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排序方式: 共有304条查询结果,搜索用时 15 毫秒
281.
Derek B. Booth David Hartley Rhett Jackson 《Journal of the American Water Resources Association》2002,38(3):835-845
ABSTRACT: For 20 years, King County, Washington, has implemented progressively more demanding structural and nonstructural strategies in an attempt to protect aquatic resources and declining salmon populations from the cumulative effects of urbanization. This history holds lessons for planners, engineers, and resource managers throughout other urbanizing regions. Detention ponds, even with increasingly restrictive designs, have still proven inadequate to prevent channel erosion. Costly structural retrofits of urbanized watersheds can mitigate certain problems, such as flooding or erosion, but cannot restore the predevelopment flow regime or habitat conditions. Widespread conversion of forest to pasture or grass in rural areas, generally unregulated by most jurisdictions, degrades aquatic systems even when watershed imperviousness remains low. Preservation of aquatic resources in developing areas will require integrated mitigation, which must including impervious‐surface limits, forest‐retention policies, stormwater detention, riparian‐buffer maintenance, and protection of wetlands and unstable slopes. New management goals are needed for those watersheds whose existing development precludes significant ecosystem recovery; the same goals cannot be achieved in both developed and undeveloped watersheds. 相似文献
282.
283.
Egg production in the arrow squid Nototodarus gouldi (Cephalopoda: Ommastrephidae), fast and furious or slow and steady? 总被引:2,自引:0,他引:2
The process of reproductive maturation and egg release was examined in the temperate shelf squid Nototodarus gouldi. The energy allocation between somatic and reproductive growth from juvenile to mature adult was investigated throughout the life span to determine the underlying energetic strategy adopted by individuals. The relative weight of the mantle, fin and digestive gland remained unchanged during ovarian development, with no significant correlations found between the mantle length (ML)-gonad residuals and the ML-mantle (r=0.01, P>0.05), ML-fin (r=0.07, P>0.05) and ML-digestive gland (r=0.07, P>0.05) residuals. This suggested that energy was not being diverted away from somatic growth during sexual development, and consequently neither muscle nor digestive gland was being utilised as an energy store. Since squid in all maturity stages contained some food in their stomachs (e.g. 66.7% of mature animals), it is likely that the cost of maturation in this species is largely being met by food intake. The energy investment in reproductive tissues was relatively low (mean gonado-somatic index for mature individuals was 9.29% - 0.40%), indicating that only small amounts of energy were being allocated to reproduction at anyone point in time, which is characteristic of a multiple-spawning strategy. Furthermore, oviduct weight was not correlated with body size (r=0.256, P>0.05), suggesting that eggs are not stored for a single release. In all except one individual, ovary weight was consistently heavier than oviduct weight, suggesting that the ovary is not being depleted of oocytes as mature ova move into the oviducts. Additionally, the ovaries of mature females contained a range of oocyte sizes with discrete peaks, indicating a continued production and development of oocyte cohorts. The presence of some individuals with stretched empty oviducts is further evidence that the reproductive strategy of N. gouldi is slow and steady, with eggs possibly being released in discrete batches over a period of time. 相似文献
284.
285.
Using a subsocial spitting spider (Scytodes pallida) as the prey and a spider-eating jumping spider (Portia labiata) as the predator, the cost of parental care is investigated. Our findings suggest that being singled out as preferred prey by P. labiata is, for egg-carrying females of S. pallida, an important cost of parental care. In survival tests, during which P. labiata was given access to egg-carrying and eggless S. pallida females, egg-carrying females were preyed on more often than eggless females. In preference tests, motionless lures instead of living S. pallida were used. The lures were made by mounting dead egg-carrying and dead eggless S. pallida females in lifelike posture in webs. In these tests, P. labiata detected and identified, by vision alone, both kinds of prey (egg-carrying and eggless), and singled out egg-carrying females as preferred prey.Communicated by M.A. Elgar 相似文献
286.
R. J. Wapner MD J. L. Simpson M. S. Golbus J. M. Zachary D. H. Ledbetter R. J. Desnick S. E. Fowler L. G. Jackson H. Lubs R. J. Mahony E. Pergament G. G. Rhoads J. D. Shulman F. De La Cruz 《黑龙江环境通报》1992,12(5):347-355
Cytogenetic data from the United States NICHD collaborative study of chorionic villus sampling (CVS) were used to evaluate the clinical significance of chorionic mosaicism. The 10 754 patients with normal cytogenetic results were compared wtih 108 patients (1.0 per cent) with placental mosaicism and 181 patients (1.6 per cent) with pseudomosaicism. Of the pregnancies intended to continue, the pregnancy loss rate was significantly greater in patients with placental mosaicism than in the cytogenetically normal cohort (8.6 vs. 3.4 per cent, p <0.05). However, there was no difference in the frequencies of abruptio placenta, preterm labour or delivery, small-for-gestational-age newborns, pregnancy-induced hypertension, or neonates with Apgar scores less than 7. 相似文献
287.
Anthony Johnson F. Susan Cowchock Marilyn Darby Ronald Wapner Laird G. Jackson 《黑龙江环境通报》1991,11(7):443-450
First-trimester maternal serum alpha-fetoprotein (AFP) and human chorionic gonadotropin (HCG) levels were measured in samples from 29 women with cytogenetically abnormal pregnancies and 145 women with cytogenetically normal pregnancies matched for gestational age, race, and sample storage time. All patients had a risk of fetal aneuploidy greater than or equal to that of a mother 35 years of age. AFP was significantly lower in samples from pregnancies affected with trisomy 21 (0.67 MoM;p <0.05), while HCG values were no different from those of matched controls. Trisomies 13 and 18 could not be distinguished from matched controls by AFP. However, levels of HCG were significantly lower in such pregnancy samples, with median values of 0.65 MoM in trisomy 13 and 0.32 MoM in trisomy 18 (p<0.05). Variations in AFP and HCG levels suggest that expressed differences between autosomal aneuploidies include differences in fetal and placenta! protein production in the first trimester. 相似文献
288.
Brian Miller Richard Reading Courtney Conway Jerome A. Jackson Michael Hutchins Noel Snyder Steve Forrest Jack Frazier Scott Derrickson 《Environmental management》1994,18(5):637-645
This paper discusses common organizational problems that cause inadequate planning and implementation processes of endangered
species recovery across biologically dissimilar species. If these problems occur, even proven biological conservation techniques
are jeopardized. We propose a solution that requires accountability in all phases of the restoration process and is based
on cooperative input among government agencies, nongovernmental conservation organizations, and the academic community. The
first step is formation of a task-oriented recovery team that integrates the best expertise into the planning process. This
interdisciplinary team should be composed of people whose skills directly address issues critical for recovery. Once goals
and procedures are established, the responsible agency (for example, in the United States, the US Fish and Wildlife Service)
could divest some or all of its obligation for implementing the plan, yet still maintain oversight by holding implementing
entities contractually accountable. Regular, periodic outside review and public documentation of the recovery team, lead agency,
and the accomplishments of implementing bodies would permit evaluation necessary to improve performance. Increased cooperation
among agency and nongovernmental organizations provided by this model promises a more efficient use of limited resources toward
the conservation of biodiversity. 相似文献
289.
Amy M. Breman Jennifer C. Chow Lance U'Ren Elizabeth A. Normand Sadeem Qdaisat Li Zhao David M. Henke Rui Chen Chad A. Shaw Laird Jackson Yaping Yang Liesbeth Vossaert Rachel H. V. Needham Elizabeth J. Chang Daniel Campton Jeffrey L. Werbin Ron C. Seubert Ignatia B. Van den Veyver Jackie L. Stilwell Eric P. Kaldjian Arthur L. Beaudet 《黑龙江环境通报》2016,36(11):1009-1019
290.
A. E. Sheindlin W. D. Jackson W. S. Brzozowski and L. H. Th. Rietjens 《Natural resources forum》1979,3(2):133-145
The paper describes research and development in the field of magnetohydrodynamic power generation technology, based on discussions held in the Joint IAEA/UNESCO International Liaison Group on MHD electrical power generation. Research and development programmes on open cycle, closed cycle plasma and liquid-metal MHD are described. Open cycle MHD has now entered the engineering development stage. The paper reviews the results of cycle analyses and economic and environmental evaluations: substantial agreement has been reached on the expected overall performance and necessary component specifications. The achievement in the Soviet Union on the U-25 MHD pilot plant in obtaining full rated electrical power of 20.4 MW is described, as well as long duration testing of the integrated operation of MHD components. Work in the United States on coal-fired MHD generators has shown that, with slagging of the walls, a run time of about one hundred hours at the current density and electric field of a commercial MHD generator has been achieved. Progress obtained in closed cycle plasma and liquid metal MHD is reviewed. Electrical power densities of up to 140 MWe/m3 and an enthalpy extraction as high as 24 per cent have been achieved in noble gas MHD generator experiments. 相似文献