The single most important reason is that enormous pressures are being brought to bear upon earth's natural ecosystems by the activities of man.  First and foremost among the spectrum of indignities we inflict upon the plants and animals with which we share the earth is the fact that as humanity uses ever more land and water to produce the food and fiber needed to sustain our growing numbers, the amounts of these precious natural resources that remain accessible to the rest of the biosphere grow ever smaller (Wallace, 2000; Tilman et al., 2001, 2002).

It takes few brain cells to realize that when squeezed for space and life's most essential liquid, something's got to give; and it's a good bet that that unfortunate something is going to be a species other than Homo sapiens.  And so it is: as a result of humanity's insatiable appetite for Lebensraum, habitat destruction has become the leading cause of species extinction (Pimm and Raven, 2000).  The problem has become so serious, in fact, that Raven (2002) has calculated that "species-area relationships, taken worldwide in relation to habitat destruction, lead to projections of the loss of fully two-thirds of all species on earth by the end of this century."

So what can be done to prevent the impending species holocaust?

Our suggestion is to allow the atmosphere's CO2 concentration to take its natural course as the Age of Fossil Fuels takes its natural course and first peaks, then declines and eventually gives way to the next generation of power-producing technologies.  We proffer this prescription for species protection in all sincerity and with supreme confidence.  Why?  Because we know - we absolutely know - that elevated levels of atmospheric CO2 enable natural vegetation to produce more biomass with less water at the same time that they help agriculturists produce more food on less land.  And with greater efficiencies of resource utilization on both sides of the not-so-delicately-balanced biospheric teeter-totter on which we sit, man and nature have at least a fighting chance of living together in some modicum of harmony in the years ahead.  With the help of additional atmospheric CO2, we need not usurp the planet's entire stock of resources in order to survive.

But, one might argue, CO2 does not come without unwanted baggage; for it is claimed by many that elevated concentrations of atmospheric CO2 have the capacity to significantly elevate earth's surface air temperature.  To this assertion we have two replies.  First, there is no compelling reason to believe that such is so.  Second, even if this claim were true, warmer temperatures are known to be conducive to supporting greater ecosystem species richness.  In a recent paper in Science, for example, Allen et al. (2002) report that "the latitudinal gradient of increasing biodiversity from poles to equator is one of the most prominent but least understood features of life on earth."  They then proceed to advance our understanding of this phenomenon by describing how they derived a model - "based on first principles of biochemical kinetics" - that "quantitatively predicts how species diversity increases with environmental temperature," after which they demonstrate that the predictions of their model "are supported by data for terrestrial, freshwater, and marine taxa along latitudinal and elevational gradients."  And those taxa include both plants and animals, as well as both endotherms and ectotherms.

As important as it is, of course, temperature is not the only variable that affects ecosystem biodiversity; ranking high on the list of secondary and third-order influences is ecosystem productivity, as is reaffirmed by the work of Jetz and Rahbek (2002) in the very same issue of Science in which Allen et al.'s paper was published.  And we hardly need to remind anyone that the most dramatic biological impact of atmospheric CO2 enrichment is provided by its aerial fertilization effect that enhances plant biomass production.

In conclusion, there are just a host of reasons why our best ally in the fight to preserve the planet's biodiversity is atmospheric CO2 and more of it.  If we are truly concerned about this greatest of threats ever to be faced by the biosphere, i.e., mankind's usurpation of the lion's share of the planet's natural resources, we will not allow the world's anti-CO2 fanatics to ignore these facts.  They must be shouted from the rooftops.

References
Allen, A.P., Brown, J.H. and Gillooly, J.F.  2002.  Global biodiversity, biochemical kinetics, and the energetic-equivalence rule.  Science 297: 1545-1548.

Jetz, W. and Rahbek, C.  2002.  Geographic range size and determinants of avian species richness.  Science 297: 1548-1551.

Pimm, S.L. and Raven, P.  2000.  Extinction by numbers.  Nature 403: 843-845.

Raven, P.H.  2002.  Science, sustainability, and the human prospect.  Science 297: 954-959.

Tilman, D., Cassman, K.G., Matson, P.A., Naylor, R. and Polasky, S.  2002.  Agricultural sustainability and intensive production practices.  Nature 418: 671-677.

Tilman, D., Fargione, J., Wolff, B., D'Antonio, C., Dobson, A., Howarth, R., Schindler, D., Schlesinger, W.H., Simberloff, D. and Swackhamer, D.  2001.  Forecasting agriculturally driven global environmental change.  Science 292: 281-284.

Wallace, J.S.  2000.  Increasing agricultural water use efficiency to meet future food production.  Agriculture, Ecosystems & Environment 82: 105-119.