In Guns, Germs, and Steel, how do domestic animals interact with plants to increase crop production?
Diamond gives the answer to this question early in Chapter 4. It is on page 88 in the paperback edition of the book, the page after Figure 4.1.
Diamond tells us that there are two ways in which large domesticated animals interacted with plants to increase crop production. First, the presence of domesticated animals gave farmers fertilizer. The dung from the animals could be used to fertilize the crops, making the crops more productive. Second, the animals could be used to pull plows. Before animal-pulled plows, people could only till the soil by digging at it with sticks. Once plows could be pulled by animals, more areas of land (and better quality land) could be cultivated.
In these two ways, large domesticated animals interacted with plants to increase crop production.