ter 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 food is a key resource for most fly species, fly species may show significantly different patterns of fluctuation than bees whose larvae are all dependent on pollen for food, reinforcing the idea that different pollinator groups may respond differently to environmental change.
Anthropogenic changes in fly species abundances may be difficult to monitor unless there are baseline data on natural population fluctuations. Although measures of the abundances of mosquitoes, tsetse flies, or pest species receive much attention (Williams et al. 1990, American Mosquito Control Association, Integrated Satellite and Field Data Analysis of ODA and Other Tsetse and Trypanosomiasis Research and Control Programmes), other types of flies are monitored less frequently. In North America, few extensive data sets exist that are specific to anthophilous flies. However, other types of studies, designed with different questions in mind, can be further analyzed to provide measure of relative abundance of anthophilous fly species. Three of these are discussed here to give the reader a sense of the types of studies that might be applicable.
In 1947, R. H. Whittaker (1952) conducted extensive sweep-net sampling of different insect communities in the Great Smoky Mountains of Tennessee and North Carolina. He sampled 16 sites, representing 13 distinct plant communities, and sorted insects to species. The 13 main sites were each sampled three times on different days in June and July of 1947. The vegetation within sites was divided into herb, shrub, and low-tree strata. Samples consisted of 50 successive sweeps within each stratum. Some supplementary collecting of large flying insects missed during sweep samples was performed. Over 8000 Diptera were collected, and the abundances of 45 families was presented for each site. Diptera ranged from 28% to 58% of the total sample per site. The scale of this survey was extensive; all but three of the sites were within the Great Smoky Mountains National Park and are likely to have had minimal impact compare