Because most species have a high reproductive capacity, populations tend to grow if environmental conditions permit. The change in the size of a population depends on the rate at which new organisms enter the population, compared with the rate at which they leave. Natality is the number of individuals added to the population by reproduction per thousand individuals in the population. Mortality is the number of individuals leaving a population by death per thousand individuals in the population. If a species enters a previously uninhabited area, its population will go through a typical pattern of growth. Figure 17.8 shows a population growth curve, which is a graph of change in population size over time. There are four recognizable portions in a population growth curve: the lag phase, the exponential growth phase, the deceleration phase, and the stable equilibrium phase.