What two factors does carrying capacity compare?

Carrying capacity refers to the maximum abundance of a species that can be sustained within a given area of habitat. When an ideal population is at equilibrium with the carrying capacity of its environment, the birth and death rates are equal, and size of the population does not change. Populations larger than the carrying capacity are not sustainable, and will degrade their habitat. In nature, however, neither carrying capacity or populations are ideal-both vary over time for reasons that may be complex, and in ways that may be difficult to predict. Nevertheless, the notion of carrying capacity is very useful because it highlights the ecological fact that, for all species, there are environmental limitations to the sizes of populations that can be sustained.