The long term effects that are due to Christopher Columbus are the effects of what we call the Columbian Exchange. The Columbian Exchange was the exchange of plants, animals, germs, and the like between the New World and the Old World which began when Columbus “discovered” the New World.
For me, the most important long term effect due to Columbus was the killing off of the vast majority of the native population of the Americas. Much of this killing was unintentional. The Europeans, of course, did kill many natives on purpose. However, the germs that they carried with them killed many more natives. The natives of the Americas had no resistance to the infectious diseases that the Europeans carried with them. When they were exposed to the diseases, they died in horrifying numbers. I would argue that these deaths, and the impact that they had on native societies, are the most important long term effect of Columbus’ “discovery” of the New World.