Symbiotic associations are not only ubiquitous in nature, but they also play fundamental roles in ecology and evolution. This paper discusses symbiosis with regards to the fitness costs and benefits conferred to the organisms involved in such interactions, and how the varying nature of these costs and benefits impinges on the way these associations are labelled. Focusing on recent research as examples, we discuss the influence of environmental factors, ontogenetic and evolutionary time, and the symbionts’ life history traits on the interaction dynamics of symbiotic associations, and argue that symbiotic interactions are highly plastic across circumstances and timescales. We conclude that many symbioses do not fit neatly into the traditional categories of mutualism, commensalism, or parasitism, and urge caution when using such labels to describe biotic interactions.
