Positive and Negative effects tourism

In my opinion tourism has more positive aspect than negatives, because it helps to the country to improve its economy, to know other cultures, to share other points of view and simply to show our country and our own culture.
In the other hand I thing that exist disadvantages in the enviroment aspect, I mean, the pollution that some foreigners make in nature places as National Parks, Reserves and Natural Monuments are even though some of them leave garbage everywhere, write on trees etc. Nobody wants foreigners like those here in Chile, don't you think?