How will the ongoing rise in the air's CO2 content affect the biodiversity or species richness of earth's many ecosystems? Will C3 plants out-compete C4 plants? Will nitrogen-fixers displace non-N-fixers? Will weeds overwhelm more desirable species? Will the greater CO2-induced growth of tall forest trees reduce the transmission of precious light to shaded understory plants and lead to their demise? A number of recent studies suggest that these imagined threats are just that - imagined - and that, if anything, earth's increasing atmospheric CO2 concentration will probably be beneficial to biodiversity, increasing the niche security of nearly all the planet's many different life forms.
With respect to the first of the biodiversity worries that have come to the fore in recent years, several experiments seem to suggest that C3 plants will not out-compete C4 plants and negate the biospheric inroads they have made over the past few million years. Although C3 plants do manifest a greater biomass response to atmospheric CO2 enrichment than do C4 plants, all else being equal, the growth response of C4 plants is much larger than what has long been believed to be the case (Wand et al., 1999). In addition, all else is generally not equal; and C4 plants appear to be more responsive to CO2 than C3 plants in terms of their ability to enhance their water and nutrient acquisition capabilities as the air's CO2 content rises (BassiriRad et al., 1998), possibly as a consequence of the greater water- and nutrient-gathering effectiveness of mycorrhizal fungal associations with C4 plants as opposed to C3 plants (Wilson and Hartnett, 1998). Hence, it was not surprising that in an eight-year CO2 enrichment study of a native tallgrass prairie, Owensby et al. (1999) found that elevated CO2 did not affect either the basal coverage of its C4 species or their relative contribution to the composition of the ecosystem.
With respect to concerns about nitrogen-fixing legumes displacing non-N-fixers in a higher-CO2 world of the future, there would also appear to be little to worry about. Although Stocklin and Korner (1999) did observe N-fixing legumes to have an initial advantage over non-N-fixers in a glasshouse CO2 enrichment study of simulated grassland ecosystems, Arnone (1999) in a much longer study of a real grassland ecosystem did not observe any such advantage to be maintained by the community's legumes after four years of differential CO2 treatment. Neither were N-fixing legumes observed to prevail over non-N-fixers in the CO2 enrichment experiments of Matthies and Egli (1999) or Navas et al. (1999).
Studies of weed responses to elevated levels of atmospheric CO2 tell much the same story. Wayne et al. (1999) detected a stimulatory effect of CO2 on the growth of the weedy field mustard plant; but it was no greater than the CO2-induced growth enhancement experienced by most crops. On the other hand, the major bracken weed of the United Kingdom and elsewhere has proven totally unresponsive to atmospheric CO2 enrichment (Caporn et al., 1999); and in a study of pasture ecosystems near Montreal, Canada, Taylor and Potvin (1997) found that elevated CO2 concentrations had no influence the number of native species returning after removal to simulate disturbance, even in the face of the introduced presence of a normally noxious weed of those ecosystems.
In the case of forest ecosystems, concern has also been misplaced with respect to the possibility that taller trees might squeeze out certain understory plants due to increased shade resulting from CO2-induced increases in the growth of upper-canopy foliage. In reviewing 15 previously published studies, for example, Kerstiens (1998) found that shade-tolerant trees were two to three times more responsive to atmospheric CO2 enrichment than were sun-loving trees. Hence, even if the sunlight transmitted through the upper-canopy foliage of a forest ecosystem were to be dramatically reduced, the growth-enhancing effects of the rise in atmospheric CO2 concentration would likely more than compensate for the reduced intensity of the transmitted solar radiation, thereby enabling the full complement of understory species to maintain viable niches in the forest ecosystem.
Studies of ryegrass and wheat by Griffiths et al. (1998) and paper and yellow birch by Catovsky and Bazzaz (1999) have also demonstrated, as stated by Arp et al. (1998) in their study of six perennial plants of the Netherlands, that "a rise in CO2 would not change the relationships between plant species in the natural environment, but would reinforce existing ones." Nevertheless, in a study of a fertile and species-rich grassland near Basal, Switzerland, Leadley et al. (1999) observed that elevated CO2 produced a marginally significant increase in ecosystem biodiversity. And in a study of certain consequences of global warming, Chadwick-Furman (1996) concluded that rising sea levels could "lead to higher coral diversity in inner reef areas," as well as in "latitudinally marginal areas, which presently are temperature limited."
Another route by which atmospheric CO2 enrichment may actually increase the species richness of an ecosystem begins with CO2-induced increases in the exudation of organic matter into the soil, which phenomenon, according to the study of Hodge et al. (1998), stimulates the proliferation of previously-dormant but viable microorganisms, including symbiotic soil fungi. These fungi, in turn, are highly selective in the species of plants they tend to support, as observed by van der Heijden et al. (1998a). In fact, van der Heijden et al. (1998b) demonstrated that increasing the number of fungal species in the soils of certain artificial ecosystems from 4 to 14 increased ecosystem plant diversity by 60%.
In addition to species richness, another aspect of biodiversity that deserves mention is genetic diversity, or diversity among genotypes of single species. Here we are happy to report that in at least eight studies of the past three years - those of Schaffer et al. (1997), Case et al. (1998), Egli et al. (1998), Kubiske et al. (1998), Luscher et al. (1998), Midgley et al. (1999), Norton et al. (1999) and Polley et al. (1999) - increasing the air's CO2 concentration induced absolutely no differential growth responses among the several genotypes of all sorts of different plant species, suggesting that none would be favored over the other in a high-CO2 world of the future.
In conclusion, it would appear that we need not worry about the ongoing rise in the air's CO2 content negatively impacting the biodiversity of earth's many ecosystems. If it is to influence genetic diversity and species richness in any way, it will in all likelihood be to enhance these desirable biospheric properties.
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