A step-by-step process for the ways in which increased heterogeneity of carbon (C) substrates from aboveground will positively influence belowground diversity is as follows (Fig. 7.2) (Hooper et al., 2000): (1) diversity of primary producers leads to diversity of C inputs below-ground, (2) C resource heterogeneity leads to diversity of herbivores and detritivores, and (3) diversity of detritivores or belowground herbivores leads to diversity of organisms at higher trophic levels in belowground food webs. The critical point is the nature and extent of trophic interactions (Hooper et al., 2000). There are three general categories of interactions by which organisms in one compartment can affect biodiversity in another one: (1) obligate, selective interactions (one-to-one linkage), through mutualism for example; (2) one-to-many species linkages, via keystones and dominants; and (3) causal richness, or many-to-many linkages. The nature and extent of these interactions varies a great deal depending on the systems studied and the spatial scales at which the mechanisms are being considered.
There is a strong interaction between ecosystem function, organismal abundance and diversity, and the nature of humus forms in soil. Ponge (2003) compared more than 20 ecosystem attributes, and the nature of the processes and organisms occurring in mull, moder, and mor soils (Table 7.3) (Ponge, 2003). The table is a useful means of comparing many soil attributes across a broad range of physical, chemical, and biological traits. It shows a marked gradient from high (mull) to low (mor) biodiversity and rapid to slow and very slow rates of humification. Not surprisingly, a key determinant of litter decomposability, phenolic content, varied inversely across the same sequence of three humus types. Of course, we have yet to see how well these generalizations hold up when including a detailed analysis of the microbial communities in all three humus types.
Studies of biodiversity should include assessments of the nature and extent of anthropogenic disturbance. In a recent multistate and provincewide study of snail distributions and diversity in 443 sites, anthropogenic disturbance was found to be a major factor in decreases in species richness in forested ("duff") versus grassland ("turf") sites. This indicates that the conservation of faunas in the former will require protection of the soil surface architecture (Nekola, 2003).
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