philous fly species in North America have been assessed for their value as pollinators (e.g., Nye and Anderson 1974, Burgett 1980, Mesler et al. 1980, Levesque and Burger 1982, Boyle and Philogène 1983, Campbell and Motten 1985, Motten 1986, Kearns and Inouye 1994, Jarlan et al. 1997a, b). A recent review (B. M. H. Larson, D. W. Inouye, and P. G. Kevan, unpublished manuscript) that details the pollen-carrying abilities, constancy, and effectiveness of fly pollinators indicates that flies may contribute significantly to the pollination of North American flowers. Even flies that are generalists in their floral visits can contribute to plant reproductive success, and may equal or rival bees as effective pollinators in some circumstances (Motten 1986, McGuire and Armbruster 1991, Kearns and Inouye 1994). The predominance of flies on certain arctic and alpine flowers makes their contributions particularly important in these environments (Downes 1965, Hocking 1968, Mani 1968, Kevan 1972, Pojar 1974, Arroyo et al. 1982, 1985, Primack 1983, Shaw and Taylor 1986, Inouye and Pyke 1988, Kearns and Inouye 1994, Elberling and Olesen 1999). Much remains to be learned about the importance of fly pollination in other habitats. In addition, our understanding of the contributions of fly pollinators is poor for those flowers that attract multiple insect species whose numbers fluctuate independently.
Most entomophilous flowers receive visits from several different types of insects (Moldenke 1976, Herrera 1988, Ellis and Ellis-Adams 1993, Petanidou and Ellis 1993, Waser et al. 1996). For example, in a study of 2200 plant species in California, 71% of the outcrossing species were visited by two potential pollinators and 49% were visited by three or more potential pollinators (Moldenke 1976); 91% of 666 phrygana plant species in a botanical reserve in Greece were visited by five or more insect species (Petanidou and Ellis 1993). Redundancy in pollination systems is probably the rule, rather than the exception. Many species of flies are generalists that visit multiple plant species (D. W. Inouye, unpublished data) and their contributions to plant reproductive success are likely to be discounted because of their reputation as ineffective pollinators (Faegri and van der Pijl 1979, Richards 1986). However, the complexity of interactions in redundant pollination systems deserves further attention. When multiple pollinator species visit the same flowers, their respective value as pollinators is interdependent (Thomson and Thomson 1992), and may differ from year-to-year or even over the course of the flowering season. An inefficient, but legitimate, pollinator could remove pollen from the system (e.g., by eating it or carrying it to flowers of a different species) and prevent an efficient pollinator from delivering that same pollen to a conspecific flower. The same inefficient pollinator may be of great value to the plant’s reproductive success when the more efficient pollinator is absent (Kearns and Inouye 1994). Petanidou and Ellis (1993) suggest that the presence of multiple pollinating species creates “a regime of selective noise” that could halt floral evolution. Redundant pollination systems and the role of flies in these systems deserve further attention.Dipteran pollinator declines: is there evidence?
Diptera tend to be small, noncharismatic, and difficult to identify. The basic biology of most species is poorly understood. These factors are likely to be responsible for the minimal conservation focus on the order. Insect conservation efforts have tended to focus on large and showy species, particularly butterflies and dragonflies (Samways 1994). As early as 1835, the Apollo butterfly (Parnassius apollo) was granted legislative protection in Bavaria. Since that time, several conservation organizations have focused attention on butterflies and, more recently, on other insect groups. For example, the IUCN (International Union for the Conservation of Nature) now has Specialist Groups on social insects, Lepidoptera, Odonata, Orthopteroidea, and wa