Populations （1）
Ecologists are interested in the interactions between organisms. Since it takes more than one organism to have an interaction, the basic unit of ecology is the population. A population is a group of individuals that interbreed and share the same gene pool. While every individual in a species has the capacity to interbreed with any other individual, a population is a group of organisms that exist in the same specific geographic locale and actually are interbreeding. All the killer whales in the ocean make up a species, but only the killer whales that actually live and migrate together—only the killer whales that actually interbreed—make up a specific population.
Populations are much more than the sum of their parts: a population displays patterns and concerns that are not applicable to an individual organism. Whereas an individual is concerned with living for as long as possible and having as many offspring as it can, a population is concerned with maintaining its number given the resources at hand.
Population Growth
A vital characteristic of a population is the rate at which it grows. The rate of population growth depends on a variety of factors, including birth rate, death rate, initial population size, and resources. With unlimited resources, a population can expand very rapidly. Two rabbits that live in Rabbit Utopia and have five male and five female offspring every four months will produce a population of 12 rabbits after four months and 72 rabbits after eight months. Sounds like nothing, right? After one year, the population will be 432 rabbits. After two years, there will be 93,312 rabbits. And after three years, the population will be more than 20 million rabbits. This rabbit population is following the trend of exponential population growth, in which there is nothing to limit the growth of a population and that population correspondingly grows by exponential factors. A graph of exponential growth looks like this:
Perhaps Rabbit Utopia can grow enough lettuce to support 20 million rabbits, but normal nature cannot. In nature, when a population is small, the resources surrounding it are relatively large and the population will grow at near exponential levels. But as populations grow larger, they need more food and take up more space, and resources become tight. Within the population, competition for food and space grows fierce, predators move in to sample some of the bounty, and disease increases. These factors slow the growth of the population well before it reaches stratospheric levels. Eventually, the rate of population growth approaches zero, and the population comes to rest at a maximum number of individuals that can be maintained within a given environment. This value is the carrying capacity of the population, the point at which birth and death rates are equal.
The carrying capacity of an environment will shift as an environment changes. When there is a drought and less vegetation, the carrying capacity of rabbits in a population will decrease since the environment will not be able to produce enough food. When there is a lot of rain and lush vegetation, the carrying capacity will increase.