SOIL FUNCTION IN A CHANGING WORLD - THE ROLE OF INVERTEBRATE ECOSYSTEM ENGINEERS

In this review the interactions between plant, animal and microbial co mponents of the soil biota are represented by a model which allocates a pivotal functional role to the large, abundant invertebrates which i ngest or manipulate both organic and mineral material, forming long-la sting microstructures. These invertebrates are designated soil ecosyst em engineers and it is argued using data on numerical and biomass dens ities, geographical distribution and known functional roles, that eart hworms and termites are the most important engineers in terrestrial ec osystems. Evidence is presented that they may exert influence on the d iversity and activity of biota in subordinate trophic levels, for exam ple litter transformers, micropredators and microfloras mediating fund amental nutrient transformations. Links between the activity and diver sity of engineers and the physical properties of soils, including stru ctural heterogeneity, stability, distribution of organic matter and in filtration and retention of water are also described. In considering t he probable effects of global change on engineers, it is hypothesized that living plants affect both the abundance and diversity of engineer s, through the quantity and quality of litter and other effects. Chang es in their communities will therefore affect engineers. Expected chan ges in temperature will expand the latitudinal distribution of termite s and favour humivorous termites and endogeic earthworm species that f eed in the soil. In some regions, however, these changes will not occu r since local fauna may not include representatives of these groups. A lthough elevated CO2 may impact engineers through effects on plant gro wth (notably an increase in C/N ratio), land use intensification, part icularly physical disturbance of forests, is of more immediate concern as changes in the functional group balance within engineers communiti es can be demonstrated. In addition, exotic species of earthworms may colonize disturbed land, with adverse effects on soil structure. Distu rbance affects termites by reducing diversity (especially of soil-feed ing forms) and some species may reach crop pest status, owing to chang es in the availability of organic matter.