The phrase "population decreases" can be replaced with a number of synonyms that convey the same meaning. One option is "population decline," which emphasizes a gradual decrease in numbers over time. Another alternative is "population reduction," which implies a deliberate effort to lower the number of individuals in a particular group or area. "Demographic shrinkage" is a more academic term that refers to a decrease in the size of a specific population subset, while "depopulation" suggests a more drastic, sudden decrease. "Population loss" and "population depletion" are other possible synonyms that can be used interchangeably with "population decreases" in a variety of contexts.