首页 | 本学科首页   官方微博 | 高级检索  
文章检索
  按 检索   检索词:      
出版年份:   被引次数:   他引次数: 提示:输入*表示无穷大
  收费全文   615篇
  免费   28篇
  国内免费   42篇
林业   90篇
农学   50篇
基础科学   68篇
  73篇
综合类   283篇
农作物   31篇
水产渔业   13篇
畜牧兽医   50篇
园艺   16篇
植物保护   11篇
  2025年   1篇
  2024年   4篇
  2023年   8篇
  2022年   6篇
  2021年   7篇
  2020年   15篇
  2019年   13篇
  2018年   11篇
  2017年   12篇
  2016年   16篇
  2015年   17篇
  2014年   27篇
  2013年   29篇
  2012年   44篇
  2011年   42篇
  2010年   46篇
  2009年   46篇
  2008年   38篇
  2007年   36篇
  2006年   31篇
  2005年   25篇
  2004年   35篇
  2003年   38篇
  2002年   26篇
  2001年   17篇
  2000年   17篇
  1999年   12篇
  1998年   9篇
  1997年   8篇
  1996年   6篇
  1995年   9篇
  1994年   4篇
  1993年   5篇
  1992年   4篇
  1991年   3篇
  1990年   7篇
  1989年   1篇
  1988年   7篇
  1987年   3篇
排序方式: 共有685条查询结果,搜索用时 15 毫秒
91.
92.
利用1999/2000年和2006/2007年2个不同时相的遥感影像对福建省沿海水域的互花米草进行解译提取,并结合地理信息数据和社会统计数据进行变化分析。研究表明:基于专家决策树的福建省互花米草信息提取法具有较高的解译精度,动态监测结果显示,7a来福建沿海水域互花米草蔓延迅速,已侵占了大片的养殖水域和滩涂资源,研究旨在为引起有关部门的高度重视并采取必要的治理措施提供理论依据。  相似文献   
93.
Uncertainty in parameter estimates from sampling variation or expert judgment can introduce substantial uncertainty into ecological predictions based on those estimates. However, in standard population viability analyses, one of the most widely used tools for managing plant, fish and wildlife populations, parametric uncertainty is often ignored in or discarded from model projections. We present a method for explicitly incorporating this source of uncertainty into population models to fully account for risk in management and decision contexts. Our method involves a two-step simulation process where parametric uncertainty is incorporated into the replication loop of the model and temporal variance is incorporated into the loop for time steps in the model. Using the piping plover, a federally threatened shorebird in the USA and Canada, as an example, we compare abundance projections and extinction probabilities from simulations that exclude and include parametric uncertainty. Although final abundance was very low for all sets of simulations, estimated extinction risk was much greater for the simulation that incorporated parametric uncertainty in the replication loop. Decisions about species conservation (e.g., listing, delisting, and jeopardy) might differ greatly depending on the treatment of parametric uncertainty in population models.  相似文献   
94.
The quality of environmental decisions should be gauged according to managers’ objectives. Management objectives generally seek to maximize quantifiable measures of system benefit, for instance population growth rate. Reaching these goals often requires a certain degree of learning about the system. Learning can occur by using management action in combination with a monitoring system. Furthermore, actions can be chosen strategically to obtain specific kinds of information. Formal decision making tools can choose actions to favor such learning in two ways: implicitly via the optimization algorithm that is used when there is a management objective (for instance, when using adaptive management), or explicitly by quantifying knowledge and using it as the fundamental project objective, an approach new to conservation.This paper outlines three conservation project objectives – a pure management objective, a pure learning objective, and an objective that is a weighted mixture of these two. We use eight optimization algorithms to choose actions that meet project objectives and illustrate them in a simulated conservation project. The algorithms provide a taxonomy of decision making tools in conservation management when there is uncertainty surrounding competing models of system function. The algorithms build upon each other such that their differences are highlighted and practitioners may see where their decision making tools can be improved.  相似文献   
95.
Uncertainty and adaptive management for biodiversity conservation   总被引:2,自引:0,他引:2  
Adaptive management of natural resources is widely supported, but in biodiversity conservation there have been few practical applications of the approach in its entirety. Some of the contributions to this special publication examine progress in the implementation of adaptive approaches into conservation policy, while others explore novel theoretical and modeling approaches that seek to accommodate the complexities of real-world applications. Several of the papers address the treatment of uncertainty in adaptive management through innovative approaches to experimentation and monitoring, use and characterisation of expert knowledge and reconciliation of differences of opinion about parameters or systems. Drawing on these contributions, we discuss the major impediments to implementing adaptive management, why adaptive management has been slow to be implemented and how this can be redressed.  相似文献   
96.
创新网络中主体的合作与资源共享必然伴随着知识转移,合作主体的选择是知识转移活动必须要考虑的问题.通过深入分析创新网络中知识转移主体合作的影响因素,提出了知识转移主体合作的评价指标体系.并在此基础上,利用多目标优化决策理论,建立了主体合作的定量决策模型.即使在知识转移主体特征变化较大的情况下,模型还能综合考虑各种因素对决策的影响,给出合理的最佳的主题选择方案.另外,模型能对优选方案进行排队,能为决策者提供丰富的决策支持信息.  相似文献   
97.
Abstract: This article focuses on a significant paradigm shift in the approaches to biodiversity conservation in the developing countries that has affected important international organisations, including the Global Environment Facility (GEF). This paradigm shift marks a movement beyond protected areas to production landscapes. It is argued that one particular project, People, Land Management and Environmental Change (PLEC), developed under the leadership of Harold Brookfield has contributed significantly to the shift. The project was instrumental in introducing the term ‘agrodiversity’ that encompasses biological diversity as well as the management diversity found in smallholder agricultural systems at all levels. The article describes the evolution of PLEC into a major international collaborative project funded through the GEF. It highlights the project's main achievements and its influence on the policy‐making in the GEF relying on the findings of independent evaluations.  相似文献   
98.
以2007年聊城地区不同月份的中巴资源影像为数据源,根据小麦不同成长周期叶绿素含量的变化规律,对其长势进行了监测,得到了该地区农作物长势优劣等级图。  相似文献   
99.
河南烟区烤烟经济性状的灰关联聚类分析评价   总被引:1,自引:0,他引:1  
为了比较河南省不同烟草种植区烤烟经济性状的差异,运用灰色关联聚类和决策分析方法分别对河南省10个烟草种植区的烤烟产量、上等烟和中等烟比例3个指标进行了综合评价。结果表明:根据不同烟区烤烟经济性状的差异,10个烟草种植区可分为4个类群:第Ⅰ类群主要包括三门峡、洛阳和济源烟区;第Ⅱ类群主要包括信阳烟区;第Ⅲ类群主要包括许昌、南阳、驻马店、平顶山和漯河烟区;第Ⅳ类主要包括商丘烟区。各类群烟区烤烟经济性状的综合表现为:许昌、南阳、驻马店、平顶山和漯河烟区表现最优,综合评价值为0.98;三门峡、洛阳和济源烟区次之,综合评价值为0.93;信阳烟区相对较低,综合评价值为0.89;商丘烟区综合评价值最低,为0.85。  相似文献   
100.
基于3D Studio MAX制作直齿圆柱齿轮模型的方法主要是使用编辑修改器对几何形体的次对象进行编辑修改,从而形成轮齿,过程稍显繁琐。然而,在此模型基础上,利用截面与放样命令却可以很简便地制作其它较复杂的齿轮模型。尤其是使用放样命令,在蜗杆及其类似模型制作方面有着事半功倍的效果。  相似文献   
设为首页 | 免责声明 | 关于勤云 | 加入收藏

Copyright©北京勤云科技发展有限公司  京ICP备09084417号