Studying the changes in densities of pocket gophers when large herbivores such as elk are excluded from mountain forests would be an example of studying ecology at which level?

Studying the changes in densities of pocket gophers when large herbivores such as elk are excluded from mountain forests would be an example of studying ecology at the community level.
A community refers to a group of organisms of various species that live in the same location and interact with each other. So, if this scientist is observing gophers and elks, they are looking at the level of communities, not anything more or less.

That is true! many fish species traveled to the u.s. because of it.

Ihaven't done that yet, so no idea,but this is not the right place to ask