The people of every nation trade with the people of nearly every other nation. We all import some things and export other things. Canadians imports bananas from Panama, and the Panamanians may in return purchase computers built in Canada. Nations are not equally rich in resources just as people are not equally talented. Because of the uneven nature of talent and natural resources, we trade to improve our standard of living.
Nations specialize in much the same way that people do.
You may live in a manufacturing community that produces a large number of shoes. It may import grain from communities that have a lot of grain but are short on shoes. According to Ehrlich, this is proof that your community is overpopulated. Thus a tiny farm community that produces enough corn to feed a small nation is considered overpopulated because it doesn't produce enough shoes for a handful of people. Ehrlich seems to be condemning the division of labor.