Are “generalists” poor quality pollinators?

Background/Question/Methods

By definition, foragers from specialist pollinator taxa should carry relatively pure pollen loads.  However individuals from generalist taxa can exhibit a variety of feeding behaviors during their foraging bouts.  For example, a generalist forager could feed from several species during a bout, or they could exhibit constancy and feed from a single plant species.  In the former case one would expect them to carry a mixed pollen load, while in the latter case they should carry a relatively pure pollen load.  Thus estimates of fitness contributions to plants by generalist taxa based on visitation patterns alone may be misleading without estimates of individual pollen loads. 

 Here I evaluate the relationship between pollinator specialization (d’ index) and individual pollen loads for a plant-pollinator network studied for two summers in California, for all the pollinator taxa, and just for bees.  Specifically I ask: 1) is there a correlation between the degree of specialization and the amount of conspecific pollen carried by individual foragers? And 2), is there a relationship between the degree of specialization and forager constancy, as estimated by the number of pollen morpho-types found on their bodies? 

Results/Conclusions

Results for the 2001 season show that the degree of pollinator specialization is positively correlated with the proportion of conspecific pollen carried by foragers (Spearman’s ρ=0.225, p= 0.013, n=121).  This relationship was even stronger when only bees were considered (ρ=0.520, p= 0.003, n=31), which have been previously shown to transport most of the pollen in this system.  However in 2002, pollinator specialization was not related to conspecific pollen loads, but rather was negatively related with the number of pollen morpho-types carried by foragers (ρ= -0.201, p= 0.029, n=118).  The same was true for bees (ρ= -0.347, p= 0.019, n=45).  The difference in these results is likely due to a drought in 2002 when significantly fewer flowers bloomed compared to 2001.  These results suggest that when resources are abundant specialists can potentially provide higher quality pollen to plants compared to generalists.  However when resources are scare, individuals must forage from a wider range of plant species, thereby reducing the quality of the pollen carried on their bodies, and presumably being deposited on stigmas.  These findings showcase the importance of considering the pollen being transported by specialist and generalist pollinator taxa for more than a single season.