Symbiosis is a close ecological relationship between the individuals of two (or more) different species. Sometimes a symbiotic relationship benefits both species, sometimes one species benefits at the other's expense, and in other cases neither species benefits.
Ecologists use a different term for each type of symbiotic relationship:
-- both species benefit
-- one species benefits, the other is unaffected
-- one species benefits, the other is harmed
-- neither species benefits
-- both species are unaffected
The following table illustrates the correct use of these terms in interactions between Species "A" and Species "B".
"+" denotes benefit to the species
"0" denotes no positive or negative effect
"-" denotes an undesirable effect of the interaction.