Rapid contemporary evolution can promote population persistence under environmental stressors and resource competition, but temporal fluctuations may alter these outcomes in complex ways. By integrating plankton experiments with mathematical modeling, we demonstrate how temporal fluctuations in environmental stressors and resource availability shape ecological outcomes through rapid adaptation. First, algal experiments revealed that large environmental fluctuations had a dual effect: they initially caused an adaptation lag, but ultimately enhanced evolutionary rescue. Our mathematical model further suggests that environmental fluctuations can promote rescue by increasing trait variance (Shibasaki & Yamamichi 2026 Evolution). Second, we showed that high genetic variance in prey defense generated predator-prey population cycles, thereby promoting fluctuation-dependent predator coexistence. Using the framework of modern coexistence theory, we demonstrate that these cycles facilitate coexistence by increasing niche differences through temporally varying resource availability (Yamamichi 2026 Biol. Lett.). Together, these findings demonstrate that environmental and resource fluctuations, rather than being mere noise, can act as key drivers that paradoxically facilitate both evolutionary rescue and multispecies coexistence.