What kinds of factors tend to limit population growth? What kinds of factors tend to facilitate it?
If populations were able to reproduce without any restrictions, similar to the way humans reproduce, the growth would be exponential. In essence, the birth and death rates of the population is unaffected by the populations size. This pattern, however, is extremely rare.
Population growth can be affected spontaneously. The factors that cause these changes are often called density-independent and are mostly due to abiotic interactions. Changes in climate, severe weather and other natural disasters that are unaffected by the population itself are examples of these factors.
Commonly, when density-independent limitations are rare, population size over time reflects logistic growth. This is when the group of organisms grow in size in an exponential pattern until some factor that limits birth rates or increases death rates is triggered by the size of the population. When an increase in the number of organisms can no longer be supported due to some aspect of the environment, the population is said to have reached its carrying capacity. In the years following this event, the population size will generally stay constant at this determined capacity.
The factors that determine the carrying capacity of a population are often called density-dependent limitations because their effect on individuals is determined by the number of organisms in the population. These are often biotic interactions, and a common example is the competition for resources. As size increases, the amount of available resources must be divided among the increased numbers and therefore creates competition among organisms of the same species.
Not all aspects of the environment inhibit population growth, though. In some cases, resources are abundant, climate is prime for production or predation is scarce, and therefore population size is not affected by biotic or abiotic limitations. These situations tend to facilitate growth.