Final answer:
The main idea of the passage is the importance of squirrels for tree reproduction, evidenced by the impact of gray squirrels on red squirrel populations and potential ecological consequences.
Step-by-step explanation:
The main idea of the passage provided is c. the importance of squirrels for tree reproduction. This is evident from the passage describing the different behaviors and impacts of squirrel species, particularly the gray squirrel's influence on the red squirrel population and how this can have wider ecological repercussions. Squirrels play a crucial role in seed dispersal for trees, and changes in squirrel populations due to competition for resources can affect this process. For example, the decline of the red squirrel due to competition from the gray squirrel could have consequences for tree reproduction in the areas they inhabit.