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

Abstract

Heatwaves can affect species abundances by changing how species interact in local communities. However, these effects remain poorly understood, especially when the outcome of species interactions is contingent on their arrival history. We studied how heatwaves affect interactions between the yeast Metschnikowia reukaufii and the bacterium Acinetobacter nectaris, both found in the floral nectar of Diplacus aurantiacus, a hummingbird-pollinated shrub native to California. Microbes were introduced to artificial nectar in different orders of arrival in the presence or absence of simulated two-day heatwaves. Heatwaves made yeast--bacterium interactions more contingent on arrival history, causing large variation in nectar acidity, a factor known to affect hummingbird preference. Specifically, in the absence of heatwaves, Acinetobacter 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, it was suppressed by Metschnikowia substantially enough to keep nectar pH at a high level. These results suggest that heatwaves shift species interactions from determinism to historical contingency, with taxonomic and functional consequences.