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Biodiversity (Fungi) -- Summary
Biodiversity aboveground is often determined by biodiversity belowground.  Such is the implication of two important papers by van der Heijden et al. (1998a, b).

In the first, van der Heijden et al. (1998a) demonstrated that plants common to European calcareous grasslands may be totally dependent upon the presence of specific species, or combinations of specific species, of arbuscular mycorrhizal (AM) fungi in the soil.  In the second, van der Heijden et al. (1998b) demonstrated that increasing the number of AM fungi in the soils of artificial ecosystems from 4 to 14 increased ecosystem plant diversity by 60%.

These findings take on special significance in light of the study of Hodge et al. (1998), who found that in microcosms planted to ryegrass, elevated atmospheric CO2 stimulated the proliferation of previously dormant but culturable rhizosphere microorganisms, including bacteria, yeast and fungi, and that these microorganisms utilized carbon at faster rates when it was obtained from plants grown at elevated CO2.

Consider the implications of these observations.  As the CO2 content of the air rises, most plants typically exhibit increases in vegetative productivity and, therefore, increases in carbon exudation to the soil.  This increased availability of soil carbon tends to increase the numbers and activities of soil bacteria, yeast and fungi, the latter of which help the plants of the ecosystem to acquire more water and nutrients [see Growth Response to CO2 (Nutrient Acquisition)] and grow even better, so that they exude even more carbon to the soil, and so on.  Along the way, certain viable but dormant microorganisms begin to grow and proliferate; and this phenomenon enables more species of plants (which depend upon the presence of specific fungi for their existence) to grow in the ecosystem, with the result that ecosystem biodiversity increases.  In addition, Wilson and Hartnett (1998) have shown that the presence of AM fungi is particularly helpful to the growth of C4 plants, which can always use a boost to keep up with the typically-greater biomass production benefits conferred upon C3 plants by atmospheric CO2 enrichment; and this phenomenon helps to insure that C4 plants are not pushed out of the picture by C3 plants as the air's CO2 content rises.

Clearly, the presence of soil fungi helps to maintain, and sometimes even increase, the biodiversity of various ecosystems; and elevated levels of atmospheric CO2 help them to better perform this important function.

Hodge, A., Paterson, E., Grayston, S.J., Campbell, C.D., Ord, B.G. and Killham, K.  1998.  Characterization and microbial utilisation of exudate material from the rhizosphere of Lolium perenne grown under CO2 enrichment.  Soil Biology and Biochemistry 30: 1033-1043.

van der Heijden, M.G.A., Boller, T., Wiemken, A. and Sanders, I.R.  1998.  Different arbuscular mycorrhizal fungal species are potential determinants of plant community structure.  Ecology 79: 2082-2091.

van der Heijden, M.G.A., Klironomos, J.N., Ursic, M., Moutoglis, P., Streitwolf-Engel, R., Boller, T., Wiemken, A. and Sanders, I.R.  1998.  Mycorrhizal fungal diversity determines plant biodiversity, ecosystem variability and productivity.  Nature 396: 69-72.

Wilson, G.W.T. and Hartnett, D.C.  1998.  Interspecific variation in plant responses to mycorrhizal colonization in tallgrass prairie.  American Journal of Botany 85: 1732-1738.