Background
A valid concern about any environmental change is what it may do to biodiversity, and in this regard, the ongoing rise in the air's CO2 content is no different from any other factor undergoing substantial change; people want to know if it will cause species that play only minor roles in earth's many ecosystems to be essentially squeezed out by the competition of more predominant plants.

What was done
In an experiment that addresses important aspects of this question, Ramseier et al. (2005) grew five annual plants that are characteristic of early old-field succession in central Europe (Centaurea cyanus L., Matricaria chamomilla L., Silene noctiflora L., Papaver rhoeas L. and Legousia speculum-veneris (L.) Chaix) at various total densities and relative frequencies that yielded 17 different frequency-density combinations in greenhouse chambers maintained at either 360 or 700 ppm for approximately 4.5 months, after which the aboveground biomass of each plant was determined and the data analyzed by various statistical procedures.

What was learned
On average, Centaurea was the most predominant plant, accounting for 54.6% of the total ecosystem final yield, followed by Matricaria with 22.9%, Silene with 16.9%, Legousia with 3.1% and Papaver with 2.7%.  In addition, the authors report that "species' initial abundance had relatively little impact on the change in community composition."  Nevertheless, they found that "elevated CO2 significantly changed community composition towards the previously more poorly performing species Silene, Legousia and Papaver."

What it means
These results would appear to suggest that subordinate species may actually experience a boost in their community standing as a result of the ongoing rise in the air's CO2 content.  In fact, Ramseier et al. remark that "subordinate species have been observed to respond more to elevated CO2 than dominant species in several studies," citing those of Newton et al. (1994), Leadley and Korner (1996), Clark et al. (1997), Navas et al. (1997), Potvin and Vasseur (1997), Berntson et al. (1998), Stocklin and Korner (1999) and Niklaus et al. (2001).  Hence, it is plausible that biodiversity may be better protected in a high-CO2 world of the future than it has been for millions of years past.

References
Berntson, G.M., Rajakaruna, N. and Bazzaz, F.A.  1998.  Growth and nitrogen uptake in an experimental community of annuals exposed to elevated atmospheric CO2.  Global Change Biology 4: 607-626.

Clark, H., Newton, P.C.D., Bell, C.C. and Glasgow, E.M.  1997.  Dry matter yield, leaf growth and population dynamics in Lolium perenne/Trifolium repens dominated pasture turves exposed to two levels of elevated CO2.  Journal of Applied Ecology 34: 304-316.

Leadley, P.W. and Korner, C.  1996.  Effects of elevated CO2 on plant species dominance in a highly diverse calcareous grassland.  Carbon Dioxide, Populations, and Communities (Korner, C. and Bazzaz, F.A., Eds.) Academic Press, San Diego, CA, USA, pp. 159-175.

Navas, M.L., Sonie, L., Richarte, J. And Roy, J.  1997.  The influence of elevated CO2 on species phenology, growth and reproduction in a Mediterranean old-field community.  Global Change Biology 3: 523-530.

Newton, P.C.D., Clark, H., Bell, C.C. and Glasgow, E.M.  1994.  Effects of elevated CO2 and simulated seasonal changes in temperature on the species composition and growth rates of pasture turves.  Annals of Botany 73: 53-59.

Niklaus, P.A., Leadley, P.W., Stocklin, J. and Korner, C.  1998.  Nutrient relations in calcareous grassland under elevated CO2.  Oecologia 116: 67-75.

Potvin, C. and Vasseur, L.  1997.  Long-term CO2 enrichment of a pasture community: species richness, dominance, and succession.  Ecology 78: 666-677.

Stocklin, J. and Korner, C.  1999.  Interactive effects of elevated CO2, P availability and legume presence on calcareous grassland: result of a glasshouse experiment.  Functional Ecology 13: 200-209.