Explain Limiting Factors on Population Practice Test

In a tide pool, a species of barnacle can attach only to bare rock. As barnacles increase, nearly all bare rock becomes covered. New barnacle larvae are present in the water, but few successfully settle and survive. Which factor is most directly limiting the barnacle population size in this tide pool?

A farmer notices that a field of corn grows poorly even though the plants receive adequate sunlight and water. Soil testing shows very low nitrogen levels. After nitrogen fertilizer is added, the corn plants grow taller and produce more ears. In this situation, which factor was limiting plant population growth and productivity?