Are organisms that can be

used as warning signals for

changes in an ecosystem.

Bioindicators

Explanation:

Indicator species and/or keystone species  

Explanation:

Indicator species are species (e.g., plants, animals, fungi) that can be utilized to determine changes in the environment. An indicator species can show the impact of pollution on an ecosystem. For example, mosses are indicator species that indicate the acidity in the soil. Moreover, keystone species are species that maintain the ecological balance in the ecosystem. When a keystone species is removed, the entire ecosystem is dramatically changed or may even cease to exist altogether. In consequence, keystone species can be used to define an entire ecosystem. For example, bees are a keynote species that contributes to balance the ecosystem by pollinating plants.