I read somewhere that the number of trainers in the city/forest depends on A. If you visit the area every day (-10 per person if you aren't) and B. Talk to them (-5 if you don't.) Each person starts at 50. If they hit 0 they disappear and can only be brought back if you do the entralink with the opposite version from yours and do challenges in the other area, talking to the people. Some of them will ask to come with you, and saying yes replenishes your people, although they're subject to the same system. More people=bigger buildings/trees and better items at the store/wider variety of Pokemon (depending on your area.) Each person actually has their own special Pokemon/item, so the "catch-em all" idea applies to these people too.
*Numbers are fabricated. I did as much as I could from memory, but they're just crap-shoot guesses. However, the system is similar in its workings to what I said. So, disregard the numbers, regard the systems.