Passage Summary: The world is running out of ways to grow more grain, but the number of people who need to eat it is still going up, so we are likely headed for a shortage.

Conclusion: A major global shortage of grain is probably going to occur.

Reasoning: Grain production has stopped growing and will be hard to increase because land is already being used efficiently, while the demand for grain continues to rise with the population.

Analysis: The statement in question serves as a premise that supports the final prediction of a grain shortage. Specifically, it acts as an intermediate conclusion: it is supported by the fact that farmland is already at maximal efficiency, and it in turn supports the final conclusion about the impending shortage. When identifying roles, notice how this claim limits the 'supply' side of the economic equation. It is a building block in a classic supply-and-demand argument.