How do predators in a boreal forest affect the ecosystem?
The most productive ecosystems are temperate and tropical forests, and the least productive are deserts and tundras.
It really doesn't matter what levels are found in different ecosystems. In some, many levels are found and in a few only three are found.
If the base is very large, many levels are found. In boreal forests, which cover about 29% of the Earth's surface, a very small base or first layer exists because of the cold climate and short growing season.
Predators affect this ecosystem just as they do others. Since this system has a base that is small, there only are a few predators. There just isn't enough food for more.
It is better to say "How does this ecosystem affect predators?" Since the predators numbers are what the system can carry not the other way around.
Even if there were more bears or eagles, only the strongest will live as there just isn't enough food for more.