Concurrent density dependence and independence in populations of arctic ground squirrels

No population increases without limit. The processes that prevent this can operate in either a density-dependent way (acting with increasing severity to increase mortality rates or decrease reproductive rates as density increases), a density-independent way, or in both ways simultaneously. However, ecologists disagree for two main reasons about the relative roles and influences that density-dependent and density-independent processes have in determining population size. First, empirical studies showing both processes operating simultaneously are rare. Second, time-series analyses of long-term census data sometimes overestimate dependence. By using a density-perturbation experiment on arctic ground squirrels, we show concurrent density-dependent and density-independent declines in weaning rates, followed by density-dependent declines in overwinter survival during hibernation. These two processes result in strong, density-dependent convergence of experimentally increased populations to those of control populations that had been at low, stable levels.     

轿车车门断面设计的原则和方法

介绍了轿车车门断面设计的基本方法,探讨了控制断面的作用和设置原则．结合车门间隙、车门密封、车门运动校核及车门结构件连接等具体设计过程,分析了车门断面设计中需要考虑的控制要素,阐述了轿车车门断面设计的基本思路和程序．