Muhly, T.B., M. Hebblewhite, D. Paton, J.A. Pitt, M.S. Boyce, and M. Musiani, 2013: “Humans strengthen bottom-up effects and weaken trophic cascades in a terrestrial food web.” PLoS ONE, v. 8, paper no. e64311, doi: 10.1371/journal.pone.0064311.
Ongoing debate about whether food webs are primarily regulated by predators or by primary plant productivity, cast as top-down and bottom-up effects, respectively, may becoming superfluous. Given that most of the world’s ecosystems are human dominated we broadened this dichotomy by considering human effects in a terrestrial food-web. We studied a multiple human-use landscape in southwest Alberta, Canada, as opposed to protected areas where previous terrestrial food-web studies have been conducted. We used structural equation models (SEMs) to assess the strength and direction of relationships between the density and distribution of: (1) humans, measured using a density index; (2) wolves (Canis lupus), elk (Cervus elpahus) and domestic cattle (Bos taurus), measured using resource selection functions, and; (3) forage quality, quantity and utilization (measured at vegetation sampling plots). Relationships were evaluated by taking advantage of temporal and spatial variation in human density, including day versus night, and two landscapes with the highest and lowest human density in the study area. Here we show that forage-mediated effects of humans had primacy over predator-mediated effects in the food web. In our parsimonious SEM, occurrence of humans was most correlated with occurrence of forage. Elk and cattle distribution were correlated with forage, and the distribution of elk or cattle and wolves were positively correlated during daytime and nighttime. Our results contrast with research conducted in protected areas that suggested human effects in the food web are primarily predator-mediated. Instead, human influence on vegetation may strengthen bottom-up predominance and weaken top-down trophic cascades in ecosystems. We suggest that human influences on ecosystems may usurp top-down and bottom-up effects.
Open Access
Hagemann, S., C. Chen, D.B. Clark, S. Folwell, S.N. Gosling, I. Haddeland, N. Hanasaki, J. Heinke, F. Ludwig, F. Voss, and A.J. Wiltshire, 2013: “Climate change impact on available water resources obtained using multiple global climate and hydrology models.” Earth System Dynamics, v. 4, pp. 129-144, doi: 10.5194/esd-4-129-2013.
Climate change is expected to alter the hydrological cycle resulting in large-scale impacts on water availability. However, future climate change impact assessments are highly uncertain. For the first time, multiple global climate (three) and hydrological models (eight) were used to systematically assess the hydrological response to climate change and project the future state of global water resources. This multi-model ensemble allows us to investigate how the hydrology models contribute to the uncertainty in projected hydrological changes compared to the climate models. Due to their systematic biases, GCM outputs cannot be used directly in hydrological impact studies, so a statistical bias correction has been applied. The results show a large spread in projected changes in water resources within the climate–hydrology modelling chain for some regions. They clearly demonstrate that climate models are not the only source of uncertainty for hydrological change, and that the spread resulting from the choice of the hydrology model is larger than the spread originating from the climate models over many areas. But there are also areas showing a robust change signal, such as at high latitudes and in some midlatitude regions, where the models agree on the sign of projected hydrological changes, indicative of higher confidence in this ensemble mean signal. In many catchments an increase of available water resources is expected but there are some severe decreases in Central and Southern Europe, the Middle East, the Mississippi River basin, southern Africa, southern China and south-eastern Australia.
Open Access
Jasechko, S., Z.D. Sharp, J.J. Gibson, S.J. Birks, Y. Yi, and P.J. Fawcett, 2013: “Terrestrial water fluxes dominated by transpiration.” Nature, v. 496, pp. 347-350, doi: 10.1038/nature11983.
Renewable fresh water over continents has input from precipitation and losses to the atmosphere through evaporation and transpiration. Global-scale estimates of transpiration from climate models are poorly constrained owing to large uncertainties in stomatal conductance and the lack of catchment-scale measurements required for model calibration, resulting in a range of predictions spanning 20 to 65 per cent of total terrestrial evapotranspiration (14,000 to 41,000 km3 per year). Here we use the distinct isotope effects of transpiration and evaporation to show that transpiration is by far the largest water flux from Earth’s continents, representing 80 to 90 per cent of terrestrial evapotranspiration. On the basis of our analysis of a global data set of large lakes and rivers, we conclude that transpiration recycles 62,000 ± 8,000 km3 of water per year to the atmosphere, using half of all solar energy absorbed by land surfaces in the process. We also calculate CO2 uptake by terrestrial vegetation by connecting transpiration losses to carbon assimilation using water-use efficiency ratios of plants, and show the global gross primary productivity to be 129 ± 32 gigatonnes of carbon per year, which agrees, within the uncertainty, with previous estimates. The dominance of transpiration water fluxes in continental evapotranspiration suggests that, from the point of view of water resource forecasting, climate model development should prioritize improvements in simulations of biological fluxes rather than physical (evaporation) fluxes.
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Friedman, A.R., Y.-T. Hwang, J.C.H. Chiang, and D.M.W. Frierson, 2013: “Interhemispheric temperature asymmetry over the 20th century and in future projections.” Journal of Climate, doi: 10.1175/JCLI-D-12-00525.1.
The temperature contrast between the northern and southern hemispheres - the interhemispheric temperature asymmetry (ITA) - is an emerging indicator of global climate change, potentially relevant to the Hadley circulation and tropical rainfall. We examine the ITA in historical observations and in Coupled Model Intercomparison Project (CMIP) 3 and 5 simulations.
The observed annual mean ITA (north minus south) has varied within a 0.8 °C range and features a significant positive trend since 1980. The CMIP multimodel ensembles simulate this trend, with a stronger and more realistic signal in CMIP5. Both ensembles project a continued increase in the ITA over the 21st century, well outside the 20th century range. We mainly attribute this increase to the uneven spatial impacts of greenhouse forcing, which result in amplified warming in the Arctic and northern landmasses. CMIP5 specific-forcing simulations indicate that before 1980, the greenhouse–forced ITA trend was primarily countered by anthropogenic aerosols. We also identify an abrupt decrease in the observed ITA in the late 1960s, which is generally not present in the CMIP simulations; it suggests that the observed drop is due to internal variability.
The difference in the strengths of the northern and southern Hadley cells co-varies with the ITA in the CMIP5 simulations, in accordance with previous findings; we also find an association with the hemispheric asymmetry in tropical rainfall. These relationships imply a northward shift in tropical rainfall with increasing ITA in the 21st century, though this result is difficult to separate from the response to global mean temperature change.
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Munang, R., I. Thiaw, K. Alverson, J. Liu, and Z. Han, 2013: “The role of ecosystem services in climate change adaptation and disaster risk reduction.” Current Opinion in Environmental Sustainability, v. 5, pp. 47-52, doi: 10.1016/j.cosust.2013.02.002.
This paper analyzes the vicious spiral between climate change impacts, ecosystem degradation and increased risk of climate-related disasters; secondly, it defines the central role of ecosystem management in climate change adaptation and disaster risk reduction and their multifaceted linkages; and thirdly, it assesses the challenges for enhanced ecosystem management for climate change adaptation and disaster risk reduction. Given the increasing importance of ecosystem services and management in adapting and responding to climate change impacts and associated disaster risks, the paper concludes that political commitment at the highest level is urgently needed if ecosystem management is to have the adequate weight it deserves in the post-2012 climate change agreement. It is further recommended that adequate financial, technological and knowledge resources be allocated for integrating ecosystem management in the climate change and disaster risk reduction portfolios, including within national policy-setting, capacity building, planning and practices, particularly in developing countries vulnerable to climate change impacts and increased risks of climate-related disasters.
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Sprague, L.A., R.M. Hirsch, and B.T. Aulenbach, 2011: “Nitrate in the Mississippi River and its tributaries, 1980 to 2008: Are we making progress?” Environmental Science and Technology, v. 45, pp. 7209-7216, doi: 10.1021/es201221s.
Changes in nitrate concentration and flux between 1980 and 2008 at eight sites in the Mississippi River basin were determined using a new statistical method that accommodates evolving nitrate behavior over time and produces flow-normalized estimates of nitrate concentration and flux that are independent of random variations in streamflow. The results show that little consistent progress has been made in reducing riverine nitrate since 1980, and that flow-normalized concentration and flux are increasing in some areas. Flow-normalized nitrate concentration and flux increased between 9 and 76% at four sites on the Mississippi River and a tributary site on the Missouri River, but changed very little at tributary sites on the Ohio, Iowa, and Illinois Rivers. Increases in flow-normalized concentration and flux at the Mississippi River at Clinton and Missouri River at Hermann were more than three times larger than at any other site. The increases at these two sites contributed much of the 9% increase in flow-normalized nitrate flux leaving the Mississippi River basin. At most sites, concentrations increased more at low and moderate streamflows than at high streamflows, suggesting that increasing groundwater concentrations are having an effect on river concentrations.
Additional and related materials are available from the US Geological Survey.
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Tesoriero, A.J., J.H. Duff, D.A. Saad, N.E. Spahr, and D.M. Wolock, 2013: “Vulnerability of streams to legacy nitrate sources.” Environmental Science and Technology, v. 47, pp. 3623-3629, doi: 10.1021/es305026x.
The influence of hydrogeologic setting on the susceptibility of streams to legacy nitrate was examined at seven study sites having a wide range of base flow index (BFI) values. BFI is the ratio of base flow to total streamflow volume. The portion of annual stream nitrate loads from base flow was strongly correlated with BFI. Furthermore, dissolved oxygen concentrations in streambed pore water were significantly higher in high BFI watersheds than in low BFI watersheds suggesting that geochemical conditions favor nitrate transport through the bed when BFI is high. Results from a groundwater–surface water interaction study at a high BFI watershed indicate that decades old nitrate-laden water is discharging to this stream. These findings indicate that high nitrate levels in this stream may be sustained for decades to come regardless of current practices. It is hypothesized that a first approximation of stream vulnerability to legacy nutrients may be made by geospatial analysis of watersheds with high nitrogen inputs and a strong connection to groundwater (e.g., high BFI).
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Park, J., H.E. Gall, D. Niyogi, and P.S.C. Rao, 2013: “Temporal trajectories of wet deposition across hydro-climatic regimes: Role of urbanization and regulations at U.S. and East Asia sites.” Atmospheric Environment, v. 70, pp. 280-288, doi: 10.1016/j.atmosenv.2013.01.033.
Dominant global patterns of urbanization and industrialization contribute to large-scale modification of the drivers for hydrologic and biogeochemical processes, as evident in Asia, Africa, and South America which are experiencing rapid population and economic growth. One manifestation of urbanization and economic development is decreases in air quality, increases in dry/wet deposition fluxes, and growing adverse impacts on public health and ecosystem integrity. We examined available long-term (1980–2010) observational data, gathered at weekly intervals, for wet deposition at 19 urban sites in the U.S., and monitoring data (2000–2009) available for 17 urban sites at a monthly scale in East Asia. Our analyses are based on data for four constituents (SO42−, NO3−, Ca2+, and Mg2+); differences in atmospheric chemistry and terrestrial sources of these constituents enabled a robust comparative analysis. We examined intra-annual variability and the long-term temporal trajectories of wet deposition fluxes to discern the relative role of anthropogenic and stochastic hydro-climatic forcing. Here, we show that: (1) temporal variability in wet deposition fluxes follows an exponential probability density function at all sites, evidence that stochasticity of rainfall is the dominant control of wet deposition variability; (2) the mean wet deposition flux, μΩ (ML−2T−1), has decreased in the U.S. over time since enactment of the Clean Air Act, with μΩ having become homogenized across varying hydro-climatic regimes; and (3) in contrast, μΩ values for East Asian cities are 3–10 times higher than U.S. cities, attributed to lax regulatory enforcement. Based on the observed patterns, we suggest a stochastic model that generates ellipses within which the μΩ temporal trajectories are inscribed. In the U.S., anthropogenic forcing (regulations) is dominant in the humid regions, while variability in hydro-climatic forcing explains inter-annual variability in arid regions. Our stochastic analysis facilitates projections of the temporal trajectory shifts in wet deposition fluxes as a result of urbanization and other land-use changes, climate change, and regulatory enforcement.
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Green, M.B., A.S. Bailey, S.W. Bailey, J.J. Battles, J.L. Campbell, C.T. Driscoll, T.J. Fahey, L.C. Lepine, G.E. Likens, S.V. Ollinger, and P.G. Schaberg, 2013: “Decreased water flowing from a forest amended with calcium silicate.” Proceedings of the National Academy of Sciences, doi: 10.1073/pnas.1302445110.
Acid deposition during the 20th century caused widespread depletion of available soil calcium (Ca) throughout much of the industrialized world. To better understand how forest ecosystems respond to changes in a component of acidification stress, an 11.8-ha watershed was amended with wollastonite, a calcium silicate mineral, to restore available soil Ca to preindustrial levels through natural weathering. An unexpected outcome of the Ca amendment was a change in watershed hydrology; annual evapotranspiration increased by 25%, 18%, and 19%, respectively, for the 3 y following treatment before returning to pretreatment levels. During this period, the watershed retained Ca from the wollastonite, indicating a watershed-scale fertilization effect on transpiration. That response is unique in being a measured manipulation of watershed runoff attributable to fertilization, a response of similar magnitude to effects of deforestation. Our results suggest that past and future changes in available soil Ca concentrations have important and previously unrecognized implications for the water cycle.
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Januchowski-Hartley, S.R., P.B. McIntyre, M. Diebel, P.J. Doran, D.M. Infante, C. Joseph, and J.D. Allan, 2013: “Restoring aquatic ecosystem connectivity requires expanding inventories of both dams and road crossings.” Frontiers in Ecology and the Environment, v. 11, pp. 211-217, doi: 10.1890/120168.
A key challenge in aquatic restoration efforts is documenting locations where ecological connectivity is disrupted in water bodies that are dammed or crossed by roads (road crossings). To prioritize actions aimed at restoring connectivity, we argue that there is a need for systematic inventories of these potential barriers at regional and national scales. Here, we address this limitation for the North American Great Lakes basin by compiling the best available spatial data on the locations of dams and road crossings. Our spatial database documents 38 times as many road crossings as dams in the Great Lakes basin, and case studies indicate that, on average, only 36% of road crossings in the area are fully passable to fish. It is therefore essential that decision makers account for both road crossings and dams when attempting to restore aquatic ecosystem connectivity. Given that road crossing structures are commonly upgraded as part of road maintenance, many opportunities exist to restore connections within aquatic ecosystems at minimal added cost by ensuring upgrade designs permit water flow and the passage of fish and other organisms. Our findings highlight the necessity for improved dam and road crossing inventories that traverse political boundaries to facilitate the restoration of aquatic ecosystem connectivity from local to global scales.
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