RESOURCE TRACKING BY BUMBLE BEES: RESPONSES TO PLANT-LEVEL DIFFERENCES IN QUALITY

Abstract

This study considers how a forager moves through its environment when food is patchily distributed and the patches differ in quality. One possibility is to move directionally among patches, staying longer in the best patches (the Hypothesis). Additionally, when foragers reuse patches in a regenerating environment, they can potentially benefit from remembering the most profitable patches, and return preferentially to these (the Hypothesis). Both hypotheses were tested with foraging bumble bees (Bombus spp.) collecting nectar in the foothills and mountains of southwest Alberta, Canada. Individually marked worker bumble bees were observed visiting flowers on focal plants of five species visited primarily by bumble bees. In three plant species (Epilobium angustifolium, Hedysarum alpinum, and Penstemon confertus), variation in nectar secretion rate was experimentally induced by defoliation and fertilization of individual plants. In the other two species (H. sulphurescens and Oxytropis monticola), bees' responses to natural variation in plant-level nectar secretion were measured. Consid- ering the bee population as a whole, plants with higher nectar production rates attracted more bees (three of five plant species) and had more of their flowers visited (three of five plant species). The Local-Experience Hypothesis was supported in three of the five plant species: individual bees stayed longer by visiting more flowers on plants with higher rates of nectar production. The Memory Hypothesis was supported in four of the five plant species; individual bees were more likely to return to plants with higher rates of nectar secretion. Overall, there was a positive correlation between the extent to which individual plants differed in their rate of nectar secretion (a species-level trait), and the strength of support for the Memory Hypothesis (measured as a standardized effect size). That is, individual bumble bees more strongly adjusted their visitation rate based on plant quality as the magnitude of natural differences in plant quality increased. Support for the Memory Hypothesis may explain why bumble bees often revisit the same plants on successive foraging trips: memory for good plants gives them a fitness advantage over naive bees using the same area. Bumble bees appear to use a relatively sophisticated spatial memory in resource tracking, which may constrain the evolution of empty-flower strategies of plant nectar secretion.