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排序方式: 共有113条查询结果,搜索用时 109 毫秒
1.
Changes in the crustacean zooplankton community composition and abundance in Lake Winnipeg (1969–2006) provide a rare opportunity to examine their response to environmental changes in the largest naturally eutrophic lake on the Canadian prairies. Since 1929, zooplankton species composition in Lake Winnipeg has changed little except for the addition of the invasive cladoceran, Eubosmina coregoni in 1994. The dominant taxa in the lake in summer include: Leptodiaptomus ashlandi, Acanthocyclops vernalis, Diacyclops thomasi, Daphnia retrocurva, Daphnia mendotae, Diaphanosoma birgei, Eubosmina coregoni, and Bosmina longirostris. Climate-accelerated nutrient loading to southern Lake Winnipeg over the last two decades has led to increased phytoplankton abundance and higher frequency of cyanobacterial blooms especially in its northern basin. Crustacean zooplankton have likewise increased especially in the North Basin, but less so in the more nutrient rich South Basin, possibly as a consequence of higher densities of pelagic planktivorous fish and light-limited primary production compared with the more transparent North basin (Brunskill et al., 1979, 1980). Calanoid copepods play a larger role in the South basin food web in contrast to cyclopoid copepods and Cladocera in the North basin. The study begins to fill the recognized gap in understanding of Lake Winnipeg's food web structure and provides a baseline for evaluating ongoing changes in the zooplankton community with the arrival of new non-indigenous taxa, e.g. Bythotrephes longimanus and Dreissena polymorpha. It reinforces previous work demonstrating that zooplankton provide valuable indices toward evaluating the health of an ecosystem. 相似文献
2.
Using data on AT&T intrastate toll prices, this study examines the effect of changing regulatory practices and increased competition on AT&T pricing behavior in the intrastate long-distance telecommunications industry. The results suggest that, with some qualification, the price of AT&T telephone service is lower in states which have the longest history of departure from rate-of-return regulation. Moreover, the regulatory climate has a far more significant effect on AT&T prices than does competition. The paper outlines several possible explanations for these results. 相似文献
3.
Brenda Moraska Lafrancois Stephen C. Riley David S. Blehert Anne E. Ballmann 《Journal of Great Lakes research》2011,37(1):86-91
Relationships between large-scale environmental factors and the incidence of type E avian botulism outbreaks in Lake Michigan were examined from 1963 to 2008. Avian botulism outbreaks most frequently occurred in years with low mean annual water levels, and lake levels were significantly lower in outbreak years than in non-outbreak years. Mean surface water temperatures in northern Lake Michigan during the period when type E outbreaks tend to occur (July through September) were significantly higher in outbreak years than in non-outbreak years. Trends in fish populations did not strongly correlate with botulism outbreaks, although botulism outbreaks in the 1960s coincided with high alewife abundance, and recent botulism outbreaks coincided with rapidly increasing round goby abundance. Botulism outbreaks occurred cyclically, and the frequency of outbreaks did not increase over the period of record. Climate change scenarios for the Great Lakes predict lower water levels and warmer water temperatures. As a consequence, the frequency and magnitude of type E botulism outbreaks in the Great Lakes may increase. 相似文献
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Estimates of the economic value of unpaid household and farm work of Alberta farm women and men are calculated using two common market alternative cost methods. Time use data for 1984 collected by Doherty and Keating (1985) from 414 Alberta grain farm families as well as Statistics Canada wage data are used. The average annual value of unpaid farm work ranges from $20,647.35 to $29,088.24 for men and from $5,017.54 to $6,604.19 for women. Conversely, unpaid household work performed by women has an average annual value of $17,460.98 to $34,617.66, while the value of men's unpaid household work ranges from $1,943.02 to $3,962.54 annually. When the values of farm and household production and the income generated by off-farm work are accounted for, there is little difference between mend's and womend's total contributions to the economic well-being of the farm family. These estimates illustrate the importance of family membersd' unpaid work to the economic well-being of the family. They also suggest that there is little basis for the historic inequity of legal and other social systems that neglect the economic value of womend's contributions to family welfare. On estime la valeur du travail domestique et agricole non rémunéré des agricultrices et des agriculteurs de l'Alberta au moyen de deux autres méthodes de calcul des coûts, dd'usage courant sur le marché. On se sert pour cela des données de 1984 sur le temps recueillies par Keating and Doherty (1985) auprès de 414 producteurs de céréales de l'Alberta et des données sur les salaires de Statistique Canada. La valeur annuelle moyenne du travail agricole non rémunéré varie de 20 647,35 $ à 29 088,24 $ pour les hommes et de 5 017,54 $ à 6 604,19 $ pour les femmes. Parallèlement, le travail domestique non rémunéré effectuÉ par les femmes vaut de 17 460,98 $ à 34 617,66 $ en moyenne par année, tandis que pour les hommes il varie entre 1 943,02 $ et 3 962,54 $. Lorsqud'on additionne la valeur de la production agricole et domestique et le revenu générÉ par le travail non agricole, on constate qud'il y a peu dd'écart entre la contribution totale de l'homme et celle de la femme au bien-être économique de la famille rurale. Ces estimations prouvent 1d'importance du travail non rémunéré des membres de la famille pour la situation économique de cette derniÈre. Elles suggèrent également qud'il nd'existe pas de véritable fondement à l'injustice historique introduite par les systèmes juridiques et sociaux qui négligent la valeur de la contribution des femmes au bien-être économique de la famille. 相似文献
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Steve Baron Kim Harris David Leaver Brenda M. Oldfield 《International Review of Retail, Distribution & Consumer Research》2013,23(4):395-414
The UK's independent food and grocery retail sector's competitive advantage of 'convenience' has been eroded in recent years as major retailers increase opening hours and develop their own convenience trading formats. As a result, large numbers of independent retailers are closing. A UK-wide survey was undertaken to identify other sources of competitive advantages, and the changes needed to stop or reverse the decline in this sector. Exploratory semi-structured interviews led to the development of a questionnaire on six major issues affecting the sector. 'Snowball' sampling, using industry partners, provided 142 respondents. The provision of a 'social shopping experience' emerged as a key competitive advantage that could be developed. Improving vertical and horizontal trading alliances, and the wider use of information technology, provided other options. 相似文献
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The purpose of the study was to empirically investigate the factors that influence US consumers' attitudes toward and use of electronic banking (e-banking) and to explore the reasons that a magnitude discrepancy exists between the impact of the specific factors on consumers' attitudes and the impact on their actual behaviours. Using the data from the 2003 Survey of Consumers commissioned by the Survey Research Center at the University of Michigan, the current study found that consumers' perceptions of the characteristics of e-banking, namely the perceived advantages and problems, had different impacts on consumers' attitudes toward and their use of such service. While the perceived problems were more important in forming consumers' attitudes toward e-banking, the perceived advantages had the greatest impact on consumers' use of such service. The conclusions and discussions section also includes theoretical and practical implications drawn from the results. 相似文献
10.
Carolyn Cordery Rachel Baskerville Brenda Porter 《Financial Accountability and Management》2011,27(4):363-384
Public sector reformers advocate contracting‐out as a means of improving cost‐effectiveness. In the health sector, market‐based contracts with for‐profit organisations can reduce equity of access and divert public funds to private gain. Such issues have prompted policy makers to seek alternative contracting strategies. This paper examines a primary health care policy whereby government contracts with private non‐profit organisations to increase efficiency and meet World Health Organisation ideals. The study found that the policy's implementation has not achieved these aims when for‐profit providers masquerade as non‐profit organisations. The implication is that governments may find it more effective to manage for structural diversity than mandate homogenisation. 相似文献