Lewis/Life - Review of Key Concepts

Review of Key Concepts - Chapter 41

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.
Each species has characteristic conditions where it lives (habitat) and resources necessary for its life activities (niche). Slight differences in the niche allow different species to share surroundings. Because of competition, a species' realized niche is often smaller than its fundamental niche.
A population grows when more individuals are added through birth or immigration than are subtracted 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.
Unrestrained growth is exponential and produces a J-shaped curve. Environmental resistance counters unrestrained growth.
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 regulate population size. Competition for a limited resource is density-dependent when members of the same species compete directly for it (scramble competition). Everyone gets less of the resource and reproduction declines. When members of a species contest territories or social dominance (contest competition), the winner usually receives the resource.
Carrying capacity is the number of individuals an environment can indefinitely support. After a period of exponential growth, a population may overshoot the carrying capacity and crash, or because of density-dependent factors, slow and level off at the carrying capacity, producing an S-shaped curve. Populations that regularly increase and decrease in size have a boom and bust cycle.
In K-selection, density-dependent factors maintain a population close to its carrying capacity. In r-selection, density-independent factors predominate, rarely reaching carrying capacity.
Several interspecific interactions influence population size. These include competition, predation, parasitism, and symbiosis. Competition between species may drive out all but the best competitors. In a natural environment, both predator and prey usually survive because of coevolved adaptations.
Many human populations are growing rapidly. 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 next century.