This is an interesting question. The answer to this question depends on the scope and desire of the study or survey. In other words, the person who is studying the population will have to set limits. A few example will be helpful.
The person who is conducting a study might only want to study the number of people on one particular island. Then the population study will only reflect this one island. If the study wanted to know about all people in one continent, say Africa, then that study will give the population of Africa. If the study wanted only the population of males in Asia, then the study will give the population of only males.
As you can see, population is defined as the total number of people that a particular study or census desires to number.