An interaction favourable to both populations but not obligatory to either is
Proto co-operation is the interaction between two living organisms of different species in which both are mutually benefitted but they can live without each other. That is, one organism does not die if there is no interaction. For example, the spur-winged plover, which eats residues from crocodile teeth by using its beak; the removal of ectoparasites from the back of bovines by some birds that eat the parasites.
Mutualism is where the relationship is obligatory. For example, Lichens
Commensalism is an interaction where one organism benefits from the relationship while the other is unaffected. For eg. hermit crab and sea anemone
Parasitoidism is a form a parasitism. In this relationship, the parasite benefits and the host is killed. For eg. Ophiocordyceps fungi that colonize ants.