 from Bañkowska’s (1980) studies relating to the “weediness” of species that dominate disturbed habitats. Areas that have been extensively sampled in the past could be targeted as sites for renewed collecting activity. Large-scale data compilations of the insect visitors to the flora of a region, such as that compiled for northwestern Europe by Ellis and Ellis-Adams (1993), could also serve as baseline data. I will discuss two large-scale North American studies of historical value.
One collection that lends itself to resampling is the product of an anthophilous insect survey conducted by Charles Robertson at the turn of the last century (see Marlin and LaBerge 2001). Robertson collected and identified insects visiting flowers within 10 miles of Carlinville, Illinois, over the course of about 30 years (Robertson 1929). He examined 441 flowering plant species and identified 446 species of Diptera visiting flowers. Changes in land use have been documented for the area since the original collection.
A major study, initiated by Andrew Moldenke (1975, 1976; see also Moldenke and Neff 1974a, b), documented insect visitors to plants of 17 different habitats in California as part of a hemispheric attempt to compare equivalency of insect communities. Over eight years, 800,000 pollinators were recorded on 2200 plant species. Over 76,000 fly specimens were collected. Pollination studies of several hundred plants confirmed whether or not insects were true pollinators of these species. Moldenke determined the number of plant species used by each major category of pollinator within each habitat. Broad categories of flies included in the study were Bombyliidae, muscoid flies, and Syrphidae. Specimens that were collected were identified to species whenever possible. Resampling sites from this study for the current presence of species would provide significant information relating to changes in fly species diversity. This study also contains semiquantitative data, because all insects visiting flowers were collected during sampling periods. Moldenke stated that the relative abundance of species collected may not always reflect the true proportional representation, but he made corrections to help account for bias. Errors in representation stem from two sources: (1) the fact that the fastest and most wary species may be underrepresented. Attempts were made to correct for this problem by factoring in the success rate of capture. (2) The most common insect species are underrepresented because collectors key in on unusual and interesting specimens. Again, attempts were made to correct for this bias (Moldenke 1975). This enormous data base might serve as the basis for large-scale resampling efforts to assess the extent of change since the early 1970s.Floral visitation studies
A number of floral visitation studies designed to address some aspect of floral biology or insect activity may also be useful in providing baseline data on species composition of dipteran visitors to specific plants (e.g., Boyle and Philogène 1983, Campbell and Motten 1985, Motten 1986, Shaw and Taylor 1986, McCall and Primack 1992).
Among visitation rate studies, site-to-site variation in fly numbers seems to be the norm (Toft 1983, Herrera 1988, Kearns and Inouye 1994). At one site, a particular fly may be the predominant pollinator, effecting high percentages of seed set, whereas in another plant population, that same species (or even the entire family of flies that it represents) is absent from the array of flower visitors (Mesler et al. 1980). Even within a single site, visitation rates can be affected not only by changes in fly abundance, but also changes in floral density, floral species available, and competition for nectar with other insects (Herrera 1988). On a shorter time scale, visitation rates are affected by temperature, wind, and light conditions (Primack 1978, Herrera 1988, McCall and Primack 1992). In general, studies that have looked at visitation patterns of pollinators over several years also note great variability among years (Herrera 1988, Schemske an