Biology Reference
In-Depth Information
(
Diglossa
; Thraupidae). Secondary robbers (commonly honeybees) use the
holes already made by others for easy access to nectar. Robbing levels can be
high and it is generally assumed that nectar robbers are not pollinators, but in
fact reviews of the literature have shown that the consequences for fruit set
may be harmful, neutral, or even beneficial (Maloof & Inouye, 2000; Irwin
et al., 2001). Certainly it is not uncommon for robbing bees to pollinate the
plants that they visit. For example, carpenter bees (
Xylocopa californica ari-
zonensis
) pierce the flowers of ocotillo (
Fouquieria splendens
) for nectar
while gathering pollen from the exserted stamens, and these bees are effec-
tive pollinators (Scott et al., 1993). Classification of avian visitors as either
“pollinators” or “robbers” of particular plant species may also be simplistic
(Arizmendi et al., 1996).
The relatively large tubular flowers of hummingbirds are particularly
prone to robbing by a variety of taxa. Those of
Justicia aurea
(Acanthaceae)
and
Macleania bullata
(Ericaceae) are each visited legitimately by two
hummingbird species, and robbed by other smaller hummingbirds and by
bees, ants, and butterflies (Willmer & Corbet, 1981; Navarro, 1999). Tempo-
ral and microclimatic differences help to divide the nectar resources among
these animals, with ectothermic and endothermic visitors foraging at differ-
ent times of day and on inflorescences in shade or in sun (Willmer & Corbet,
1981).
Robbing changes the quality of nectar as well as its quantity (Maloof &
Inouye, 2000; Irwin et al., 2001), thus increasing the variance in nectar
rewards and influencing the foraging behaviour of pollinators. The concen-
tration of the residual nectar is likely to increase through exposure to the
atmosphere: this confirms the value of a long corolla in reducing evapora-
tion. In hummingbird-pollinated
Ipomopsis aggregata
, flowers punctured by
bumblebees at the base of the corolla had a nectar concentration of 30%,
compared to 20% in unrobbed flowers (Pleasants, 1983). (Nectar concentra-
tions are expressed as % w/w throughout this chapter.) Apart from the
evaporation effect, robbed flowers may also have elevated amino acid con-
centrations, due to both mechanical transmission (as on the tongues of
bumblebees) and damage to floral tissue (Willmer, 1980).
Do flowers adapt to robbing by producing extra nectar? Replenishment
after legitimate removal serves to maintain a constant standing crop in flow-
ers that receive multiple visits (Castellanos et al., 2002). The flowers of
Tillandsia
species (Bromeliaceae) replenish their nectar after hummingbird
visits have been simulated by repeated nectar removal, although this re-
sponse is by no means universal (Ordano & Ornelas, 2004). These authors
