Separating pollinator constancy into flowers and rewards: A new look at an old phenomenon

Background/Question/Methods

Many pollinator species show floral constancy, restricting foraging to one or a few flowering species, but tend to be nutritional generalists, collecting both pollen and nectar during a foraging bout. To better understand the interplay between floral reward composition and bee foraging behavior, we seperate and definie reward constancy as a pollinator’s short-term tendency to forage for a single reward type, distinct from floral constancy. Using captive bumblebee colonies (Bombus impatiens) we measured individual foraging patterns  on a “single-species” array of artificial flowers offering both pollen and nectar rewards. We sought to determine whether 1) foragers show reward constancy, 2) reward constancy changes during foraging bouts, and 3) reward composition (ratio of pollen to nectar), affects constancy.  We  assigned foragers to treatments of 'single species' arrays of artificial flowers, all of which contained 1.8-2.2 mg of cherry (Prunus serotina) pollen and either 2,4,or 8 ml of scented sucrose solution.  For each bee on a single foraging trip, we recorded the overall sequence of landings and resources collected from each flower. We used traditional floral constancy indices to assess whether individuals showed non-random patterns of transferring between and among reward collection, and whether constancy was influenced by reward composition across treatments. 

Results/Conclusions

 Bees showed reward constancy even when foraging on a single ‘flower’ type.  Bateman’s and Jacob's index were  0.537 ±0.117 and 0.257 ± 0.062 respectively. These indices range from -1 (completely inconstant) to 1 (completely constant) and account for minor individual and group preferences. There was no significant differences between the fit of models accounting for nutritional ratio and null models ( df= 2. Χ²=0.245,p>0.05); reward ratio did not predict constancy.  Opposingly, the probability of bees making  ‘constant’ transfers (nectar to nectar or pollen to pollen) did increase during a trip (log-odds=1.06 df=1,Χ²=27.23, p<0.001).

These findings raise new question about the ecological role that nutritionally diverse floral rewards play in attracting pollinators. Rather than ‘one-stop shopping’, i.e. collecting both pollen and nectar to minimize travel between flowers, foragers tended to collect one resource at a time.  Our findings that reward constancy increases across a trip is consistent with traditional hypotheses of floral constancy, which predict a period of floral sampling followed by devotion to one or a few types of flower.  We observed a similar pattern in bees foraging floral rewards regardless of floral display; bees become more constant as they become familiar with the nutritional landscape.