Biology Reference
In-Depth Information
Petanidou, 2005) may serve to limit the role of selection in shaping nectar
characteristics.
The main nectar characteristics are volume, concentration, and sugar con-
tent, odour, colour, and taste, which may relate to the concentration and
composition of dissolved sugar and non-sugar solids, such as amino acids,
minerals, or phenolics (Thorp et al., 1975; Baker & Baker, 1983; Olesen
et al., 1998; Adler, 2000a; Raguso, 2004; Petanidou, 2005; Petanidou et al.,
2006). Of all nectar traits, the one that has received the most attention is
quantity (volume, also in combination with sugar concentration), probably
because of the ease of obtaining measurements (see Zimmerman, 1988 and
Corbet, 2003 for reviews). In addition, many studies have focused on the
qualitative aspect of nectars, with special attention given to the evolutionary
and ecological significance of the two major components, sugars and amino
acids (for reviews, see Petanidou, 2005; Petanidou et al., 2006; Nicolson &
Thornburg 2007, Chapter 5 in this volume).
As nectar is an aqueous solution, its secretion depends largely on water
availability. This implies that in areas with extreme water deficits, such as
desert and other arid climate regions, plants may face major costs in secret-
ing nectar. In the Mediterranean region, largely influenced by an extensive
summer drought, plants are expected to face severe water stress when flow-
ering towards the onset of, or during, the dry season. This drought regime
may strongly affect secretion and other nectar attributes, and could select
against nectar being produced as the sole reward (Herrera, 1985; Petanidou
& Vokou, 1990). Scrutinizing previous literature, Petanidou and Lamborn
(2005) discussed evidence for the importance of pollen versus nectar in
Mediterranean pollination systems, which is supported by the low number
of butterflies as exclusive nectar consumers (Petanidou & Ellis, 1993), the
high abundance of typically low-nectar-producing species (Petanidou &
Smets, 1995) and the high numbers of nectarless deceptive orchids (Dafni &
Bernhardt, 1990; Dafni & O'Toole, 1994) found in this region.
In this chapter, I examine the factors that may shape nectar characteris-
tics—such as quantity (volume) and quality (sugar and amino acid
composition and concentration)—in Mediterranean habitats. These factors
may be ecological (abiotic and biotic), phylogenetic, or co-evolutionary,
with different pollinator guilds imposing selection. I address (i) the relative
importance of the above factors in shaping nectar secretion, (ii) whether nec-
tar is important in enhancing pollinator/bee diversity in Mediterranean
communities, and (iii) whether Mediterranean communities differ from other
plant communities in nectar composition. I conclude by focusing on the
