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991.
The idea that natural selection can be meaningfully applied at the group level may be more important than previously thought. This perspective, a modern version of group selection, is called multilevel selection. Multilevel selection theory could incorporate previous explanations for the evolution of cooperation including kin selection. There is general agreement that natural selection favors noncooperators over cooperators in the case of an unstructured population. Therefore, the evolution of cooperation by multilevel selection often requires positive assortment between cooperators and noncooperators. The question is how this positive assortment can arise in the ecological meaning. We constructed an individual-based model of multilevel selection and introduced migration and evolution. The results showed that positive assortment was generated especially when a migration strategy was adopted in which individuals respond specifically to bad environmental conditions. It was also shown that the founder effect in the evolutionary process could further facilitate positive assortment by working with migration. We analyzed assortment by using relatedness defined in group-structured populations. The fact that cooperation was achieved by such migration and by the founder effect highlights the importance of sensitiveness to the ecological environment and of fluctuations in group size, respectively. 相似文献
992.
温度对奶牛粪便温室气体排放的影响 总被引:3,自引:0,他引:3
采用封闭罐-气相色谱法对4种不同温度处理下奶牛粪便温室气体排放速率进行测定.结果表明,温度是影响奶牛粪便温室气体排放的主要因子,温度较低时奶牛粪便温室气体排放速率低,而且随着奶牛粪便处理温度升高而增大.在5、15、25和35℃4个处理中,奶牛粪便CO2排放速率两两之间差异显著;CH4排放速率除了5和15℃处理差异不显著之外,其他两两处理之间差异显著;35℃处理时N2O排放速率与其他温度的排放速率差异均显著,其他两两处理之间差异不显著. 相似文献
993.
994.
995.
The ecological toxicity of cadmium (Cd, 10 mg kg−1 of dry weight soil) and butachlor (10, 50 and100 mg kg−1 of dry weight soil) in both their single and combined effects on soil urease and phosphatase was studied after 1, 3, 7, 14,
21 and 28 days exposure under controlled conditions in paddy and phaeozem soils. The results showed that Cd reduced the activities
of urease and phosphatase at early incubation time (1–7 days), while the reduction almost disappeared at the end of the incubation.
The effect of Cd on phosphatase was more pronounced than that on urease. The activities of urease and phosphatase were reduced
by butachlor, while urease activity was significantly (P < 0.05 or P < 0.01) improved when the concentrations of butachlor were 10 and 50 mg kg−1 at the end of the incubation. When Cd (10 mg kg−1) was combined with butachlor (50 and 100 mg kg−1), the activities of urease and phosphatase became lower than without combination at early incubation time, which indicated
that the toxicity of Cd significantly increased (P < 0.05 or P < 0.01). However, when Cd (10 mg kg−1) was combined with butachlor (10 mg kg−1), the activities of urease and phosphatase became higher than those without combination at the end of the incubation, which
indicated that the toxicity of Cd decreased. It was indicated that the combined effects depended largely on the incubation
time and the concentration ratio of Cd and butachlor. In addition, it was showed that the combined effects of butachlor and
Cd appeared different in paddy from phaeozem, which may be related to the different properties of these soils. 相似文献
996.
沙漠化地区退耕还林政策的生态经济效应分析——以民勤县为例 总被引:15,自引:2,他引:13
退耕还林是沙漠化地区生态环境治理的主要内容。论文以沙漠化严重的民勤县为例,采用环境社会学的研究方法,对民勤县农户的退耕还林意愿和退耕还林政策的生态经济效应在农户中的响应进行了典型调查和抽样调查,并搜集了相关数据。统计分析表明:退耕还林涉及到农户土地利用方式的变化,农户的退耕意愿主要取决于退耕后经济利益的得失;农户虽然在近期承受退耕地收益减少的负担,但长期可以得到退耕还林的保肥效果和减少风沙灾害损失的生态经济效果。退耕还林具有典型的外部经济性,政府应该为退耕还林的成本全部买单,同时,在调整退耕地结构、优化退耕模式的过程中还要协调好退耕还林与农户增收的关系。 相似文献
997.
本文运用一步熔融共混法制备了含有十溴联苯醚(DBDPO)或十溴二苯乙烷(DBDPE)和C16改性蒙脱土(MMT)的高抗冲聚苯乙烯(HIPS)复合材料,并采用X射线衍射(XRD)、透射电镜(TEM)、UL 94垂直燃烧和锥型量热等试验手段对其燃烧性能和相态进行了表征.结果标明,DBDPO/E存在时仍可获得具有插层结构的HIPS纳米复合材料.由于十溴和蒙脱土两种体系间的良好协同效应,这些材料的阻燃性提高、燃烧后的热释放速率下降.文中对其潜在机理进行了探讨.这种协同效应可用于指导发展环保性和阻燃性兼顾的HIPS纳米复合材料. 相似文献
998.
999.
中国西北地区气候变暖对农业的影响 总被引:91,自引:3,他引:88
选取西北地区资料年代较长的171个地面测站1961~2003年日平均气温资料,计算历年≥0℃、≥10℃积温和<0℃负积温,深入研究西北地区热量资源对气候变暖的响应及其对农业生产的影响。结果表明:1987~2003年比1961~1986年的平均值明显增高,尤以最低气温增幅最大,这说明最低气温的变化比最高气温的变化更敏感,西北地区气候变暖主要来自最低气温升高的贡献。冬季升温幅度大于夏季,<0℃负积温绝对值明显减少。西北地区20世纪80年代后期气候明显变暖,热量资源增加,喜温作物面积扩大,越冬作物种植区北界向北扩展,对牧区牲畜越冬度春有利。西北地区气候变暖对农业的负面影响大于正面影响。 相似文献
1000.