At the root of every ecology is a biome filled with co-dependant organisms. Things which just evolved together over time - frogs hiding in trees, Egrets riding on oxen, ramoras riding on sharks, cows using rumen bacteria to digest grass, etc.
I did not see a symbiosis tag in here for developing co-dependent relationships between organisms. As of this post, I have added one. Please feel free to design your creatures' mutual relationships with this tag!
This is too broad for biology, there is not necessarily a biological reason for symbiosis. Sometimes the organisms are smart enough to just know what works best, like a tree frog and a tree.
At this writing there are 280 questions in [biology] asking about symbiosis in one form or another, and they always combine it with another almost arbitrary tag which wastes 2 of you 5 tags. I think it warrants a tag.