Translation:The whole population depends on them.
My book, "501 Spanish Verbs", Barrons, Fifth Edition, 2003, states that, "to depend on" always uses the preposition "de" after the verb and before the noun or pronoun if that is the dependent element. "501 Spanish Verbs" has several pages of verbs that take specific prepositions. It is really helpful.
I found this on a website. When context doesn't make its meaning clear, the possessive pronoun can be omitted and replaced by a prepositional phrase such as de él (instead of "his") or de ellos (instead of "theirs").
Have a look at this website to see the various uses od de ellos.