خيارات البحث
النتائج 1 - 10 من 2,005
Population genetics of an opportunistic parasitoid in an agricultural landscape
1998
Vaughn, T.T. | Antolin, M.F. (Department of Biology, Colorado State University, Fort Collins, CO 80523 (USA))
Population genetics of an opportunistic parasitoid in an agricultural landscape
1998
Vaughn, T.T. | Antolin, M.F.
Parasitoid insects that use different hosts can have a subdivided population structure that corresponds to host use. A subdivided population structure may favour local adaptation of subpopulations to small-scale environmental differences and may promote their genetic divergence. In this paper, heritable Random Amplified Polymorphic DNA (RAPD) markers visualized by single strand conformational polymorphisms (SSCP) analysis were used to examine the population structure of the parasitoid wasp Diaeretiella rapae (Hymenoptera: Braconidae) in an environment where two aphid hosts are available for oviposition. We found 11 codominant and 34 dominant RAPD polymorphisms that conformed to Mendelian segregation patterns. A nested analysis of variance indicated extensive genetic differentiation among six populations of D. rapae that were sampled for two years. Effective migration rates (Nm) between populations ranged from 1.2-1.6 per year, indicating a relatively low dispersal rate. Genetic distances were also calculated between populations and the resulting trees indicated that populations less than 1.0 km from each other were genetically differentiated. Our results indicate that D. rapae populations are genetically subdivided on a small spatial scale that corresponds to host-use patterns.
اظهر المزيد [+] اقل [-]Water quality of an intensive agricultural watershed in Quebec النص الكامل
1998
Lapp, P. | Madramootoo, C.A. | Enright, P. | Papineau, F. | Perrone, J.
Under a watershed based approach being examined by the Quebec Ministry of Agriculture to accelerate the adoption of conservation practices, a study on the impacts of agricultural practices on the St. Esprit watershed was initiated in the fall of 1993. The water quality of this 26 km2 intensive agricultural watershed was studied over an 18 month period. Water samples taken at the outlet of the watershed were analyzed for nitrate, phosphate, suspended sediment, and atrazine. Water quality data were analyzed to establish seasonal trends in pollutant concentration and load in the watercourse. Spring snowmelt was identified as a significant period of pollutant material export. All pollutant materials displayed seasonal variability in the export process. Peak pollutant concentrations were associated with high flow events. Mean observed pollutant concentrations did not exceed drinking water quality standards.
اظهر المزيد [+] اقل [-]Economic evaluation of riparian buffers in an agricultural watershed النص الكامل
1998
Qui, Z. | Prato, T.
This study determines the most cost effective spatial pattern of farming systems for improving water quality and evaluates the economic value of riparian buffers in reducing agricultural nonpoint source pollution in a Midwestern agricultural watershed. Economic and water quality impacts of alternative farming systems are evaluated using the CARE and SWAT models, respectively. The water quality benefits of riparian buffers are estimated by combining experimental data and simulated water quality impacts of farming systems obtained using SWAT. The net economic value of riparian buffers in improving water quality is estimated by total watershed net return with riparian buffers minus total watershed net return without riparian buffers minus the opportunity cost of riparian buffers. Exclusive of maintenance cost, the net economic value of riparian buffers in reducing atrazine concentration from 45 to 24 ppb is $612,117 and the savings in government cost is $631,710. Results strongly support efforts that encourage farmers to develop or maintain riparian buffers adjacent to streams.
اظهر المزيد [+] اقل [-]Scaling of plant species richness in an agricultural mosaic landscape
1998
Wagner, H.H.
Policy instruments to control multi-nutrients in an agricultural watershed
1998
Lintner, A. | Weersink, A.J. (Department of Economics, Wilfrid Laurier University, Waterloo, Ontario (Canada))
An analysis of agricultural price and research policies in Chile
1998
Ortiz, J. (Organizacion de Preinversion de America Latina y El Caribe, OPALC, Casilla 17-030-4646, Quito (Ecuador))
An analysis of agricultural price and research policies in Chile
1998
Ortiz, J.
The interactions between agricultural price and research policies are formally addressed in this article in order to avoid biased estimates of the benefits to Chile from indigenous and international agricultural research and development. Government pricing policies influenced the innovative behavior of the main Chilean public research institution and reduced the benefits from cost-reducing research benefits relative to those obtained under free trade conditions. Despite the existence of market distortions caused by commodity price policies, Chilean welfare gains from public agricultural research were positive. However, these net economic surplus changes would have been higher if output price interventions would have been aimed at reducing distortions and putting producer prices more in line with international prices.
اظهر المزيد [+] اقل [-]An economic study of the agricultural foreign trade in Egypt
1998
Abd-El-Ghani, M. (Ministry of Agriculture, Cairo (Egypt). Desert Research Centre)
Trends in the management of an agricultural and food complex
1998
Novak, K. (Mendelova Zemedelska a Lesnicka Univ., Brno (Czech Republic))
