Fluctuating environmental conditions are thought to be important for the maintenance of species diversity, and yet our understanding of the relative contribution of different fluctuation-dependent coexistence mechanisms (the temporal storage effect and relative nonlinearity of competition) in real systems is limited. Using experiments and simulations, we show that both mechanisms consistently affect coexistence and that, contrary to long-held assumptions, the effect of relative nonlinearity can be larger in magnitude. These results may be general in that the simultaneous emergence of both mechanisms rests on two factors common to nearly all ecological systems, from the human gut to the soil microbiome: variable environmental conditions and saturating population growth rates. Understanding the origins and maintenance of biodiversity remains one of biology’s grand challenges. From theory and observational evidence, we know that variability in environmental conditions through time is likely critical to the coexistence of competing species. Nevertheless, experimental tests of fluctuation-driven coexistence are rare and have typically focused on just one of two potential mechanisms, the temporal storage effect, to the neglect of the theoretically equally plausible mechanism known as relative nonlinearity of competition. We combined experiments and simulations in a system of nectar yeasts to quantify the relative contribution of the two mechanisms to coexistence. Resource competition models parameterized from single-species assays predicted the outcomes of mixed-culture competition experiments with 83% accuracy. Model simulations revealed that both mechanisms have measurable effects on coexistence and that relative nonlinearity can be equal or greater in magnitude to the temporal storage effect. In addition, we show that their effect on coexistence can be both antagonistic and complementary. These results falsify the common assumption that relative nonlinearity is of negligible importance, and in doing so reveal the importance of testing coexistence mechanisms in combination. (link to publication)