Biology Reference
In-Depth Information
for pollinators acts as a selective agent in nectar production, the result being
a compromise between producing enough to attract pollinators but not so
much that they are unwilling to move to other plants (Klinkhamer & De
Jong, 1993); in other words, keeping them “hungry but faithful” (Willmer &
Stone, 2004). This strategy is taken to the extreme in nectarless orchids,
where the absence of nectar is thought to promote cross-pollination and nec-
tar supplementation has been shown to reduce it (e.g., Jersakova & Johnson
2006). Pollinators vary their foraging behaviour in several ways in response
to variation in nectar: visit frequency to plants, probing time per flower,
number of flowers probed, and flight distance after departure. These re-
sponses to nectar are well known for foraging bees and hummingbirds, but
have now been recorded in flies as well (Jersáková & Johnson, 2006). The
foraging behaviour of pollinators is beyond the scope of this chapter, but see
Rathcke (1992) for a broad review of pollinator responses to both average
nectar volume per flower and within-plant variation in nectar per flower, and
the consequences for pollination success.
The standing crop is the amount of nectar that pollinators actually en-
counter in flowers, and its correlation with nectar production will be strong if
pollinator visits are rare, but weak when pollinator pressure is high and
standing crops are low and variable (Zimmerman, 1988; Cresswell & Galen,
1991). In studying the nectar dynamics of Neotropical hummingbird-
pollinated plants, McDade and Weeks (2004a, b) found high variation in
nectar volume even in protected flowers, while in open flowers continuous
harvesting of nectar by the birds meant that the remaining volumes were
usually insufficient for refractometer readings.
2
NOT ALL FLORAL NECTAR DRINKERS
ARE POLLINATORS
2.1
Generalization and specialization in pollination
systems
In the classic syndromes of pollination biology, pollinators are inferred from
a suite of floral characters, including rewards (Faegri & van der Pijl, 1979;
Ollerton & Watts, 2000). The match of flower and pollinator is particularly
clear in ornithophilous flowers (those with traits characteristic of bird polli-
nation), which tend to be large, tubular, and red, with no detectable odour
and copious amounts of dilute nectar. Another well-defined syndrome is me-
littophily (bee pollination), characterized by scented zygomorphic flowers of
varied colours (often blue or purple), with moderate amounts of concentrated
