Would you consider a large city to be a population or a community?
The answer depends. A city can be considered a "population" or a "community" according to circumstances.
A population is a group of organisms of same species living in the same area at the same time.
A community,however, is a group of different species living and interacting within a specific area.
A large city would be considered a "population" as long as only one species is being discussed/studied e.g humans in London, ravens in Mirpur, goats in Lahore etc.
The same city would be considered a "community" if more than one species are being discussed at a time e.g humans,plants bacteria,ants etc. in London.
So, basically the answer depends on what are we studying about the city.