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521.
Newcombe AC Gustafson DI Fuhrman JD van Wesenbeeck IJ Simmons ND Klein AJ Travis KZ Harradine KJ 《Journal of environmental quality》2005,34(3):1004-1015
The Acetochlor Registration Partnership conducted a prospective ground water (PGW) monitoring program to investigate acetochlor [2-chloro-N-(ethoxymethyl)-N-(2-ethyl-6-methylphenyl)-acetamide] transport to ground water at eight sites. The distribution of soil textures among these sites was weighted toward coarser soil types, while also including finer-textured soils that dominate most corn (Zea mays L.)-growing areas of the United States. Each site consisted of a 1.2-ha test plot adjacent to a 0.2-ha control plot. Suction lysimeters and monitoring wells were installed at multiple depths within each test and control plot to sample soil-pore water and near-surface ground water. Irrigation was applied to each site during the growing season to ensure water input of 110 to 200% of average historical rainfall. Acetochlor dissipated rapidly from surface soils at all sites with a DT(50) (time for 50% of the initial residues to dissipate) of only 3 to 9 d, but leaching was not an important loss mechanism, with only 0.25% of the 15,312 soil-pore water and ground water samples analyzed containing parent acetochlor at or above 0.05 microg L(-1). However, quantifiable residues of a soil degradation product, acetochlor ethanesulfonic acid, were more common, with approximately 16% of water samples containing concentrations at or above 1.0 microg L(-1). A second soil degradation product, acetochlor oxanilic acid, was present at concentrations at or above 1.0 microg L(-1) in only 0.15% of water samples analyzed. The acetochlor PGW program demonstrated that acetochlor lacks the potential to leach to ground water at detectable concentrations, and when applied in accordance with label restrictions, is unlikely to move to ground water at concentrations hazardous to human health. 相似文献
522.
Vanlaar W 《Traffic injury prevention》2005,6(4):311-316
OBJECTIVES: This commentary illustrates the advantages of multilevel modeling compared to statistical techniques that ignore hierarchies, based on two empirical traffic safety examples. METHODS: The common concept shared by different definitions of multilevel modeling is identified and illustrated. Each definition defines multilevel modeling in its own way but they all refer to hierarchies. Conceptual issues inherently related to hierarchies are then pointed out and illustrated. RESULTS: Broadly speaking there are two important consequences of ignoring a hierarchical structure in the data. The first consequence, underestimation of standard errors, is illustrated with data from an observational study on seatbelt behavior. Two effects that were significant at the 5% level in a single-level model were no longer found to be significant in a two-level model. The single-level model is therefore bound to lead to erroneous conclusions regarding variables that could have an impact on seatbelt use and, ultimately, on increasing the level of traffic safety. The second consequence, related to contextual information, is illustrated with data from a roadside survey on drink driving. Of particular interest is the relationship between Traffic Count, an aggregated level 2 explanatory variable and Odds of drink driving, an individual level 1 dependent variable. CONCLUSIONS: Like every statistical technique, multilevel models should be used with caution and reservation. However, given certain limitations, multilevel modeling is very useful and valuable to traffic safety research. 相似文献
523.
524.
Andrew R. Blaustein 《Behavioral ecology and sociobiology》1980,6(3):247-255
Summary The influence of vole cycles on the demography, spatial organization, and abundance of potential rodent competitors was studied in three California rodents in the laboratory and in the field. Population densities ofMicrotus californicus were inversely correlated with reproductive success in two potential competitors (Reithrodontomys megalotis andMus musculus). Additionally, high-density vole populations forced these species into suboptimal habitats. During low-density vole populations,Reithrodontomys andMus left their refugia, expanded their usage of habitat, were more active, and their reproductive success was greater. Thus, this work suggests that during vole population peaks, competition with sympatric species of mice is severe. Conversely, when vole populations decline, competition is relaxed. The temporal and spatial organization of the three-species complex supports models of nonequilibrium and fugitive species coexistence. 相似文献
525.
Development of a phosphorus index for pastures fertilized with poultry litter--factors affecting phosphorus runoff 总被引:1,自引:0,他引:1
DeLaune PB Moore PA Carman DK Sharpley AN Haggard BE Daniel TC 《Journal of environmental quality》2004,33(6):2183-2191
Currently, several state and federal agencies are proposing upper limits on soil test phosphorus (P), above which animal manures cannot be applied, based on the assumption that high P concentrations in runoff are due to high soil test P. Recent studies show that other factors are more indicative of P concentrations in runoff from areas where manure is being applied. The original P index was developed as an alternative P management tool incorporating factors affecting both the source and transport of P. The objective of this research was to evaluate the effects of multiple variables on P concentrations in runoff water and to construct a P source component of a P index for pastures that incorporates these effects. The evaluated variables were: (i) soil test P, (ii) soluble P in poultry litter, (iii) P in poultry diets, (iv) fertilizer type, and (v) poultry litter application rate. Field studies with simulated rainfall showed that P runoff was affected by the amount of soluble P applied in the fertilizer source. Before manure applications, soil test P was directly related to soluble P concentrations in runoff water. However, soil test P had little effect on P runoff after animal manure was applied. Unlike most other P indices, weighting factors of the P source components in the P index for pastures are based on results from runoff studies conducted under various management scenarios. As a result, weighting factors for the P source potential variables are well justified. A modification of the P index using scientific data should strengthen the ability of the P index concept to evaluate locations and management alternatives for P losses. 相似文献
526.
Concern over nonpoint-source phosphorus (P) losses from agricultural lands to surface waters has resulted in scrutiny of factors affecting P loss potential. A rainfall simulation study was conducted to quantify the effects of alternative P sources (dairy manure, poultry manure, swine slurry, and diammonium phosphate), application methods, and initial soil P concentrations on runoff P losses from three acidic soils (Buchanan-Hartleton, Hagerstown, and Lewbeach). Low P (12 to 26 mg kg(-1) Mehlich-3 P) and high P (396 to 415 mg kg(-1) Mehlich-3 P) members of each soil were amended with 100 kg total P ha(-1) from each of the four P sources either by surface application or mixing, and subjected to simulated rainfall (70 mm h(-1) to produce 30 min runoff). Phosphorus losses from fertilizer and manure applied to the soil surface differed significantly by source, with dissolved reactive phosphorus (DRP) accounting for 64% of total phosphorus (TP) (versus 9% for the unamended soils). For manure amended soils, these losses were linearly related to water-soluble P concentration of manure (r2 = 0.86 for DRP, r2 = 0.78 for TP). Mixing the P sources into the soil significantly decreased P losses relative to surface P application, such that DRP losses from amended, mixed soils were not significantly different from the unamended soil. Results of this study can be applied to site assessment indices to quantify the potential for P loss from recently manured soils. 相似文献
527.
Sourcing sediment using multiple tracers in the catchment of Lake Argyle,Northwestern Australia 总被引:5,自引:0,他引:5
Control of sedimentation in large reservoirs requires soil conservation at the catchment scale. In large, heterogeneous catchments,
soil conservation planning needs to be based on sound information, and set within the framework of a sediment budget to ensure
that all of the potentially significant sources and sinks are considered.
The major sources of sediment reaching the reservoir, Lake Argyle, in tropical northwestern Australia, have been determined
by combining measured sediment fluxes in rivers with spatial tracer-based estimates of proportional contributions from tributaries
of the main stream entering the lake, the Ord River. The spatial tracers used are mineral particle magnetics, the strontium
isotopic ratio, and the neodymium isotopic ratio. Fallout of 137Cs has been used to estimate the proportion of the sediment in Lake Argyle eroded from surface soils by sheet and rill erosion,
and, by difference, the proportion eroded from subsurface soils by gully and channel erosion. About 96% of the sediment in
the reservoir has come from less than 10% of the catchment, in the area of highly erodible soils formed on Cambrian-age sedimentary
rocks. About 80% of the sediment in the reservoir has come from gully and channel erosion. A major catchment revegetation
program, designed to slow sedimentation in the reservoir, appears to have had little effect because it did not target gullies,
the major source of sediment. Had knowledge of the sediment budget been available before the revegetation program was designed,
an entirely different approach would have been taken. 相似文献
528.
Andrew Simon Massimo Rinaldi 《Journal of the American Water Resources Association》2000,36(1):133-150
ABSTRACT: The bess area of the midwestern United States contains thousands of miles of unstable stream channels that are undergoing system‐wide channel‐adjustment processes as a result of (1) modifications to drainage basins dating back to the turn of the 20th century, including land clearing and poor soil‐conservation practices, which caused the filling of stream channels, and consequently (2) direct, human modifications to stream channels such as dredging and straightening to improve drainage conditions and reduce the frequency of out‐of‐bank flows. Today, many of these channels are still highly unstable and threaten bridges, other structures, and land adjacent to the channels. The most severe, widespread instabilities are in western Iowa where a thick cap of bess and the lack of sand‐and gravel‐sized bed sediments in many channels hinders downstream aggradation, bed‐level recovery and the consequent reduction of bank heights, and renewed bank stability. In contrast, streams draining west‐central Illinois, east‐central Iowa, and other areas, where the bess cap is relatively thin and there are ample supplies of sand‐and gravel‐sized material, are closer to recovery. Throughout the region, however, channel widening by mass‐wasting processes is the dominant adjustment process. 相似文献
529.
We report a long-term study of offspring sex ratios in the cooperatively breeding superb fairy-wren Malurus cyaneus. Detailed study of this species had revealed a suite of potentially strong selection pressures on the sex ratio. First, females
gain substantial fitness benefits from the presence of helpers; so females without male helpers would benefit from any strategy
that increased the probability of recruiting help, such as overproduction of sons (local resource enhancement hypothesis),
but large numbers of helper males compete among themselves, favouring the production of daughters (local resource competition).
Second, daughters fledged early in the season have far greater chances of recruitment to the breeding population than late-fledged
daughters, so mothers would benefit from production of daughters early in the breeding season (early bird hypothesis). Third,
extra-group mate choice imposes strong sexual selection on males, suggesting that females mating with attractive sires could
benefit from investing in sons (sexual selection hypothesis). However, the predictions from these and other sex ratio hypotheses
were rejected. The only convincing evidence for manipulation of the sex ratio was a slight bias towards sons (11 sons to 10
daughters) that occurred regardless of context. This result does not support current theory. 相似文献
530.
Given the growing popularity of indicators among policy-makers to measure progress toward conservation and sustainability goals, there is an urgent need to develop indicators that can be used accurately by both specialists and nonspecialists, drawing from the knowledge possessed by each group. This paper uses a case study from the Kalahari, Botswana to show how participatory and ecological methods can be combined to develop robust indicators that are accessible to a range of users to monitor and enhance the sustainability of land management. First, potential environmental sustainability indicators were elicited from pastoralists in three study sites. This knowledge was then evaluated by pastoralists, before being tested empirically using ecological and soil-based techniques. Despite the wealth of local knowledge about indicators, this knowledge was thinly spread. The knowledge was more holistic than published indicator lists for monitoring rangelands, encompassing vegetation, soil, livestock, wild animal, and socioeconomic indicators. Pastoralist preferences for vegetation and livestock indicators match recent shifts in ecological theory suggesting that livestock populations reach equilibrium with key forage resources in semiarid environments. Although most indicators suggested by pastoralists were validated through empirical work (e.g., decreased grass cover and soil organic matter content, and increased abundance of Acacia mellifera and thatching grass), they were not always sufficiently accurate or reliable for objective degradation assessment, showing that local knowledge cannot be accepted unquestioningly. We suggest that, by combining participatory and ecological approaches, it is possible to derive more accurate and relevant indicators than either approach could achieve alone. 相似文献