od 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 compared to private lands studied over 50 years ago.
Another large-scale survey was conducted by Siemann et al. (1996; Long Term Ecological Research, University of Minnesota) in 1992 in Cedar Creek, Minnesota. Sweep samples of 48 grassland fields were conducted 7-9 times during the growing season, with 100 sweeps per sampling date. 89,596 insect specimens of 1167 species were collected, and 90% of the specimens were identified to species. The data set was used for analysis of diversity, abundance, and body size relationships of insects, but it might lend itself to reanalysis for patterns in anthophilous fly distribution. LTER sites could be targeted as locations for these types of surveys, to be repeated on a long-term basis.
In 1984, D. Inouye and M. Soulé initiated several long-term monitoring projects related to organisms occurring in the area of the Rocky Mountain Biological Laboratory (RMBL) in Colorado (latitude N 38.5°, 106° W). Malaise trapping has been continued since that time (D. W. Inouye, unpublished data). Samples were collected weekly throughout the growing season from June through late August at Gothic Research Meadow (elevation 2912 m). In 1991, trapping began at two additional sites in the area (Cement Creek, elevation 2830 m; and Washington Gulch, 2978 m). The malaise trap catches were sorted to insect order, and the numbers of flies in certain families of interest were recorded for each sample. Relative weather data were recorded for each 72-h collecting period, and could be supplemented by more detailed weather data from the RMBL weather station. The peak number of Diptera within each years’collections ranges from a low of 280 in 1995 to over 1665 individuals in 1985. Preserved specimens are in storage for future extended analysis.Dipteran species composition
Differences in dipteran species composition between natural and disturbed areas can be dramatic, and can serve as excellent indicators of environmental degradation (Bañkowska 1980). In an extensive 5-yr survey of syrphid biogeography in Poland,