If you were a scientist studying the use of and competition for water between two species of nocturnal frogs, you would be a specialist at which level of biology?

You would be a specialist at the ecosystem level of biology.
The ecosystem has to do with a community consisting of organisms who interact with each other and with their surroundings. So, if this scientists wants to see which species of frogs would win, he would have to observe them at the level of the ecosystem to see their interactions within their community.