From the context I interpret inert as not actively acting toward harming me per se as the different creatures are acting in their own self interests (as am I). May not be consistent with the dictionary definition but that's the neat thing about literature, one can use words in a different context to invoke a different perspective.

The rocks (part of the bush as a whole) can cause me great injury, and they are not actively attempting to harm me.

The bear (part of the bush as a whole) can cause me great harm, and is not attempting to harm me personally as it is taking the easiest opportunity for food or safety. If something else is easier prey, it will go for that instead leaving me alone.

It's my job to make it as inconvenient for something to see me as prey, so it'll go chew on someone else...