Final answer:
The American alligator is the keystone species in the Everglades marsh.
Step-by-step explanation:
The keystone species in the Everglades marsh is the American alligator (Alligator mississippiensis). As a keystone species, the alligator has a significant impact on the ecosystem. It plays a crucial role in controlling the populations of other species, such as fish and birds, by preying on them and creating habitat through its nesting activities.