Indicator species of plants occur commonly enough that it is not difficult to find them. Rare species can be indicator species, but, they are more difficult to find. When rare species are indicator species they become a focus of protection in a different and less common way. Such species are "spotted" on a nature walk and provide reassurance that the forest is doing well.

Indicator species of animals are not usually large animals. Larger animals have territorial needs and can extend between forests. In a walk through a forest, animal footprints are interesting to note as each species is a part of the community, but, more common smaller animals are better as indicator species because they become residents to the forest and require the forest health to maintain their lives and lifecycles.

In referencing the Northern Spotted Owl before it is easy to understand why this species is so important. It survives in a specific type of forest, thrives on the animals it hunts and of course, those hunted animals rely on the entire ecosystem of the forest to thrive. When the species preyed upon is thriving the predator does well, too.

So, a good animal indicator will probably be a predator that hunts the forest and thrives at the top of the food chain or food web if that is easier to understand.