The overall population of a species has increased. What could have occurred to allow this to happen?

The answer is immigration and/or birth.

Population change depends on several factors: birth, death, immigration and emigration. If population size is increased, then the death rate must be less than birth rate or the immigration rate. Emigration must be less than immigration rate or death rate. So, to increase population it is necessary for birth and immigration to be higher than death and emigration.