Environment Systems and Decisions - Globally we are experiencing a decline in aggregate natural capital. Many primary industries and enterprises are highly dependent on renewable and non-renewable... 相似文献
This paper introduces a new argument to the debate about the role of environmental taxes in modern tax systems. Some environmental taxes, particularly taxes on gasoline or electricity, are more difficult to evade than taxes on labor or income. When the tax base is shifted in a revenue-neutral manner toward these environmental taxes, the result is a net reduction in the amount of tax evasion. Using a carbon tax as a motivating example, the “tax evasion effect” is shown to sharply reduce the welfare cost of controlling emissions. A simple computable general equilibrium model suggests that the impact of considering tax evasion can be large: costs are lowered by 28% in the United States, by 89% in China, and by 97% in India. In countries with high levels of pre-existing tax evasion, a carbon tax will pay for itself through improvements in the efficiency of the tax system. 相似文献
The face is an important visual trait in social communication across many species. In evolutionary terms, there are large and obvious selective advantages in detecting healthy partners, both in terms of avoiding individuals with poor health to minimise contagion and in mating with individuals with high health to help ensure healthy offspring. Many models of sexual selection suggest that an individual's phenotype provides cues to their quality. Fluctuating asymmetry is a trait that is proposed to be an honest indicator of quality, and previous studies have demonstrated that rhesus monkeys gaze longer at symmetric faces, suggesting preferences for such faces. The current study examined the relationship between measured facial symmetry and measures of health in a captive population of female rhesus macaque monkeys. We measured asymmetry from landmarks marked on front-on facial photographs and computed measures of health based on veterinary health and condition ratings, number of minor and major wounds sustained and gain in weight over the first 4?years of life. Analysis revealed that facial asymmetry was negatively related to condition-related health measures, with symmetric individuals being healthier than more asymmetric individuals. Facial asymmetry appears to be an honest indicator of health in rhesus macaques, and asymmetry may then be used by conspecifics in mate-choice situations. More broadly, our data support the notion that faces are valuable sources of information in non-human primates and that sexual selection based on facial information is potentially important across the primate lineage. 相似文献
The earlier flowering times exhibited by many plant species are a conspicuous sign of climate change. Altered phenologies have caused concern that species could suffer population declines if they flower at times when effective pollinators are unavailable. For two perennial wildflowers, Tradescantia ohiensis and Asclepias incarnata, we used an experimental approach to explore how changing phenology affects the taxonomic composition of the pollinator assemblage and the effectiveness of individual pollinator taxa. After finding in the previous year that fruit set varied with flowering time, we manipulated flowering onset in greenhouses, placed plants in the field over the span of five weeks, and measured pollinator effectiveness as the number of seeds produced after a single visit to a flower. The average effectiveness of pollinators and the expected rates of pollination success were lower for plants of both species flowering earlier than for plants flowering at historical times, suggesting there could be reproductive costs to earlier flowering. Whereas for A. incarnata, differences in average seed set among weeks were due primarily to changes in the composition of the pollinator assemblage, the differences for T. ohiensis were driven by the combined effects of compositional changes and increases over time in the effectiveness of some pollinator taxa. Both species face the possibility of temporal mismatch between the availability of the most effective pollinators and the onset of flowering, and changes in the effectiveness of individual pollinator taxa through time may add an unexpected element to the reproductive consequences of such mismatches. 相似文献
The ecology of photosynthetic organisms is influenced by the need to adjust the photosynthetic apparatus to variable light
environments (photoacclimation). In this study, we quantified different components of the photoacclimation process for a reef-building
coral (Turbinaria mesenterina, Lamarck, 1816): including, variation in absorption cross-section, size of photosynthetic units, turnover time, chlorophyll
content, and colony respiration. We used these calibrations to characterize this species’ light niche, and to determine the
sensitivity of the niche boundaries to different processes of photoacclimation. Results showed that the breadth of the light
niche was most sensitive to the size of the photosynthetic unit, absorption cross-section, and rates of respiration. Habitats
with the highest light availability did not lead to maximal energy acquisition. This was because, although corals acclimated
to high light have high rates of photosynthesis per unit chlorophyll, their chlorophyll content was strongly reduced. This
suggests that potential energetic benefits that could be achieved through increased light harvesting (i.e., increased chlorophyll
content) in high-light habitats are outweighed by costs associated with photoprotection. Such costs appear to place an upper
bound on the habitat distributions of coral species. Our approach reveals how the photophysiological processes involved in
photoacclimation interact to determine the light niche. 相似文献
Over the past decade an increasing number of studies have experimentally manipulated the number of species in a community and examined how this alters the aggregate production of species biomass. Many of these studies have shown that the effects of richness on biomass change through time, but we have limited understanding of the mechanisms that produce these dynamic trends. Here we report the results of an experiment in which we manipulated the richness of freshwater algae in laboratory microcosms. We used two experimental designs (additive and substitutive) that make different assumptions about how patches are initially colonized, and then tracked the development of community biomass from the point of initial colonization through a period of 6-12 generations of the focal species. We found that the effect of initial species richness on biomass production qualitatively shifted twice over the course of the experiment. The first shift occurred as species transitioned from density-independent to dependent phases of population growth. At this time, intraspecific competition caused monocultures to approach their respective carrying capacities more slowly than polycultures. As a consequence, species tended to over-yield for a brief time, generating a positive, but transient effect of diversity on community biomass. The second shift occurred as communities approached carrying capacity. At this time, strong interspecific interactions caused biomass to be dominated by the competitively superior species in polycultures. As this species had the lowest carrying capacity, a negative effect of diversity on biomass resulted in late succession. Although these two shifts produced dynamics that appeared complex, we show that the patterns can be fit to a simple Lotka-Volterra model of competition. Our results suggest that the effects of algal diversity on primary production change in a predictable sequence through successional time. 相似文献
Despite the ubiquity and importance of microbes in nature, little is known about their natural population dynamics, especially for those that occupy terrestrial habitats. Here we investigate the dynamics of the yeast-like fungus Aureobasidium pullulans (Ap) on apple leaves in an orchard. We asked three questions. (1) Is variation in fungal population density among leaves caused by variation in leaf carrying capacities and strong density-dependent population growth that maintains densities near carrying capacity? (2) Do resident populations have competitive advantages over immigrant cells? (3) Do Ap dynamics differ at different times during the growing season? To address these questions, we performed two experiments at different times in the growing season. Both experiments used a 2 x 2 factorial design: treatment 1 removed fungal cells from leaves to reveal density-dependent population growth, and treatment 2 inoculated leaves with an Ap strain engineered to express green fluorescent protein (GFP), which made it possible to track the fate of immigrant cells. The experiments showed that natural populations of Ap vary greatly in density due to sustained differences in carrying capacities among leaves. The maintenance of populations close to carrying capacities indicates strong density-dependent processes. Furthermore, resident populations are strongly competitive against immigrants, while immigrants have little impact on residents. Finally, statistical models showed high population growth rates of resident cells in one experiment but not in the other, suggesting that Ap experiences relatively "good" and "bad" periods for population growth. This picture of Ap dynamics conforms to commonly held, but rarely demonstrated, expectations of microbe dynamics in nature. It also highlights the importance of local processes, as opposed to immigration, in determining the abundance and dynamics of microbes on surfaces in terrestrial systems. 相似文献
Thirteen sediment samples from different locations in the Niger Delta region of Nigeria were analyzed for the presence of 16 polynuclear aromatic hydrocarbons (PAHs) via gas chromatography/mass spectrometry. The specific target compounds for this study included naphthalene, acenaphthylene, acenaphthene, flourene, phenanthrene, anthracene, flouranthene, pyrene, benz[a]anthracene, chrysene, benzo[b]fluoranthene, benzo[k]flouranthene, benzo[a]pyrene, benzo[ghi]perylene, dibenz[a,h]anthracene, and indeno[1,2,3-cd]pyrene. Four isotopically labeled polynuclear aromatic hydrocarbons (acanaphthene-d10, phenanthrene-d10, chrysene-d12 and perylene-d12) were used for internal standardization. All 16 PAHs were found in most of the thirteen samples with concentration ranging from 0.1 microg/kg to 28 microg/kg. It was also found that the 5 and 6-ring PAHs were present in higher concentrations than all the other compounds, indicating their high resistance to microbial degradation. 相似文献