Boundary lines in symbiosis forms

Symbiosis can take different forms (parasitism, mutualism, commensalism, etc.) but boundaries between different types of symbiotic interactions are not well defined. The kinds of symbiotic associations between organisms cannot however be restricted to isolated and distinct categories. These associations are part of a broad continuum in which it is difficult to know where one type of association ends and another begins. Moreover, different scientists use the same term to mean different things or different terms to mean the same thing. This can obscure what is biologically important and what is not. This communication proposes a new classification scheme, which simply and comprehensively illustrates relationships between the various kinds of associations. The scheme illustrates relationships clearly and highlights the continuum between types of associations. It further indicates where modifications to the scheme are possible over time. The classification of the association between two organisms can be reduced to two factors: 1) the impact incurred by the host (benefit or damage) and 2) the relative duration of the association (RDA), i.e. the ratio of the duration of the association to the life expectancy of the symbiont. The conceptual figure provides concrete examples and illustrates some relationships that can change during different life stages. This figure should help teachers and students in the understanding of symbiosis, and could be a starting point for future discussions in the continuously developing research fields studying ecological and evolutionary implications of symbiotic relationships.