Seasonal Temperature Fluctuation Enables Persistence of a Model Multi-Trophic System.

Seasonal thermal fluctuation is a dominant environmental force in temperate ecosystems, yet its role in shaping community dynamics and persistence remains poorly understood. We addressed this question by tracking high-resolution seasonal dynamics of a model system composed of host Drosophila flies and two parasitoid species attacking hosts at larval and pupal stages respectively. Both parasitoid species increased in abundance steadily during the spring. The pupal parasitoid declined in summer but resurged in autumn, whereas the larval parasitoid declined after a summer peak. The host population remained relatively stable after an initial spring increase. Modelling predicted that the system would collapse under constant cool environments due to excessive parasitism on hosts, whereas pupal parasitoids would be excluded under constant warmth. These predictions were validated by controlled laboratory mesocosm experiments. Life history assays clearly revealed that cool and warm seasons favour different parasitoid species: cooler temperatures resulted in higher mortality of immature larval parasitoids due to host immunity, whereas warmer temperatures narrowed the host exposure window and increased mortality of immature pupal parasitoids. These results demonstrate that seasonal temperature fluctuation maintains a fragile multi-trophic motif by preventing resource overexploitation and competitive exclusion, highlighting that intra-annual periodic forcing sustains biodiversity.