Mutualism: It confers benefits on both the interacting species. It is an obligate association of two organisms where two organisms often live together and are interdependent on each other. E.g. Lichen shows a symbiotic relationship between fungi and algae. Such a population interaction is represented as (+,+).
Proto-cooperation is an association between individuals of two species, each of which are benefited by the presence of the other but can live equally well without association. E.g. The plover enters the mouth of a crocodile to feed on the parasitic leeches. The bird gets food and the crocodile gets rid of the blood-sucking parasites. Such a population interaction is represented as (+,+).
Amensalism: It is an interaction between two organisms of different species in which one species inhibits the growth of other species by secreting certain chemicals. The organism which inhibits the growth of another organism is called amensal. E.g. Penicillium secretes penicillin that inhibits the growth of large number of bacteria. Such a population interaction is represented by (0,-).
Commensalism: Interaction in which one species benefits and other is neither harmed nor benefited. The species which is benefited is termed commensal and the other species is called host. e.g. Epiphytes growing on other plants. Such a population interaction is represented by (+,0).