There are contrasting hypotheses about the influence of mutualisms on species richness.
We provide a synthetic framework for how mutualisms influence species richness at the ecological and evolutionary scales.
Mutualisms can promote or restrict species richness depending on their function, level of dependence, and specificity.
Because the outcomes of mutualisms are highly dependent on environmental variables, we forecast that the effects of mutualisms on species richness are also strongly influenced by biotic, abiotic, and geographic variables across ecological and evolutionary timescales.
Although our review depicts the complex and multifaceted impact of mutualisms on species richness, it also highlights a key gap: our understanding of the relative importance of the mechanisms through which mutualisms affect biodiversity. We suggest new methodological approaches to fill this gap.