If you were a scientist studying how the building of dams by a beaver population in a forest affected river water flow, soil nutrient content, and the dynamics of other populations that rely on the river water, such as fish and amphibians, you would be a specialist at what level of biology?

The scientist would be an ecologist i.e. an expert in ecology.

Explanation:

The scenario in question investigates the affect of a biotic factor (beaver population) on various abiotic factors such as water flow and soil nutrient content.

In this context, ecology is the scientific study of interactions between biotic and abiotic components of an ecosystem.