Comparative nectar-foraging behaviors and efficiencies of an alien and a native bumble bee

MARCELO AIZEN; MARIANA LOZADA; CAROLINA MORALES

BIOLOGICAL INVASIONS

SPRINGER

Año: 2011

1387-3547

Resource preemption by alien organisms can contribute to their invasion success and the demise of functionally equivalent native species, particularly when opportunistic foraging by aliens results in more efficient exploitation.  In forests of the southern Andes, the only native bumble bee and major pollinator, Bombus dahlbomii, has declined almost to extinction during the past 15 years as the alien B. ruderatusincreased in abundance.  To explore the causes of this displacement we compared the behavior and efficiency of both bumble bees while they harvested nectar from flowers of Alstroemeria aurea, the main summer food resource in the forests of NW Patagonia.  We compared the nectar content of flowers that bees selected, recently visited and rejected with that of randomly-chosen neighboring flowers and assessed differences in visitation rates.  The native bumble bee selects flowers with abundant nectar and mostly exploits nectar-rich flower patches by rejecting a higher proportion of flowers with little or no nectar.  On the other hand, the alien bumble bee discriminated less with respect to sugar content per visited flower, but visited more flowers per minute.  In contrast to expectation, the opportunistic foraging of the alien bumblebee was not more efficient and therefore cannot explain displacement of the native species by exploitative competition for food.  Workers of the native bumble bee harvested ~70% more sugar per unit of time than those of the alien species in absolute terms, and a similar amount when sugar harvested is expressed as a percentage of body mass.  These findings suggest that the displacement of the native species by the alien must be caused by other factors, such as the associated introduction of novel diseases or parasites.