The relative importance of non-hymenopteran pollinators varies across habitats. Relatively few specialized pollination systems involving non-hymenopteran insects are known from Europe and eastern North America, where pollination is commonly carried out by opportunistic social bees (Johnson & Steiner 2000). In these areas, many flowering plants receive visits from a range of insect taxa, including bees, beetles, flies, and lepidopterans, which vary in their importance as pollinators (Herrera 1987; Fishbein & Venable 1996; Waser et al. 1996). However, cole-opterans, dipterans, and lepidopterans also specialize on plants that are in turn specialized for them with respect to floral morphology, phenology, and type of reward, and that in some cases depend exclusively on them for pollination. These systems are particularly common in tropical and southern hemisphere temperate habitats (Johnson & Steiner 2000).
Plants pollinated primarily by beetles occur in temperate regions (Dafni et al. 1990; Englund 1993), but are more abundant in the tropics
(Young 1986; Momose et al. 1998). Flower-visiting dipterans are abundant in montane and Arctic areas, where they are probably major pollinators (Kearns 1992). Long-tongued flies in the family Nemestrinidae are important and sometimes exclusive pollinators of a range of South African taxa (Johnson & Steiner 1997; Manning & Goldblatt 1997), and bombyliids are key pollinators in semi-arid regions (Johnson & Midgley 1997; Johnson & Dafni 1998). Hawkmoths are particularly important in habitats with warm temperatures at dusk, as flight activity is limited on cold nights (Martínez del Rio & Búrquez 1986; Haber & Frankie 1989; Willmott & Búrquez 1996; Nilsson et al. 1997), and butterfly-pollinated species occur in both temperate and tropical habitats (Gilbert 1972; Levin & Berube 1972; Cruden & Hermann-Parker 1979). The relatively greater role of flower-visiting non-hymenopterans in tropical, alpine, and desert areas may contribute to an under-appreciation of their importance as pollinators by temperate biologists.
Quantitative data on pollinator performance, including amounts of pollen removed and deposited, extent of pollen carryover, distances flown between plants, etc., are scarce for all groups except bees (e.g., Wilson & Thomson 1991). Beetles often travel long distances between successively visited plants (e.g., mean of 18.2 m in a temperate system and 83 m in a tropical system), and so may be particularly important in effecting cross-pollination (Young 1986; Englund 1993). Flies can carry substantial amounts ofpollen (Yeboah Gyan & Woodell 1987; Kearns 1992), and move relatively short (0.2-2.7 m) distances between plants (Schmitt 1983; Olesen & Warncke 1989; Widen & Widen 1990). Moths can deliver significant amounts of pollen to stigmas (Pettersson 1991; Willmott & Búrquez 1996), and may fly long distances (up to 400 m) between successively visited plants (Linhart & Mendenhall 1977). Some studies have concluded that temperate-zone butterflies serve mainly as nectar thieves, as they carry relatively small amounts of pollen (Wiklund et al. 1979; Venables & Barrows 1985), while others have documented relatively large pollen loads and ascribe a significant role in pollination to these insects (Levin & Berube 1972; Courtney et al. 1982; Murphy 1984). Pollen dispersal distances by butterflies in temperate systems are on the order of less than 1 meter up to about 12 meters (Levin & Kerster 1968; Schmitt 1980; Waser 1982). In a tropical system, mean pollen dispersal distances for five Heliconius species ranged between 23 and 76 m, with a maximum distance of 350 m (Murawski & Gilbert 1986). Long-distance inter-plant movements by beetles, moths, and butterflies are likely to result in more cross-pollination and lower genetic microdifferentiation within populations of plants pollinated by these insects (Schmitt 1980; Young 1986; Herrera 1987; Englund 1993).
Floral constancy, commonly thought of as a characteristic of bee pollination, has also been reported for beetles, flies, and lepidopterans. Several studies of beetle pollinators report floral constancy, though few if any actually quantify both the flowers chosen by the beetles as well as the background of available flowers from which they made their choice (Pellmyr 1985; De Los Mozos Pascual & Domingo 1991; Englund 1993; Listabarth 1996). Syrphid flies foraging in a mixed array show marked constancy to a given floral species (Goulson & Wright 1998), and many species of butterflies are constant when foraging in a mixed patch of real or artificial flowers (Murphy 1984; Lewis 1986, 1989; Goulson & Cory 1993; Kandori & Ohsaki 1996; Goulson et al. 1997).
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