In Community Ecology, there are three (3) types of species-species interactions: Competition [-, -]; Predator-Prey [+, -]; Mutualism [+, +].
For studies in Population Ecology, we can interpolate these three types of interactions from the Community-level to the Population-level--interactions between individuals of the same species (inter-individual interactions*). Thus, the three (3) types of inter-individual interactions become: Competition [-, -]; Selfish or Social Parasitism [+, -]; Social [Cooperation (+, +), Altruism (-, +)]. This schema can, also, be employed to analyze inter-group interactions.
When quantified, these 3 types of inter-individual interactions can be employed to classify all interactions, effecting the decomposition of variability.
As for inter-specific interactions, inter-individual interactions may be Facultative or Obligate.