-ciose. For example, there are up to 170 species in one Order of mites, the Oribatida, in the forest floor of one watershed in western North Carolina. Hansen (2000) measured increased species richness of Oribatids as she experimentally increased litter species richness in experimental enclosures from one to two, four, and finally seven species of deciduous tree litter. This was attributed to the greater physical and chemical diversity of available microhabitats, which is in accord with the mechanisms suggested earlier by Anderson (1975).

Only 30-35% of the Oribatids in North America have been adequately described (Behan-Pelletier and Bissett, 1993), despite many studies carried out over the last 20-30 years. The studies suggest that there may be more than 100,000 undescribed species of oribatid mites yet to be discovered. Particularly in many tropical regions, Oribatids and other small arthropods are very little known in both soil and tree canopy environments (Behan-Pelletier and Newton, 1999; Nadkarni et al., 2002). This difficulty is compounded by our very poor knowledge of identities of the immature stages of soil fauna, particularly the Acari and Diptera. Solution of this problem may require considerable application of molecular techniques to more effectively work with all life stages of the soil fauna (Behan-Pelletier and Newton, 1999; Coleman, 1994a; Freckman, 1994). We concur with Behan-Pelletier and Bissett (1993): "Advances in systematics and ecology must progress in tandem: sys-tematics providing both the basis and predictions for ecological studies, and ecology providing information on community structure and explanations for recent evolution and adaptation." Chapin et al. (2000) note that 12% of birds and nearly 20% of mammals are considered threatened with extinction, and that from 5 to 10% of fish and plants are similarly threatened. With many of the soil invertebrates yet undescribed, it is impossible to affix a numerical value to losses of these members of the biota.

There are currently 70,000 species of fungi described (Table 7.1). By assuming that a constant ratio of species of fungi exists to those plant species already known, Hawksworth (1991b, 2001) calculated that there may be a total of 1.5 million species of fungi described when this mammoth classification task is completed.

Indeed, it may be possible to gain insights into biotic functions below-ground by considering a "universal" set of functions for soil and sediment biota that include the following: degradation of organic matter,

TABLE 7.1. Comparison of the Numbers of Known and Estimated Total Species Globally of Selected Groups or Organisms

Group

Known species

Estimated total species

Percentage known

Vascular plants

220,000

270,000

0 0

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