The tropical acacia tree has hollow thorns, nectar and protein-producing leaves. All perfect for the stinging ants that live inside the thorns and eat the nectar and the leaves.
And what's in it for the tree? The ants keep birds and other pests away, as well as killing off small shoots that might grow into competitive trees (ht: Jerry Coyne).
The ecosystem combines two elements that can't live without one another. Each produces something the other needs.
Too often, businesses (and freelancers) focus on making it on their own. In fact, the secret of being indispensable is making it together.