I dont think this is just bcoz of population but this is something a situation created by us. at the moment, there are few places in the top cities having tree or good gardens. everywhere constructions are going on and tree are going to hell. if you do proper plantation and take charge to let them grow, environment might give you a lot. fresh air and the earthquake resistent earth. population is something that nobody can stop making child, only we have to see that howmany are coming every day and what we have to do for let them survive with us.