I believe the answer would be transmission.
How would removing your umbrella in a rainstorm affect your ability to stay dry? The definition of a keystone species is a species that has a crucial role in an ecosystem. Removing one gear from a clock would stop the clock from working. Removing/changing a major contributer to a balance and natural ecosystem can potentially collapse the entire system altogether.
The lining of the uterus, and the fallopian tube.
