Markdown Summary:

Different organisms can serve as indicator species for various environmental factors. For example, lichens, spotted owls, mayflies, salmon, marsh periwinkles, river otters, salamanders, and E. coli are all examples of indicator species. Amphibians such as frogs and toads are strong indicators of pollution due to their permeable skin. The northern spotted owl is currently listed as an endangered species. Monarch butterflies are an indicator species for North America. Eleven species were identified as indicator species for red spruce forests, while fewer were identified for northern hardwood forests.