 generalists that visit multiple plant species (D. W. Inouye, unpublished data) and their contributions to plant reproductive success are likely to be discounted because of their reputation as ineffective pollinators (Faegri and van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 water beetles (Samways 1994).
There is no documented evidence of recent declines of North American fly pollinators. However, I am not aware of any studies that have specifically addressed this issue. Only one fly is listed as endangered on the U.S. Fish and Wildlife Service endangered species list (US Listed Insect Species Profiles), and that is a nectarivorous fly, Rhaphiomidas terminatus abdominali (Apioceridae). That only one dipteran is listed is not likely to be indicative of the actual number that are rare, endangered, or in decline, but perhaps is more indicative of our lack of knowledge in this area. Far more research on the importance of fly pollination, fly abundance,
and community composition will be necessary to make any broad generalizations about the conservation status of fly pollinators in North America.
There are two important components to the issue of pollinator declines. One is an absolute change in abundance and the second is a change in species composition. Different sets of difficulties are associated with addressing each of these issues.Dipteran abundance
Different species of insects in the same community can show independent fluctuation patterns ranging from small to extreme, making it difficult to ascribe a single environmental process to changes in abundance (Wolda 1992). The complex and varied life histories of flies may obscure patterns in fluctuations if one does not understand differences in the basic biology of different species. For example, larval habits of flies range from predatory through saprophytic and parasitic. When we are concerned with the abundance of anthophilous flies, we generally think of adult forms that feed at flowers. However, larval food supplies could be more important in producing differences in fluctuations among species (Bañkowska 1980). Different life styles, different larval habitats, and differences in the regional distribution (broad or restricted ranges) could also result in different patterns of population stability (Owen and Gilbert 1989). If larval fo