Density-mediated, context-dependent consumer-resource interactions between ants and extrafloral nectar plants

Interspecific interactions are often mediated by the interplay between resource supply and consumer density. The supply of a resource and a consumer's density response to it may in turn yield context-dependent use of other resources. Such consumer-resource interactions occur not only for predator-prey and competitive interactions, but for mutualistic ones as well. For example, consumer-resource interactions between ants and extrafloral nectar (EFN) plants are often mutualistic, as EFN resources attract and reward ants which protect plants from herbivory. Yet, ants also commonly exploit floral resources, leading to antagonistic consumer-resource interactions by disrupting pollination and plant reproduction. EFN resources associated with mutualistic ant-plant interactions may also mediate antagonistic ant-flower interactions through the aggregative density response of ants on plants, which could either exacerbate ant-flower interactions or alternatively satiate and distract ants from floral resources. In this study, we examined how EFN resources mediate the density response of ants on senita cacti in the Sonoran Desert and their context-dependent use of floral resources. Removal of EFN resources reduced the aggregative density of ants on plants, both on hourly and daily time scales. Yet, the increased aggregative ant density on plants with EFN resources decreased rather than increased ant use of floral resources, including contacts with and time spent in flowers. Behavioral assays showed no confounding effect of floral deterrents on ant-flower interactions. Thus, ant use of floral resources depends on the supply of EFN resources, which mediates the potential for both mutualistic and antagonistic interactions by increasing the aggregative density of ants protecting plants, while concurrently distracting ants from floral resources. Nevertheless, only certain years and populations of study showed an increase in plant reproduction through herbivore protection or ant distraction from floral resources. Despite pronounced effects of EFN resources mediating the aggregative density of ants on plants and their context-dependent use of floral resources, consumer-resource interactions remained largely commensalistic.     

Groundwater depletion in India: Social losses from costly well deepening

We develop a dynamic groundwater model that incorporates both groundwater pumping and investment in deeper wells and apply the model to the arid, alluvial aquifer region of Northern India that is experiencing rapid depletion. We compute the potential benefits of regulating groundwater use by comparing the net benefits of groundwater under optimal management to the net benefits under a common pool regime with two different cost structures: one with flat electricity tariffs, which are widespread in India, and a second with full marginal cost electricity pricing. Using numerical simulation, we find that the opportunity to invest in deeper wells significantly exacerbates the common pool problem and suggests the potential for large benefits (66% of common pool benefits) from optimally managing groundwater use or new drilling. Flat tariffs exacerbate the problem, but large gains (almost 23%) remain even if farms are charged the full marginal cost of electricity.     

Ecological and evolutionary mechanisms for low seed: ovule ratios: need for a pluralistic approach?

Central to the ecology and evolution of a broad range of plants is understanding why they routinely have submaximal reproduction manifested as low seed : ovule and fruit : flower ratios. We know much less about the processes responsible for low seed : ovule ratios than we do for fruit : flower ratios. Current hypotheses for low seed : ovule ratios are largely drawn from those for fruit : flower ratios, including proximate (ecological) causes of pollen limitation, resource limitation, and pollen quality, as well as the ultimate (evolutionary) hypothesis of "bet hedging" on stochastic pollination. Yet, such mechanisms operating on fruit : flower ratios at the whole-plant level may not best explain low seed : ovule ratios at the individual-flower level. We tested each of these proximate and ultimate causes for low seed : ovule ratios using the specialized pollination mutualism between senita cacti (Pachycereus schottii) and senita moths (Upiga virescens). Seed : ovule ratios were consistently low (approximately 0.61). Such excess ovule production by senita likely has a strong genetic component given the significant differences among plants in ovule number and the consistency in ovule production by plants within and among flowering seasons. Excess ovule production and low seed : ovule ratios could not be explained by pollen limitation, resource limitation, pollen quality, or bet hedging. Nevertheless, phenotypic selection analyses did show significant selection gradients for increased ovule number, suggesting that other evolutionary processes may be responsible for excess ovule production and low seed : ovule ratios. In contrast, low fruit : flower ratios at the whole-plant level were explained by an apparent equilibrium between pollen and resource limitation. Thus, mechanisms responsible for low fruit : flower ratios at the whole-plant level are not necessarily in accord with those of low seed : ovule ratios at the individual-flower level. This suggests that we may need to adopt a more pluralistic approach to seed : ovule ratios and consider alternative hypotheses, including a greater array of proximate and ultimate causes. Initial results of this study suggest that floral allometry, selection on correlated floral traits, stigma clogging with pollen grains, and style clogging with pollen tubes may provide promising avenues for understanding low seed : ovule ratios.     

Risk-cost trade-offs in environmental risk management decision-making

Risk-cost trade-offs are deemed to provide a basis for selecting between those risk reductions which are economic and those which are non-economic. Insufficient work has been done in environmental risk management decision-making practice to validate this assumption, or to develop an analytical framework within which it may be employed. Empirical evidence from the literature is offered for the existence of risk-cost trade-offs and the relationship of risk-cost trade-offs to the benefits side of the management decision-making issue is discussed. The risk-cost trade-off curve alone does not provide a complete basis for selecting between economic and non-economic risk-cost combinations. When combined with information on the willingness-to-pay for risk reductions, the risk-cost trade-off curve may be used to distinguish between risk reduction possibilities in a risk management decision-making context. Results allow for a more developed theoretical decision- making framework that, when amended for uncertainty, produces decision criteria useful for environmental risk management decision-making.     

多金属硫化物矿区不同品种水稻根际土壤酶活的变化

选取酸性矿山废水污灌形成的多重金属污染水稻土,采用多格层根际箱模拟水稻根际环境,研究高镉积累水稻长香谷fOryzasativaL．cv．Choukoukoku）和低镉积累水稻金农丝苗（OryzasativaL．cv．Jinnongsimiao）2个品种水稻生育中后期（分蘖期、孕穗期、扬花期、蜡熟期）根际土壤脲酶、酸性磷酸酶和过氧化氢酶活性的变化特征。结果表明：高镉积累水稻比低镉积累水稻各时期根际土壤脲酶活性低,随着生育期的延长,2个品种水稻土壤脲酶活性先降低后升高（分蘖期酶活性最高）,2种水稻土壤脲酶活性均在根际s0层达到最高,但其他各层土壤脲酶活性与根系距离无显著关系。随着水稻生育期的延长,高镉积累水稻根际土壤酸性磷酸酶活性先升高（在孕穗期酶活性达最高）,随后降低;低镉积累水稻土壤酸性磷酸酶活性则在分蘖期最高,之后逐渐降低。除分蘖期高镉积累水稻土壤酸性磷酸酶活性低于低镉积累水稻酶活性外,其他2个时期前者酸性磷酸酶酶活性均高于后者的酶活。2个品种水稻土壤过氧化氢酶活性随生育期延长不断升高,在整个生育后期,高镉吸收水稻比低镉吸收水稻各时期土壤过氧化氢酶活性高,但在蜡熟期时两者无显著差异;在各时期2种水稻过氧化氢酶活性在根系S0层达到最大,随着与水稻根系距离的增加呈先降低后增加的趋势。可见,在多重金属污染矿区,土壤酶活性明显受水稻品种、水稻生长时期和水稻根系的影响。该结果可以为研究多重金属污染矿区分析土壤酶活性与不同重金属积累程度的水稻品种作物相关关系提供数据支持,给农业生产及土壤修复提供依据。     

Benefits of the development and dissemination of climate-smart rice: <Emphasis Type="Italic">ex ante</Emphasis> impact assessment of drought-tolerant rice in South Asia

In the era of climate change, rice (Oryza sativa L.) production is increasingly affected by the rising frequency and severity of various abiotic stresses such as flood and drought. In some cases, 100 % yield loss can be attributed solely to these stresses. Hence, there is an urgent need to develop climate-smart rice that is more resilient to abiotic stresses. As the development and dissemination of climate-smart rice varieties entail costs, this article examines the net economic benefits of developing and disseminating successful drought-tolerant rice in South Asia where rice area prone to drought far outweighs area with other abiotic stresses. Drought causes major rice yield losses not only in South Asia but also in other parts of Asia and Africa. Using the ORYZA2000 (ORYZA model series were developed at the International Rice Research Institute (IRRI) in the early 1990s in collaboration with Wageningen University and Research Centre (WUR) and 16 national agricultural research and extension systems in Asia. ORYZA2000 is an update and integration of earlier versions of the ORYZA model. More detailed documentation of ORYZA2000 can be found in (http://books.irri.org/9712201716_content.pdf)) crop growth simulation model, we demonstrate that a successful drought-tolerant rice variety can provide yield gains in South Asia of a minimum of 1.71 % to a maximum of 8.96 % when there is no change in the climate and under different climate scenarios projected by Canadian General Circulation Models (CGCMs). Moreover, our economic analysis shows that the economic benefits from the successful development and dissemination of droughttolerant rice more than outweigh the research investments needed to develop a variety. Although the research, development, and dissemination of a climate-smart drought-tolerant rice variety in the South require USD 84 million, our economic model indicates that rice production will be higher by a minimum of 6 million tons worth USD 3.3 billion, and rice prices will be lower by a minimum of USD 72 per ton when a drought-tolerant variety is adopted in South Asia (as compared to the case without this new variety). This can lead to improved rice food security and better nutritional outcomes for the poor.     

Analysis of culturable and unculturable microbial community in bensulfuron-methyl contaminated paddy soils

To investigate the influence of bensulfuron-methyl(BSM)on culturable microbial quantities and unculturable microbial community structures,conventional and molecular biological methods were employed in five BSM treated soils with three replications,respectively. The results obtained with traditional culture-dependent methods showed that a low-level of BSM had slight and transient effects on culturable microorganisms;nevertheless,high concentration of BSM resulted in a dramatic decrease in bacterial colony fo...