环境科学交叉关系学科7章重点句和词汇.doc

第7章 种群原理 Population Principles A population can be defined as a group of individuals of the same species inhabiting an area. Some important ways in which populations differ include natality (birthrate), mortality (death rate），sex ratio, age distribution, growth rates, density, and spatial distribution. In human populations, natality is usually described in terms of the birthrate, the number of individuals born per l000individuals per year. In human population studies, mortality is usually discussed in terms of the death rate, the number of people who die per 1000 individuals per year. The population growth rate is the birthrate minus the death rate. The sex ratio refers to the relative numbers of males and females. The age distribution, the number of individuals of each age in the population, greatly influences the population growth rate. Population density is the number of organisms per unit area. When the population density is too great, all individuals within the population are injured because they compete severely with each other for necessary resources. The carrying capacity of an area is the number of individuals of a species that can be maintained in an area, over time, without harming the habitat. The combination of factors that sets the carrying capacity for an area is called environmental resistance. For most populations, four categories of limiting factors are recognized as components of environmental resistance that set the carrying capacity. These are: (1) the availability of raw materials, (2) the availability of energy, (3) the accumulation of waste products and their means of disposal, and (4) interactions among organisms. K-strategists are organisms that typically reach a stable population as the population reaches the carrying capacity. K-strategists usually occupy relatively stable environments and tend to be large organisms that have relatively long lives, produce few offspring, and provid