How can planting a large area of just one variety of plant cause problems for both a farmer and an ecosystem?

Particular plants use particular nutrients from the soil. If you were to consistently plant a large number of plants, of the same species, in the same area. The plant will use all of the nutrients it needs, before the nutrients are replenished. This can cause a decrease in the farmer's yield. By doing this you also create a habbitat for pests. Some pests live off to certain plants, so if your not planting other crops, the pests will thrive. Plating a variety of crops is good because some plants will repel pests from other crops that the pests may be attracted too.

when you breathe in, or inhale, your diaphragm contracts (tightens) and moves downward. this increases the space in your chest cavity, into which your lungs expand. the intercostal muscles between your ribs also enlarge the chest cavity. they contract to pull your rib cage both upward and outward when you inhale.