Sometimes, completely different species or organisms forge close or interdependent relationships, to the advantage of at least one of the parties involved. This interaction is called a symbiotic relationship.
It’s not easy to come up with a narrow definition of symbiosis. In fact, symbiotic relationships can be divided into several groups:
All organisms or species involved in the relationship not only benefit from it, but it’s also essential or indispensable for their survival.
All parties involved in the relationship benefit from it, as it improves their biological fitness, but it’s not strictly necessary for their survival.
Only one party involved in the relationship benefits from it, without harming the other one.
When a species or organism (parasite or host) benefits from the relationship at the expense of the other party (host), usually harming them.