In CO2 enrichment studies of ryegrass microcosms and wheat microcosms, Griffiths et al. (1998) observed no changes in the rhizosphere microbial community structure, or soil microbial biodiversity, of either system. Likewise, in a study of six perennial plants common to The Netherlands and grown under conditions of variable soil nitrogen and water supply, Arp et al. (1998) concluded that "a rise in CO2 would not change the relationships between plant species in the natural environment, but would reinforce existing ones." Similarly, Catovsky and Bazzaz (1999) studied the effects of elevated CO2 levels on co-occurring paper and yellow birch trees subjected to varying degrees of soil moisture stress, finding that the current distributions of these temperate forest species should only be reinforced by the ongoing rise in the air's CO2 content. In a study of a fertile and species-rich grassland near Basal, Switzerland, however, Leadley et al. (1999) observed that elevated CO2 did produce changes in the relative composition of the ecosystem; but the changes were such as to produce a marginally significant increase in ecosystem biodiversity.
On another note, Kerstiens (1998) reviewed 15 previously published studies of CO2-induced growth responses of trees of differing shade-tolerance, finding that elevated CO2 caused greater relative biomass increases in shade-loving species than in sun-loving species. In fact, in more than half of the studies analyzed, shade-tolerant trees exhibited CO2-induced relative growth increases that were two to three times greater than those of shade-intolerant trees. Hence, it is likely that forest biodiversity will not be adversely affected by the ongoing rise in the air's CO2 content; for the understory shade-loving trees, which are overshadowed by the taller sun-loving trees, should respond much better to the rise in the air's CO2 content and not be out-competed by their taller light-loving neighbors, which will obviously not be out-competed by them either.
Finally, in a study of potential consequences of predicted global warming, Chadwick-Furman (1996) concludes that rising sea levels may allow "more water circulation between segregated lagoons and outer reef slopes," which could "increase the exchange of coral propagules between reef habitats and lead to higher coral diversity in inner reef areas." She also notes that the warming itself may positively impact coral diversity, especially in reefs that are in "latitudinally marginal areas, which presently are temperature limited."
In conclusion, this diverse assortment of studies suggests that the ongoing rise in the air's CO2 concentration will likely not have a negative influence on ecosystem biodiversity. In fact, in some cases it may even have a positive impact.
References
Arp, W.J., Van Mierlo, J.E.M., Berendse, F. and Snijders, W. 1998. Interactions between elevated CO2 concentration, nitrogen and water: effects on growth and water use of six perennial plant species. Plant, Cell and Environment 21: 1-11.
Catovsky, S. and Bazzaz, F.A. 1999. Elevated CO2 influences the responses of two birch species to soil moisture: implications for forest community structure. Global Change Biology 5: 507-518.
Chadwick-Furman, N.E. 1996. Reef coral diversity and global change. Global Change Biology 2: 559-568.
Griffiths, B.S., Ritz, K., Ebblewhite, N., Paterson, E. and Killham, K. 1998. Ryegrass rhizosphere microbial community structure under elevated carbon dioxide concentrations, with observations on wheat rhizosphere. Soil Biology and Biochemistry 30: 315-321.
Kerstiens, G. 1998. Shade-tolerance as a predictor of responses to elevated CO2 in trees. Physiologia Plantarum 102: 472-480.
Leadley, P.W., Niklaus, P.A., Stocker, R. and Korner, C. 1999. A field study of the effects of elevated CO2 on plant biomass and community structure in a calcareous grassland. Oecologia 118: 39-49.