Today, humans co-opt about 40% of the terrestrial net primary productivity of the earth. This means that of all the solar energy that is converted into plant biomass by photosynthesis, humans take a large share through agriculture and other harvesting. Just a few thousand years ago, humans took only a few percent of the earth’s productivity. As the human population grows, there will likely be less and less energy left for other organisms that do not directly feed us. Some people concerned with the future of life on earth wonder where we might find a balance.