17) What is the difference between a keystone and an indicator species and why are they impo for scientists to know about?
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Explanation:

A keystone species is a species important to the environment such that when eradicated, much of its surrounding ecosystem will take a long time to recover from the drastic changes.  For example, sea otters are a keystone species of the kelp forests in the east part of the Pacific Ocean.  When they were almost hunted to extinction for the fur trade, the ecosystem suffered kelp loss as a result of overpopulation of sea urchins, which eat kelp.

An indicator species is a species that helps scientists measure the overall health of its ecosystem.  For example, scientists have used wood stork populations in the Everglades to measure the health of it, determining whether the habitat is suitable for many other species of wading birds.