  Explaining coexistence in species-rich communities of primary producers
remains a challenge for ecologists because of their likely competition for
shared resources. Following Hutchinson's seminal suggestion, many theoreticians
have tried to create diversity through a fluctuating environment, which impairs
or slows down competitive exclusion. However, fluctuating-environment models
often only produce a dozen of coexisting species at best. Here, we investigate
how to create richer communities in fluctuating environments, using an
empirically parameterized model. Building on the forced Lotka-Volterra model of
Scranton and Vasseur (Theor Ecol 9(3):353-363, 2016), inspired by phytoplankton
communities, we have investigated the effect of two coexistence mechanisms,
namely the storage effect and higher intra- than interspecific competition
strengths (i.e., strong self-regulation). We tuned the intra/inter competition
ratio based on empirical analyses, in which self-regulation dominates
interspecific interactions. Although a strong self-regulation maintained more
species (50%) than the storage effect (25%), we show that none of the two
coexistence mechanisms considered could ensure the coexistence of all species
alone. Realistic seasonal environments only aggravated that picture, as they
decreased persistence relative to a random environment. However, strong
self-regulation and the storage effect combined superadditively so that all
species could persist with both mechanisms at work. Our results suggest that
combining different coexistence mechanisms into community models might be more
fruitful than trying to find which mechanism best explains diversity. We
additionally highlight that while biomass-trait distributions provide some
clues regarding coexistence mechanisms, they cannot indicate unequivocally
which mechanisms are at play.
