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251.
Energy analysis of nonmarket values of the Mississippi Delta 总被引:1,自引:0,他引:1
An energy analysis was used to estimate nonmarket values under various land cover scenarios in the Mississippi Delta. Land
loss since 1900 has led to a decline in nonmarket values from $3.1 billion/year in 1900 to $2.5 billion in 1990, resulting
in a total loss of $29.4 billion. This loss is concentrated in the Barataria-Terrebonne basins, where nonmarket value has
dropped from $1.6 billion/year in 1956 to $1.3 billion/year in 1988. Although values are projected to increase in the Atchafalaya
basin (from $723 million/year in 1988 to $756 million/year in 2058), total nonmarket value for the Louisiana coast is projected
to decrease to $2.1 billion/year under currently approved levels of restoration. 相似文献
252.
253.
Sharks are apex predators, and their evolutionary success is in part due to an impressive array of sensory systems, including
vision. The eyes of sharks are well developed and function over a wide range of light levels. However, whilst close relatives
of the sharks—the rays and chimaeras—are known to have the potential for colour vision, an evolutionary trait thought to provide
distinct survival advantages, evidence for colour vision in sharks remains equivocal. Using single-receptor microspectrophotometry,
we measured the absorbance spectra of visual pigments located in the retinal photoreceptors of 17 species of shark. We show
that, while the spectral tuning of the rod (wavelength of maximum absorbance, λmax 484–518 nm) and cone (λmax 532–561 nm) visual pigments varies between species, each shark has only a single long-wavelength-sensitive cone type. This
suggests that sharks may be cone monochromats and, therefore, potentially colour blind. Whilst cone monochromacy on land is
rare, it may be a common strategy in the marine environment: many aquatic mammals (whales, dolphins and seals) also possess
only a single, green-sensitive cone type. It appears that both sharks and marine mammals may have arrived at the same visual
design by convergent evolution. The spectral tuning of the rod and cone pigments of sharks is also discussed in relation to
their visual ecology. 相似文献
254.
Quantifying climate change mitigation potential in the United States Great Plains wetlands for three greenhouse gas emission scenarios 总被引:1,自引:0,他引:1
255.
Hungate BA Johnson DW Dijkstra P Hymus G Stiling P Megonigal JP Pagel AL Moan JL Day F Li J Hinkle CR Drake BG 《Ecology》2006,87(1):26-40
Experimentally increasing atmospheric CO2 often stimulates plant growth and ecosystem carbon (C) uptake. Biogeochemical theory predicts that these initial responses will immobilize nitrogen (N) in plant biomass and soil organic matter, causing N availability to plants to decline, and reducing the long-term CO2-stimulation of C storage in N limited ecosystems. While many experiments have examined changes in N cycling in response to elevated CO2, empirical tests of this theoretical prediction are scarce. During seven years of postfire recovery in a scrub oak ecosystem, elevated CO2 initially increased plant N accumulation and plant uptake of tracer 15N, peaking after four years of CO2 enrichment. Between years four and seven, these responses to CO2 declined. Elevated CO2 also increased N and tracer 15N accumulation in the O horizon, and reduced 15N recovery in underlying mineral soil. These responses are consistent with progressive N limitation: the initial CO2 stimulation of plant growth immobilized N in plant biomass and in the O horizon, progressively reducing N availability to plants. Litterfall production (one measure of aboveground primary productivity) increased initially in response to elevated CO2, but the CO2 stimulation declined during years five through seven, concurrent with the accumulation of N in the O horizon and the apparent restriction of plant N availability. Yet, at the level of aboveground plant biomass (estimated by allometry), progressive N limitation was less apparent, initially because of increased N acquisition from soil and later because of reduced N concentration in biomass as N availability declined. Over this seven-year period, elevated CO2 caused a redistribution of N within the ecosystem, from mineral soils, to plants, to surface organic matter. In N limited ecosystems, such changes in N cycling are likely to reduce the response of plant production to elevated CO2. 相似文献
256.
Microenvironmental time-activity patterns in Chongqing, China 总被引:1,自引:0,他引:1
Yu Zhao Shuxiao Wang Gangcai Chen Fei Wang Kristin Aunan Jiming Hao 《Frontiers of Environmental Science & Engineering in China》2009,3(2):200-209
An investigation using recall questionnaires was conducted in winter and autumn 2006 to evaluate the time-activity patterns
in Chongqing, China. The average time spent in seven microenvironments (MEs) including outdoors, transit, living room, bedroom,
kitchen, classroom/office, and other indoors were found to be about 3.5, 1.1, 2.5, 9.7, 1.4, 4.2, and 1.7 h per day, respectively.
According to the results of a nonparametric test, the sampling period and day of week were significant for the variation of
the time spent in all MEs except for transit and outdoors. The time budget was analyzed using a general linear model (GLM),
which exhibited significant variability by demographic factors such as gender, age, residence, education, and household income. 相似文献