Ecosystem Biodiversity
From ClimateWiki
How will the biodiversity of Earth’s ecosystems respond to predicted changes in the planet’s climate? This question is among the top concerns about potential CO2-induced global warming, and it has been addressed in several different ways.
Studying the direct effects of elevated CO2 itself, Lau et al. (2010) grew thale cress (Arabidopsis thaliana) either alone or together with the C3 grass Bromus inermis or the C4 grass Andropogon gerardii in small pots placed within open-field FACE arrays at the Cedar Creek Ecosystem Science Reserve, Minnesota (USA). These were maintained at atmospheric CO2 concentrations of either 368 or 560 ppm from the time of emergence to the time of plant senescence. At the time of harvest, the CO2-induced stimulation of A. thaliana biomass was approximately 42 percent when it was grown alone, but 46 percent when it was grown together with A. gerardii and 50 percent when grown together with B. inermis, while corresponding stimulation values were 1 percent, 3 percent, and 4 percent for leaf number; 15 percent, 17 percent, and 21 percent for plant height; 11 percent, 21 percent, and 20 percent for stem number; and 25 percent, 43 percent, and 39 percent for fruit number. All of this indicates greater CO2-induced benefits for A. thaliana when grown in competitive mixtures with other species. Lau et al. conclude, “elevated CO2 reduces the effects of competition on mean fitness ... and minimizes the strength of competition as a selective agent.” Therefore, it may logically be expected (all else being equal) that ecosystem species richness or biodiversity should at least remain stable, or possibly even increase, in response to continued increases in the air’s CO2 content.
In a related study, Peter B. Reich of the University of Minnesota (2009) wrote, “levels of N [nitrogen] deposition and CO2 have risen in recent decades and are expected to increase further,” and in the case of natural ecosystems, he writes, “the suppression of diversity by increasing N availability is almost ubiquitous,” while stating “evidence of CO2 effects on species richness is scarce and shows mixed results, with positive, neutral, and negative responses seen in the few published reports.”
In what Collins (2009) appropriately describes as “a rare gem in long-term ecological research,” Reich presided over the ten-year-long BioCON study conducted at the Cedar Creek Long-Term Ecological Research site, where, as Reich describes it, “species richness was measured in 48 experimental grassland plots (each 2 m by 2 m) planted in 1997 with 16 perennial species [four species from each of four functional groups (C4 grasses, C3 grasses, legumes and non-legume forbs)] and treated since 1998 with all combinations of ambient and elevated atmospheric CO2 (ambient and +180 ppm delivered by means of a free-air CO2 enrichment technique) and ambient and enriched N (ambient and +4 g N m-2 year-1 delivered as ammonium nitrate in three equal doses each year),” while several plant physiological processes and properties were measured throughout each growing season.
Reich found that at the ambient soil N concentration, elevated CO2 had minimal impact on observed species richness (-2 percent), while at the ambient atmospheric CO2 concentration, elevated N decreased species richness by fully 15 percent over the last seven years of the ten-year-long study. But when the elevated soil N concentration was combined with the elevated atmospheric CO2 concentration, species richness declined by only 5 percent, leading Reich to conclude, “elevated CO2 reduces losses of plant diversity caused by nitrogen deposition.” This was such an important finding that he made it the title of his paper. With levels of nitrogen deposition “expected to increase further,” as Reich notes, the Earth’s natural ecosystems will benefit from the fact that the atmosphere’s CO2 concentration is rising in tandem with the increasing level of N deposition that is being experienced throughout the world.
Regarding possible indirect effects of rising CO2 concentrations, some researchers contend that CO2-induced global warming will be so fast and furious that many species of plants will not be able to migrate poleward in latitude or upward in altitude at rates required to keep them within the geographically shifting temperature regimes to which they have been adapted. Thus, they claim, many species will be driven to extinction, and the species richness of various ecosystems will be greatly reduced.
As some examples of these contentions, Dyer (1995) wrote, “the magnitude of the projected warming is considerable” and “the rate at which it is predicted to occur is unprecedented,” and consequently, “there is genuine reason for concern that the extent of range shifts will exceed the dispersal abilities of many plant species.” Malcolm and Markham (2000) similarly wrote, “rapid rates of global warming are likely to increase rates of habitat loss and species extinction,” and “many species may be unable to shift their ranges fast enough to keep up with global warming.” Malcolm et al. (2002) added, “migration rates required by the warming are unprecedented by historical standards, raising the possibility of extensive, and in many cases, catastrophic, species loss.” In his 26 April 2007 testimony to the Select Committee of Energy Independence and Global Warming of the United States House of Representatives, NASA’s James Hansen echoed these fears, saying “greenhouse gas emissions threaten many ecosystems,” contending “very little additional forcing is needed ... to cause the extermination of a large fraction of plant and animal species,” claiming “polar species can be pushed off the planet, as they have no place else to go,” and stating “life in alpine regions ... is similarly in danger of being pushed off the planet.”
An enlightening reality check on these doomsday scenarios was provided by Le Roux and McGeoch (2008), who examined patterns of altitudinal range changes in the totality of the native vascular flora of sub-Antarctic Marion Island (46°54’S, 37°45’E) in the southern Indian Ocean, which warmed by 1.2°C between 1965 and 2003. These South African researchers found that between 1966 and 2006, there was “a rapid expansion in altitudinal range,” with species expanding their upper elevational boundaries by an average of 70 meters. And because, as they describe it, “the observed upslope expansion was not matched by a similar change in lower range boundaries,” they emphasize “the flora of Marion Island has undergone range expansion rather than a range shift,” noting “the expansion of species distributions along their cooler boundaries in response to rising temperatures appears to be a consistent biological consequence of recent climate warming,” citing references to several other studies that have observed the same type of response.
An important consequence of the stability of lower-range boundaries of species together with expanding upper-range boundaries is a greater overlapping of ranges, which results in greater local species richness or biodiversity everywhere up and down various altitudinal transects. As a further consequence, le Roux and McGeoch indicate “the present species composition of communities at higher altitudes is not an analogue of past community composition at lower altitudes, but rather constitutes a historically unique combination of species,” a new world significantly richer in species in a greater number of locations than in the recent past.
Working on the outskirts of Jena, Germany, Steinbeiss et al. (2008) sowed 20-m by 20-m plots of soil with seeds of either one, two, four, eight, 16, or 60 species of either one, two, three, or four plant functional groups (grasses, small herbs, tall herbs, and legumes), creating 16 replicate plots per species level except for the 16-species level plots (14 replicates) and the 60-species level plots (four replicates) in April 2002, just before soil carbon content sampling was performed, and after which similar sampling was repeated in April 2004 and April 2006.
During the first two years of their study, soil carbon storage was limited to the top 5 cm of soil, while below 10 cm depth, carbon was actually lost. After four years, however, carbon stocks had increased significantly within the top 20 cm of the soil. However, and “more importantly,” in the words of Steinbeiss et al., “carbon storage significantly increased with sown species richness in all depth segments and even carbon losses were significantly smaller with higher species richness.” Consequently, they concluded, “plant species richness ... accelerate[d] the build-up of new carbon pools within four years,” and “higher plant diversity mitigated soil carbon losses in deeper horizons.”
The researchers state their findings suggest “higher biodiversity might lead to higher soil carbon sequestration in the long-term,” and, therefore, “the conservation of biodiversity might play a role in greenhouse gas mitigation.” It also should be added that this phenomenon represents a previously unrecognized negative feedback, since studies such as that of Le Roux and McGeoch have demonstrated global warming typically leads to higher local and regional biodiversity wherever the process has been studied throughout the world.
The study of Odland et al. (2010) provides what can be seen as the icing on the cake when it comes to refuting James Hansen’s wild contention of mountaintop species being “pushed off the planet” by global warming. Based on their documentation of plant species diversity on 13 mountain summits in southern Norway—in a reenactment of what Lye (1973) had done more than three decades earlier—and their assessment of regional warming over the intervening years, the three scientists sought to ascertain how plant species richness may have changed in response to what turned out to have been a significant increase in local temperatures. They found average summer temperatures had risen by approximately 1.3°C between the times of the two studies, and over that period plant taxa richness had risen by an average of 90 percent, with two of the summits experiencing increases of fully 200 percent.
Odland et al. state the average rise they identifed “is in accordance with similar studies in both Scandinavia and southern Europe (Kullman, 2007a,b; Parmesan, 2005; Pauli et al., 2007),” but the 200 percent increase in taxa richness they documented on two of the summits “is exceptional.” And because the latter result is also true, it can validly be called an exceptional truth. And giving credit where credit is due, the three Norse researchers conclude “the present increase in species richness is mainly a result of recent climatic change.”
Not all ecosystems, however, respond so dramatically to warming, though still positively, as evidenced by the study of Hudson and Henry (2010). They employed open-top chambers to passively warm an evergreen-shrub heath, which was dominated by several shrub species and bryophytes, by 1.0–1.3°C over a period of 15 years (when there was also a significant background warming) in the vicinity of Alexandra Fiord, Nunavut, Canada (79°N). In what they describe as “the longest-running passive warming experiment in the Canadian Arctic,” the two University of British Columbia researchers found “experimental warming did not strongly affect vascular plant cover, canopy height or species diversity, but it did increase bryophyte cover by 6.3% and decrease lichen cover by 3.5%,” although they note “temporal changes in plant cover were more frequent and of greater magnitude than changes due to experimental warming.” These findings thus prompted them to state, “this evergreen-shrub heath continues to exhibit community-level resistance to long-term experimental warming.”
In about the only logical conclusion that could be reached on the basis of their observations, Hudson and Henry state their findings “support the view that only substantial climatic changes will alter unproductive ecosystems,” such as the one they studied. In further support of this statement they note other plant communities also have “exhibited strong resistance to simulated climate change manipulations (e.g. Grime et al., 2008), where resistance is defined as the ability of a community to maintain its composition and structure in the face of environmental change.” They amplify this conclusion by stating, “at other Arctic sites, lichen, bryophyte and evergreen-shrub dominated heaths were [also] less responsive to experimental warming than other plant communities,” citing the studies of Hollister et al. (2005), Jonsdottir et al. (2005), and Wahren et al. (2005).
In the first of two studies of an agricultural crop that looks at the phenomenon in question from a very different point of view, Yang et al. (2009) write, “rice (Oryza sativa L.) is unequivocally one of the most important food crops that feed the largest proportion of the world’s population,” and they note “the demand for rice production will continue to increase in the coming decades, especially in the major rice-consuming countries of Asia, Africa and Latin America, due to the population explosion and cropland reduction.” Hence, they state “as sufficient intraspecific variation in yield response [of rice] exists under field conditions, there is a pressing need to identify genotypes which would produce maximum grain yield under projected future CO2 levels.” In other words, they are looking to go beyond nature in determining what will grow where, in order to take best advantage of what Earth’s changing environment has to offer.
Working with that same goal in mind at the National Institute for Agro-Environmental Sciences in Tsukuba, Japan, Lou et al. (2008) grew plants of four different rice cultivars—Dular (a traditional indica variety), IR72 (an improved indica variety), Koshihikari (a temperate japonica variety), and IR65598 (a new variety not yet released to farmers)—within growth chambers in submerged pots filled with a fertilized soil collected from the plough layer of a paddy field in Chiba Prefecture, Japan, at two atmospheric CO2 concentrations: ambient (~370 ppm) and elevated (~570 ppm). This protocol revealed the extra 200 ppm of CO2 reduced the ultimate grain yield of Dular (by 0.7 percent), while it increased the final grain yield of IR72 by 8.0 percent, that of Koshihikari by 13.4 percent, and that of IR65598 by 17.7 percent.
Shortly thereafter—working at the FACE facility at Yangzhou City, Jiangsu Province, China—Yang et al. (2009) focused on a single two-line inter-subspecific hybrid rice variety (Liangyoupeijiu), produced as part of “a nationwide mega project” to develop what they call “super” hybrid cultivars that would “further break the yield ceiling.” In their three-year CO2-enrichment study, which employed the same CO2 levels as the study of Lou et al., they found a much greater grain yield stimulation: a 28.4 percent CO2-induced increase under a low nitrogen fertility treatment of 12.5 g N m-2 and a 31.7 percent CO2-induced increase under a high nitrogen fertility treatment of 25 g N m-2.
In discussing their findings, Yang et al. state their hybrid cultivar “appears to profit much more from elevated CO2 than inbred japonica cultivars,” which does indeed seem to be the case, as both Japanese and Chinese FACE studies of inbred japonica cultivars have found CO2-induced grain yield enhancements only on the order of 13 percent for a 200 ppm increase in the air’s CO2 concentration. Therefore, noting “there is a pressing need to identify genotypes which could optimize harvestable yield as atmospheric CO2 increases,” Yang et al. conclude, “on the basis of available FACE data on rice,” the hybrid rice cultivar Liangyoupeijiu “appears to be particularly promising.”
References
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Dyer, J.M. 1995. Assessment of climatic warming using a model of forest species migration. Ecological Modelling 79: 199–219.
Grime, J.P., Fridley, J.D., Askew, A.P., Thompson, K., Hodgson, J.G. and Bennett, C.R. 2008. Long-term resistance to simulated climate change in an infertile grassland. Proceedings of the National Academy of Sciences of the United States ofAmerica 105: 10028–10032.
Grime, J.P., Brown, V.K., Thompson, K., Masters, G.J., Hillier, S.H., Clarke, I.P., Askew, A.P., Corker, D., and Kielty, J.P. 2000. The response of two contrasting limestone grasslands to simulated climate change. Science 289: 762–765.
Hollister, R.D., Webber, P.J., and Tweedie, C.E. 2005. The response of Alaskan arctic tundra to experimental warming: differences between short- and long-term responses. Global Change Biology 11: 525–536.
Hudson, J.M.G. and Henry, G.H.R. 2010. High Arctic plant community resists 15 years of experimental warming. Journal of Ecology 98: 1035–1041.
Jonsdottir, I.S., Magnusson, B., Gudmundsson, J., Elmarsdottir, A., and Hjartarson, H. 2005. Variable sensitivity of plant communities in Iceland to experimental warming. Global Change Biology 11: 553–563.
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Reich, P.B. 2009. Elevated CO2 reduces losses of plant diversity caused by nitrogen deposition. Science 326: 1399–1402.
Steinbeiss, S., Bessler, H., Engels, C., Temperton, V.M., Buchmann, N., Roscher, C., Kreutziger, Y., Baade, J., Habekost, M., and Gleixner, G. 2008. Plant diversity positively affects short-term soil carbon storage in experimental grasslands. Global Change Biology 14: 2937–2949.
Wahren, C.H.A., Walker, M.D., and Bret-Harte, M.S. 2005. Vegetation responses in Alaskan arctic tundra after 8 years of a summer warming and winter snow manipulation experiment. Global Change Biology 11: 537–552.
Yang, L., Liu, H., Wang, Y., Zhu, J., Huang, J., Liu, G., Dong, G., and Wang, Y. 2009. Yield formation of CO2-enriched inter-subspecific hybrid rice cultivar Liangyoupeijiu under fully open-air condition in a warm sub-tropical climate. Agriculture, Ecosystems and Environment 129: 193–200.
