Specialists drive biodiversity scaling in symbiotic relationships

The stunning diversity of symbiotic life forms, and their unique vulnerability to extinction, emerge from the close relationship between host and symbiont species richness. The general form of this relationship should be linear, but simulation studies have shown that it becomes sub-linear (and even power law-like) when sampling within a host-symbiont network. Here, we resolve this paradox with a new mathematical model of scaling in bipartite graphs, based on the independent behavior of specialist (single-host) and generalist (multi-host) symbionts. Using this model, we show that specialists constrain the architecture of ecological networks, and at global scales, drive the accumulation of symbiont biodiversity. By definition, specialists also face the highest risk of coextinction with their hosts and -- despite substantial uncertainty about their true richness -- we show that specialist symbionts could easily account for the majority of threatened species on Earth. Our study reveals that symbiosis remains one of the most poorly-understood building blocks of ecosystem function and evolutionary diversification, and serves as a reminder that foundational macroecological principles are still waiting to be discovered from first principles.