Final answer:
A sink population is sustained by immigration, which is the movement of individuals into the population from neighboring subpopulations.
Step-by-step explanation:
A sink population is supported by immigration into the sink by individuals from neighboring subpopulations. Populations gain individuals through births and immigration, while they lose individuals through deaths and emigration. These factors together determine the population's growth rate. Therefore, when a population, known as a "sink," is unable to sustain its numbers through births alone and experiences higher rates of death, it depends on the influx of individuals from other populations to maintain its numbers. This movement into an area is immigration. In contrast, emigration is the movement of individuals out of a population, which decreases its size and growth rate.