Non-native species can enter an ecosystem as unwelcome invaders, but they can also provide benefits to native organisms with which they co-exist. Little is known about the extent to which native bees can incorporate pollen from non-native plants into their diets, and the costs or benefits associated with their use. Many wild bees are dietary specialists, collecting pollen exclusively from certain plant families, and many specialists prefer pollen from the sunflower family (Asteraceae). In high-elevation Rocky Mountain meadows, non-native common dandelion (Taraxacum officinale; Asteraceae) is an abundant floral resource that begins to flower in early June, while most native Asteraceae species flower later in the summer. The earlier availability of Taraxacum could provide a fitness advantage to those Asteraceae-specialists that use it, because bee eggs laid late in the season typically fail to survive the winter. To test this hypothesis, we (1) investigated whether individual bees that use Taraxacum pollen produce more early-season offspring (eggs) than individuals that do not, and (2) compared the survival of bee larvae experimentally reared on Taraxacum pollen to that of larvae reared on pollen of native Asteraceae species.
Results/Conclusions
Bees that used Taraxacum pollen produced an average of 2 more offspring each, which represents a significant (~30%) increase in fitness relative to bees that avoided Taraxacum entirely. However, larvae experienced a significantly greater risk of death (1.6x greater) when reared on Taraxacum pollen than when reared on non-Taraxacum Asteraceae pollen—an effect that appears to negate the early-nesting benefits of Taraxacum usage. Consequently, common dandelion appears to represent an ecological trap for these North American Asteraceae-specialist bees; however, the trap may become less effective if rising temperatures reduce the extent to which bee emergence precedes flowering of native Asteraceae species. The factors that contribute to making Taraxacum pollen a poor food source even for Asteraceae specialists are still under investigation.