 Bañkowska (1980) compared the syrphid fauna of multiple natural areas with those from areas subjected to mild, moderate, and intensive anthropogenic pressures. She documented a lower species diversity in urban and agricultural areas. Both were dominated by syrphid species with broad distributional ranges, good colonizing abilities, and high ecological tolerances. In comparison, natural habitats were species-rich and characterized by shifting proportions of species, as one moved from one habitat toward another. There were characteristic species associations for each distinct habitat type. Syrphid fly species composition closely followed patterns of food supply and habitat condition.
The following example illustrates Bañkowska’s findings: In total, 73 species were collected in the city of Warsaw, compared to 128 species in natural areas containing Warsaw’s pre-urban habitat of oak-hornbeam forest. Only 66 species inhabited urban parks, and 46 species inhabited green areas around housing estates. The proportion of phytophagous and terrestrial saprophagous species dropped significantly, with only four species of phytophages present near the housing estates. These four were pests that eat ornamental plants, or weed-eating species. Urban areas were dominated by four syrphid species with broad geographic ranges
Clearly, this type of information on dipteran species composition will be invaluable in determining whether North American fly pollinators are in decline. Bañkowska’s comparison of species composition in disturbed and natural habitats is a good model for the type of study needed in North America. This type of study could be tied to a second project estimating the pollination value of different dipteran species.
In Japan, a broad-scale, four-year survey compared all insect visitors to roughly 100 plant species in each of three different habitats (Inoue et al. 1990, Kakutani et al. 1990, Kato et al. 1990). The habitats included a university campus with mostly exotic vegetation, an undisturbed oak forest, and a botanically rich mosaic containing both native deciduous and planted coniferous forest. This project involved the collection and identification of several thousands of flower-visiting insects or arachnids in each habitat. Information on flowering and insect phenology and plant breeding systems was collected as well. Again, the site of greatest human disturbance was poorest in species numbers (Kakutani et al. 1990). The total number of arthropod species collected on the plants of the university campus was 37% of the total of the oak forest and 23% of the total mosaic of natural and planted forest (Kakutani et al. 1990).
To look at anthropogenic effects on dipteran species composition, one can compare different sites as in the studies just described, or one can look within a site at how species composition changes over time with changes in habitat. The later type of study involves resampling historical data sets for the continued presence of species. If we attempt to use historical data sets, we need to remember that they may not represent complete lists of the species that were present. Resampling efforts may produce more or fewer species, depending upon the effort and techniques employed. Disney (1986) recounts a survey of insects conducted by the Yorkshire Naturalists’ Union from 1954 to 1958, in which 436 species of Diptera were collected from the Malham Tarn in England (Flint 1963). Renewed attempts to produce a comprehensive list of Diptera in the 1970s resulted in a species list in excess of 1000. Disney attributes the increase in numbers to the use of multiple collecting methods, intensified efforts, and the availability of better keys for recognizing or separating species.
If we recognize the limitations of resampling, large collections from circumscribed locales can still provide useful data on the historical presence of dipteran species; these data can be compared with modern-day presence/absence data. Although presence/absence data are not quantitative, they can provide the same sort of information derived