human
Question: Which of the following organisms have the greatest effect on an ecosystem because of the changes they make to their environment?

Choices:
A) bees building a hive in a hollow tree
B) wasps building a nest in a leafy bush
C) beavers building a dam across a stream
D) fish digging a burrow on a river bottom
gpt
The correct answer is 2: beavers building a dam across a stream.