How does the active compound in poison ivy protect the plant from predators?
it is a strong acid that buns the skin.
it dissolves skin cells and causes pain.
it starts an immune system response that causes a rash.
it makes the skin more sensitive to oxygen.

The answer is It starts an immune system response that causes a rash.

The active compound in poison ivy that protects it from predators is called urushiol. When this compound gets in contact with predator's skin, it activates immune system response of the predator. It is not harmful in low doses, but in higher doses, it is able to suppress some fatty acids, prostaglandins, and other molecules, which further affect body's immune response.

Other organisms can change their behavior to take the place of the plants in the ecosystem.