<span>I believe it is light, but this is kind of a confusing question. Technically there is a independent and dependent variable in the question. The variable being manipulated is light, and the earthwork activity depends on the light. </span>
Answer: Cows put in a pasture of fresh grass will eat it down too a point it would regrow fast.. Unless the pasture is not large enough for the number of cattle. Then they will eat it into the ground and kill it.
Explanation:
The first one is commensalism. The second one is mutualism. The last one is parasitism.