Biology Reference
In-Depth Information
nectar. Unrelated plants show this phenotypic convergence because they share
a functional group of animals as pollinators (Fenster et al., 2004).
Implicit here is the assumption of floral adaptation to pollinators and
hence specialization in plant-pollinator interactions (Herrera, 1996; Waser
et al., 1996). These topics have been the subject of considerable recent debate,
much of it focussed on geographic and phylogenetic differences in levels of
specialization (Waser et al., 1996; Johnson & Steiner, 2000; Fenster et al.,
2004; Ollerton et al., 2006). The paradox is that visitors to flowers are much
more diverse than the syndrome classification suggests. Many plants are pol-
linated by several animal species, sometimes taxonomically distant from one
another (Herrera, 1996). Not surprisingly, the number of plant-pollinator
interactions observed increases with sampling effort in both time and space.
Meticulous field observations are necessary, and complete sampling requires
nocturnal observations, should not ignore vertebrates, and should be carried
out over the whole flowering season and over many years (Ollerton et al.,
2006). Herrera (1988) found that
Lavandula latifolia
(Lamiaceae) in south-
eastern Spain was visited by nearly 85 bee, fly, and butterfly species over its
extended flowering period. On a finer time scale, temporal changes in vol-
ume, concentration, viscosity, and composition ensure that attractiveness to a
succession of different pollinators changes throughout the day, which fa-
vours generalization in pollination systems (Corbet et al., 1979). A plant
species growing in different habitats may be exposed to quite different polli-
nator populations (Bernardello et al., 1994; Hingston & McQuillan, 2000).
In addition, only a few of the many visitors may be pollinators, and meas-
urements of pollination effectiveness (e.g., Fishbein & Venable, 1996) are
much more time-consuming than rough categorization. However, for present
purposes, we are interested in all nectar consumers, regardless of whether
they are effective pollinators, opportunistic visitors, ant guards, or robbers
that deplete the nectar rewards.
2.2
Nectar robbing and nectar theft
Robbing causes further blurring of the distinctions between pollination sys-
tems, and may be an added influence on the rates of nectar production by
flowers. The protection of nectar in long narrow corolla tubes is designed to
exclude unwanted visitors, but such animals may puncture the base of the
corolla and steal nectar. Following the terminology of Inouye (1980), these
nectar robbers can be contrasted with nectar thieves, which access the nectar
by the normal route, but are morphologically unsuited (usually too small,
e.g., flower mites or ants) to effect pollination. Primary nectar robbers in-
clude carpenter bees and bumblebees, and birds such as the flowerpiercers
