Biomonitors

bioindicators, are a group of species whose population, functions, and status can reveal the status of the environment.

One example could be the small crustaceans that are present in many areas of the ocean. Since they are present in many areas and are of one species, they can be used to determine if an ecosystem.

How? It's simple, compare their usual behavior, physiological, and biological states to their current one in the environment they're in. Should something change, it could be determined that something was happening to the surrounding environment, which has the potential to become devastating to the ecosystem.