Organisms can be used as bio-indicators. These sites contain various resources such as images, video clips, lesson plans to help students learn the concepts of bio-indicators and how to use them to monitor environmental changes.
Here is a case study of how frogs are bio-indicators of water pollution. Click on the video link on the right side of this page to listen to Dr. Hayes explain why we should worry about the presence of artificial hormones in the environment.
Here is a lesson plan about how leaf stomata can be used as bio-indicators of environmental change. This lesson plan includes background information and many useful reference resources. NOTE: This site includes ads.