The impact of agriculture has been profound on humanity, most clearly in terms of population. This is because breeding plants and animals have significantly increased the availability of human consumable calories per square kilometre. One way to think about it is that we replaced things that were not consumable by humans with things that were. Through techniques like irrigation, we were also able to make things grow where they might not have before. Having a large population nearby made it worthwhile for farmers to grow more food than they needed for themselves, as they could trade this surplus for other goods. For non-farmers, this meant that they could focus on making other goods and trading these goods for food and other things. People could specialize—focus on doing one thing, which led to increased productivity. Increased productivity led to the creation of better buildings, tools, weapons, and to the rise of governments to oversee this activity and military forces to protect people and resources.