What architectural element does the term "keystone species" reference?

Answer

The wedge-shaped stone at the apex of an arch.

The term is derived from architecture, specifically referencing the keystone, which is the wedge-shaped stone placed at the top of an arch; its removal causes the entire structure to lose its integrity and collapse.

What architectural element does the term "keystone species" reference?
speciesEcologyRolekeystone species