The Relationship: A squirrel (the commensal) and a tree (the host).

How it works:

* Squirrel benefits: The squirrel uses the tree for shelter and nesting. The tree provides a safe and protected space, helping the squirrel avoid predators and harsh weather conditions.

* Tree is unaffected: The squirrel's presence doesn't harm or benefit the tree. The tree is simply a resource the squirrel utilizes.

Why this is commensalism:

* One organism benefits (squirrel) while the other is neither harmed nor helped (tree).

This is a classic example of commensalism in a boreal forest. There are many other examples, such as:

* Birds using tree cavities for nesting: Similar to the squirrel example, the birds benefit from the shelter, while the tree remains unaffected.

* Mosses and lichens growing on trees: The mosses and lichens find a suitable place to grow, but they don't impact the tree's health significantly.