Fig. 1.

Predictions on how species coexistence is determined by differences in organisms’ size (Δ size) through the lenses of different coexistence theories. Under the niche theory perspective (left column panels), if size differences (x-axis) relate well to niche differences (ND, top panel), coexistence is possible when niche differences (i.e., size differences) among species are large (bottom panel). The niche perspective does not consider relative fitness differences. Under the neutral perspective (central column panels), all species are equivalent in terms of both niche and fitness, irrespective of their sizes. Differences in size should not affect the likelihood of coexistence. The competitive outcome in the neutral scenario is influenced by stochasticity, meaning that a pair of species may sometimes coexist and sometimes not. The empty dots and arrows illustrate such stochastic variability among replicates for a same pair of species. The modern coexistence perspective (right column panels) proposes that if size differences relate well to both niche (ND) and fitness differences (RFD), coexistence is possible when niche differences are large enough to compensate for fitness differences. Compared to the neutral scenario, the competitive outcome of the modern coexistence theory is not influenced by stochasticity and should be consistent among replicates in a same pair of species