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.
Pollen supplementation data
All data were collected at the Landels-Hill Big Creek Reserve, Monterey County, CA, U.S.A.
First file: “Pollen supplementation data.csv”. The column headings represent: “Year”= year of study; “Patch”= location; “Plant”= identity of individual plant; “Aborted.hp”=whether the pollen-supplemented flower aborted (y) or set fruit and produced seeds (n); “Weight.hp”= weight in mg of capsule from pollen-supplemented flower; “Aborted.control”=whether the control (unsupplemented) flower aborted (y) or set fruit and produced seeds (n); “Weight.control”= weight in mg of capsule from control (unsupplemented) flower; “PLindex”=pollen limitation index, defined as the ratio of weights for supplemented relative to control capsules ; “lntranPLindex”=natural log-transformed PL index; TotalScroph= total abundance of Scrophularia californica flowers on the transect for this patch, in units of 500 flowers; “Vvisit”= mean Vespula pennsylvanica visitors per transect count divided by total abundance of S. californica in units of 500 flowers; “Avisit”= mean Apis mellifera visitors per transect count divided by total abundance of S. californica in units of 500 flowers; “Bvisit”= mean Bombus visitors per transect count divided by total abundance of S. californica flowers in units of 500 flowers; “Growprecip”= total growing-year precipitation (Sept. 1- Aug. 31), in cm; “Time”=timing of pollen-supplementation experiments, measured in how many days later the supplementations began than in the earliest year (2007, on June 23).
Control flower data
Second file: “Control flower data.csv”. The column headings represent: “Year”= year of study; “Patch”= location; “Plant”= identity of plant; “Weight”= capsule weight in mg (for control plants with two unsupplemented flowers, this value represents the mean capsule weight); TotalScroph= total abundance of Scrophularia californica flowers on the transect for this patch, in units of 500 flowers; “Vvisit”= mean Vespula pennsylvanica visitors per transect count divided by total abundance of S. californica in units of 500 flowers; “Avisit”= mean Apis mellifera visitors per transect count divided by total abundance of S. californica in units of 500 flowers; “Bvisit”= mean Bombus visitors per transect count divided by total abundance of S. californica flowers in units of 500 flowers; “Growprecip”= total growing-year precipitation (Sept. 1- Aug. 31), in cm; “Time”=timing of pollen-supplementation experiments, measured in how many days later the supplementations began than in the earliest year (2007, on June 23).
Pollen transfer effectiveness data
Third file: “Pollen transfer effectiveness data.csv”. These data were all collected in 2003. The column headings represent: “Sample”=the specimen identifier; “Species”= visitor species (Apis= Apis mellifera, Bombus= Bombus spp., Vespula= Vespula pennsylvanica); “Pollen”=were pollen grains found (y) or not (n) on the stigma; “Estgrains”=estimated number of pollen grains deposited (some numbers are fractions because values for stigmas that were more difficult to score represent means of two different counts).
Visitation data 2003-2014
Fourth file: “Visitation data 2003-2014”. This file includes all patches with Scrophularia californica monitored for pollinator visitation from 2003-2014. The column headings represent: “Year”= year of study; “Patch”= location; TotalScroph= total abundance of Scrophularia californica flowers on the transect for this patch, in units of 500 flowers; “Vvisit”= mean Vespula pennsylvanica visitors per transect count divided by total abundance of S. californica in units of 500 flowers; “Avisit”= mean Apis mellifera visitors per transect count divided by total abundance of S. californica in units of 500 flowers; “Bvisit”= mean Bombus visitors per transect count divided by total abundance of S. californica flowers in units of 500 flowers.