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

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. Reduced soil moisture can disrupt mutualistic species
interactions, potentially shifting them toward antagonism and negatively
impacting plant fitness.  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.
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 neighbor plants.
However, the Partial Least Squares Path Model revealed that only flower
size (an estimate of nectar accessibility) had a significant effect on the
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%.  Synthesis: This study provides new
insight into how soil moisture influences plant-animal interactions,
influencing 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.