The continuum concept remains a useful framework for studying mycorrhizal functioning

Background: Recent studies have questioned the validity of the mutualism-parasitism continuum of mycorrhizal function. This paper re-evaluates the continuum model and analyzes these concerns. Scope and Conclusions: Three insights arise from this analysis. First, the continuum model defines mycorrhizal function as an emergent property of complex interactions. The model identifies resource trade and symbiotic control as key determinants of the costs and benefits of the symbiosis for plants and fungi, and the interaction of these factors with the environment ultimately controls mycorrhizal function. Second, analysis of carbon costs and phosphorus benefits is too narrow a focus to accurately predict mycorrhizal function. Analysis of plant and fungal fitness responses in ecologically and evolutionarily relevant systems are required to elucidate the full range of nutritional and non-nutritional factors embodied within mycorrhizal functioning. Finally, the definition of the term 'parasitism' has evolved. Some fields of science maintain the original definition of a nutritional relationship between host and parasite while other fields define it as a +/- fitness relationship. This has generated debate about whether the continuum of mycorrhizal functioning should properly be called a positive-negative response continuum or a mutualism-parasitism continuum. This controversy about semantics should be resolved, but it does not overturn the continuum concept.