Individuals come and go, but it is the population that perpetuates the species.
Old members of a population die, and new ones are born. The individuals making up a population may be totally different in several years from those there today, but the species continues. One of the characteristics of life is that it reproduces itself. Since nothing lives forever, there is always a turnover of generations. In a stable population, the birth rate equals the death rate, and the total numbers do not change.