Data from: Effects of long-term variation in pollinator abundance and diversity on reproduction of a generalist plant

1. Quantifying the benefits of pollinator abundance and diversity for
plant reproduction is a critical challenge, especially given recent
pollinator declines. Many studies test for pollen limitation (PL), but
huge gaps remain in our understanding of how often and why plants
experience PL. Most studies are limited to a few sites or years. Little
existing research mechanistically links pollinator visitation with plant
reproduction. Also, buffering against variability in visitation is a key
but largely untested potential benefit of pollinator diversity. Long-term
studies pairing visitation data with PL experiments are essential to
addressing these gaps, but extremely rare. 2. I measured pollinator
visitation, responses to pollen supplementation, and fruit weights in the
generalist plant Scrophularia californica across multiple sites for 8
years. I determined how often S. californica experienced significant PL
and tested mechanistic models predicting PL and fruit (capsule) weight
from visitation. I then used those models to simulate pollinator loss and
evaluate the relative effects of changes in visit quantity and quality. I
explored the stabilizing benefits of visitor diversity by quantifying both
covariation among pollinators and effects of simulated species loss on
variation in PL and capsule weights. 3. Scrophularia showed significant PL
in only 26.9% of patch/year combinations tested. Visitation by the most
abundant (Apis mellifera) and effective (Vespula pennsylvanica)
pollinators strongly predicted PL and capsule weights. Both response
curves were highly non-linear, but PL saturated more with increased
visitation. Pollinator loss simulations predicted substantially increased
PL and reduced capsule weights, mostly because of lower visit numbers
rather than changes in visit quality. Visitation rates of individual
pollinators varied substantially and without correlation. Simulated
pollinator loss increased the spatial and temporal skew in visitation,
resulting in higher predicted coefficients of variation for PL and capsule
weights. 4. Synthesis. Previous work predicts a non-linear relationship
between pollinator visitation and plant reproduction, but this is the
first long-term study I am aware of that quantifies such a pattern. The
analysis of covariance in pollinator visitation combined with my
simulation results provide novel empirical support for another important
hypothesis: that multiple pollinators can provide stabilizing benefits for
plant reproduction.