Showing papers by "Guy F. Midgley published in 2004"

Extinction risk from climate change

Abstract: Climate change over the past approximately 30 years has produced numerous shifts in the distributions and abundances of species and has been implicated in one species-level extinction. Using projections of species' distributions for future climate scenarios, we assess extinction risks for sample regions that cover some 20% of the Earth's terrestrial surface. Exploring three approaches in which the estimated probability of extinction shows a power-law relationship with geographical range size, we predict, on the basis of mid-range climate-warming scenarios for 2050, that 15-37% of species in our sample of regions and taxa will be 'committed to extinction'. When the average of the three methods and two dispersal scenarios is taken, minimal climate-warming scenarios produce lower projections of species committed to extinction ( approximately 18%) than mid-range ( approximately 24%) and maximum-change ( approximately 35%) scenarios. These estimates show the importance of rapid implementation of technologies to decrease greenhouse gas emissions and strategies for carbon sequestration.

Relating plant traits and species distributions along bioclimatic gradients for 88 leucadendron taxa

Abstract: Convergence between species niches and biological traits was investigated for 88 Leucadendron taxa in the Cape Floristic region. First, niche separation analysis was performed to relate species' niche positions/breadths with bioclimatic gradients. These gradients of aridity, seasonality of water availability, heat, and cold stress explained almost all variation in niche distributions. Species present in zones of extreme aridity or temperature exhibited narrower niche breadths than species situated in moderate sites, suggesting that stress-tolerant species do not occupy broad environmental ranges. Second, species niche positions were related to selected biological traits. Species of arid sites had significantly lower blade areas than did species of moist sites, confirming a functional trade-off between stress tolerance and productivity for leaf design. Species dispersal mode was correlated to species niche positions on the aridity gradient, suggesting allometrically determined cor- relations between leaf design and the design of reproductive structures. Species niche positions were also correlated with flowering traits, with species that initiate flowering in winter found under Mediterranean climate influence and species that initiate flowering in spring in sites with greater summer rainfall input. By interrelating species niche positions on bioclimatic gradients with selected biological traits, we explored a novel biogeographical approach to understanding species distributions.

Climate change and birds: perspectives and prospects from southern Africa.

Abstract: Global climate warming, now conclusively linked to anthropogenically-increased CO2 levels in the earth's atmosphere, has already had impacts on the earth's biodiversity and is predicted to threaten more than 1 million species with extinction by 2050. Climate change in southern Africa is expected to involve higher temperatures and lower rainfall, with less predictability and a greater frequency of severe storms, fires and El Nino events. The predicted changes to birds in Africa — the continent most at risk from climate change — have hardly been explored, yet birds and many other vertebrates face uncertain futures. Here, in one of the first focused analyses of the correlates of climate change vulnerability in southern African birds, we offer a wide-ranging perspective on which species may be most at risk, and explore which traits may influence the adaptability or extinction risk of bird species. Our review suggests that small nomadic species with short generation times may be least at risk. While larger-bod...

Photosynthetic and gas exchange characteristics of dominant woody plants on a moisture gradient in an African savanna

Abstract: We determined key photosynthetic gas exchange parameters, and their temperature dependence, in dominant woody plants at four savanna sites on a moisture gradient in Botswana, southern Africa. Leaf stable carbon and nitrogen (N) isotope and morphological measures were made concurrently. Sampling of these predominantly non-N-fixing species took place during an exceptional rainfall season, representing nearoptimum conditions for primary production at these sites. The mean specific leaf area and leaf size were positively related to mean annual rainfall (MAR); species with larger leaves of lower density were more abundant in wetter sites. Almost all species at all sites showed high net light-saturated photosynthetic rates (Amax @10lmolCO2m � 2 s � 1 ) due both to high CO2 carboxylation (Vc,max) and RubP-regeneration capacity (Jmax). These high rates were associated with high values of leaf [N]. Across all sites, the temperature response of Amax showed no clear optimum, and a gradual drop from 251 Ct o 351C, without notable temperature limitation at leaf temperatures in excess of 351C. Dark respiration rate (Rday) across all species and sites increased exponentially with increasing leaf temperature. Species sampled at selected sites revealed a negative relationship between leaf d 13 C (stable carbon isotope ratio) and MAR, suggesting higher leaf-level water-use efficiency at drier sites when integrated over the life of the leaf. At wetter sites, specific leaf [N] was lower and photosynthetic nitrogen-use efficiency increased, a pattern reflected at the ecosystem level by less 15 N enrichment of leaves at these sites. Taken together, the results suggest a switch from water-use to nitrogen-use efficiency constraints with increasing moisture availability. These constraints impact leaf form and function significantly, and may emerge at the ecosystem level in aspects of water and N cycling.

Effects of atmospheric CO2 concentration and defoliation on the growth of Themeda triandra

Abstract: The effects of elevated atmospheric carbon dioxide (CO2) concentration (700 μmol mol−1) on defoliated (three clippings at 3-week intervals) and undefoliated plants were determined for the C4 grass Themeda triandra, Forsk. The elevated CO2 concentration significantly increased leaf regrowth following defoliation, and total leaf production was greatest in this treatment. Shoot biomass of undefoliated plants was also increased under the elevated CO2 concentration treatment. The primary effect of the elevated CO2 concentration in both defoliated and undefoliated plants was an increase in individual leaf length and mass of dry matter, linked to a higher leaf water content and increased photosynthetic rates at the canopy level. Photosynthetic down-regulation at the leaf level occurred, but this was compensated for by increased assimilation rates and greater canopy leaf area at the elevated CO2 concentration. Increases in leaf and sheath growth of defoliated plants in the elevated CO2 concentration treatment were lost following a final 3-week reversion to ambient CO2 concentration, but occurred in plants exposed to the elevated CO2 concentration for the final 3-week period only. In conclusion, elevated atmospheric CO2 concentration increases shoot growth via increased leaf extension, which is directly dependent on stimulation of concurrent photosynthesis. CO2 responsiveness is sustained following moderate defoliation but is reduced when plants experience reduced vigour as a result of maturation or high frequency of defoliation.

Xylem hydraulics and angiosperm success: A test using broad-leafed species

Abstract: Publisher Summary This chapter examines the ways in which the hydraulic properties of broad-leafed gymnosperms differ from those of angiosperms with a similar tree growth form, and whether such differences are correlated with leaf physiology or if the leaf physiology could potentially influence competitive ability. Angiosperms attain higher conductance and photosynthetic capacity than gymnosperms. The lower hydraulic conductances and lower transpiration rates of the gymnosperms are associated with lower stomatal conductances to diffusion. The hydraulic conductivities of broad-leafed deciduous angiosperms are higher than those of broad-leafed gymnosperms growing in the same or similar environments and these differences in hydraulic characteristics are associated with differences in leaf physiology, with the angiosperms having higher photosynthetic capacities and light-saturated photosynthetic rates. Hydraulic limitations are likely to emerge as constraints on plant growth and there is evidence of a general trend of high whole plant hydraulic conductivities being associated with fast growth rates of pioneer species. An additional possible factor in the competitive potential of angiosperms is that our casual (unrecorded) observations suggest that the growth form of angiosperm trees is more plastic than that of gymnosperms.