Multitrophic assembly: a perspective from modern coexistence theory

Ecological communities encompass rich diversity across multiple trophies. While modern coexistence theory has been useful in understanding community assembly, its traditional formalism only allows for the study of assembly within a single trophic level. Here, using an expanded definition of niche and fitness differences applicable to multi-trophic communities, we study how diversity within and across trophics affect species coexistence. Specifically, we investigate how assembly in one trophic level impacts the coexistence of three types of communities: (1) the single-trophic subcommunity with species at that level, (2) the single-trophic subcommunity with species at an adjacent level, and (3) the entire multitrophic community. We find that while coexistence mechanisms are similar for single-trophic communities, they differ for multitrophic ones. We also find that fitness differences primarily constrain diversity in lower-level tropics, while niche differences primarily constrain diversity in higher-level tropics. Empirical data corroborates our predictions about multitrophic structures. Our work provides needed theoretical expectation of multitrophic communities within modern coexistence theory.