We now know how hurricanes originate and how they affect ecology.
Explanation:
A sentence fragment is a clause that is missing one of the main components that would make it complete, which makes it impossible to be understood on its own. If you read the fragment "How they affect ecology" on its own, you will not obtain the necessary information for that clause to make sense. However, it does make sense when connected to the previous clause, "We now know how hurricanes originate." We can tell the fragment is supposed to be a part of this idea, of what "we know". All we need to do is connect both parts to correct the fragmented sentence:
We now know how hurricanes originate and how they affect ecology.
