They can make up a symbiotic relationship with an ecosystem. For example, a tree may have a fungus on it, where the fungus feeds off some of the trees food. This is commensalism, where only one thing in the symbiotic relationship benefits and the other is caused no harm.
There is parasitism, where one thing is harmed and the other thing benefits in the symbiotic relationship. Then there is mutualism where both things in the symbiotic relationship benefit.