Heatwaves strengthen historical contingency in yeast--bacterium interactions in floral nectar

Heatwaves can affect species abundances by changing how species interact with one another in local communities. These effects can be complex and remain poorly understood, especially in cases where the outcome of species interactions is contingent on the history of species arrival. We studied how heatwaves affect interactions between the yeast Metschnikowia reukaufii and the bacterium Acinetobacter nectaris, both commonly found in the floral nectar of Diplacus aurantiacus, a hummingbird-pollinated shrub native to California. The microbes were introduced to artificial nectar in different orders of arrival in the presence or absence of simulated two-day heatwaves. We found that heatwaves made yeast-bacterium interactions more contingent on arrival history, thereby causing large variation in nectar acidity, a factor known to affect hummingbird preference and seed production. In the absence of heatwaves, Acinetobacter always became abundant regardless of arrival history, suppressing Metschnikowia and reducing nectar pH. In contrast, in the presence of heatwaves, Acinetobacter dominance depended on arrival history and heatwave timing. If Acinetobacter arrived after Metschnikowia during a heatwave, Metschnikowia suppressed Acinetobacter substantially enough to keep nectar pH at a high level. These results suggest that heatwaves shift species interactions from determinism to historical contingency, with both taxonomic and functional consequences.