Soil moisture influences nectar robbing and plant fitness in a primrose species

1. Low soil moisture, a prominent indicator of a changing climate, has overarching impacts on plant reproduction both directly and indirectly by affecting the visitation rate of pollinators. However, the role of soil moisture in modifying the behaviors of species interacting with plants is less well explored, as are the mechanisms associated with those changes in behavior. If species behavior changes from mutualistic to antagonistic, low soil moisture has the potential to disrupt mutualisms, with plant fitness consequences.2. We investigated the effects and underlying mechanisms of soil moisture on the intensity of nectar robbing (a common behavioral interaction between plants and pollinators) in natural populations of Primula florindae on the southeast Tibetan Plateau. To understand the relationships between soil moisture and nectar robbing and the mechanisms involved, we explored within-population variation in nectar robbing, floral and nectar traits, and the diversity and evenness of the co-flowering plant community in areas with natural variation in soil moisture. We then used an experimental approach to test how nectar robbing affected male and female components of P. florindae reproduction.3. We found that plants growing in low moisture soil experienced higher nectar robbing. Low soil moisture was associated with reduced flower size, nectar production, and the diversity of nectariferous plants. However, the Partial Least Squares Path Model revealed that only flower size (an estimate of nectar accessibility) had a significant effect on intensity of nectar robbing, with low soil moisture triggering a morphological mismatch between flowers and the primary pollinators bumblebees, prompting them to shift to nectar robbing behaviors. Nectar robbing reduced all components of male and female plant reproduction explored by up to 45%.4. Synthesis: This study provides new insight into how soil moisture influences plant-animal interactions, changing the behavior of floral visitors from mutualism to antagonism, and highlights that future soil water availability caused by global climate change may not only directly affect plants but also indirectly affect plants through changes in species interactions.