| 42.1 A Population Is All Individuals of a Single Species in a Particular Place
1. A population is a group of organisms of the same species living in a geographic region. Ecology considers relationships between organisms and their living and nonliving environments. It includes the relations of individuals in populations and their interactions with individuals of other species in communities.
2. Demographic characteristics such as population density and population dispersion are static measures of a population. 42.2 Several Factors Affect Population Growth
3. A population grows when more individuals are added through birth or immigration than leave due to death or emigration. Population growth depends upon the initial size of the population, how many individuals are added and at what rate, the age at which individuals begin to reproduce, and the age structure of the population.
4. Unrestrained growth is exponential and produces a J-shaped curve. 42.3 Factors That Regulate Population Size
5. Environmental resistance counters unrestrained growth by increasing mortality rates and/or reducing birthrates.
6. In response to environmental resistance, the population may stabilize at its carrying capacity, the number of individuals an environment can indefinitely support. After a period of exponential growth, a population may overshoot the carrying capacity and crash. Alternatively, density-dependent factors may slow growth so the population size levels off at the carrying capacity, producing an S-shaped logistic growth curve.
7. Environmental resistance includes density-independent factors, which kill a fraction of the population regardless of its size. Density-dependent factors, which have a greater effect on large populations, can also regulate population size.
8. Populations that regularly increase and decrease in size have a boom and bust cycle.
9. Different life histories reflect a trade-off between the number of offspring and parental investment. K-selected species invest heavily in rearing relatively few young. In contrast, r-selected species produce many offspring but do not expend much energy on each.
42.4 Human Population Growth
10. Human population growth has not been steady and occurs unevenly in different parts of the world. Global human population growth will level off towards the end of the 21st century.