Oceanic salt spray and herbivore pressure contribute to local adaptation of coastal perennial and inland annual ecotypes of the Seep Monkeyflower (Mimulus guttatus)

Abstract

Identifying the environmental factors responsible for natural selection across different habitats is crucial for understanding the process of local adaptation. Despite its importance, only a few studies have successfully isolated the environmental factors driving local adaptation in nature. In this study, we evaluated the agents of selection responsible local adaptation of the monkeyflower Mimulus guttatus to coastal and inland habitats in California. We implemented a manipulative field reciprocal transplant experiment at coastal and inland sites, where we excluded aboveground stressors in an effort to elucidate their role in the evolution of local adaptation. We found that excluding these stressors, most likely a combination of salt spray and herbivory, completely rescued inland plant fitness when transplanted to coastal habitat. In contrast, the exclosures in inland habitat provided limited fitness benefit for either coastal or inland plants. We have previously established that low soil water availability belowground is the most important agent of selection in inland habitat. Therefore, our study demonstrates that a distinct set of selective agents are responsible for local adaptation at opposite ends of an environmental gradient.