 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 and Horvitz 1989, Kearns and Inouye 1994, Pellmyr and Thompson 1996).
When spatiotemporal variability is large, resampling will be difficult. However, broad patterns of change in the types of floral visitors, such as those detected by Bañkowska (1980), may be apparent in disrupted habitats. Resampling of habitats that have remained intact could be combined with sampling disturbed areas containing the same plant species.
To give the reader a sense of what sorts of studies exist and how variable the data can be, I mention a few of the North American studies of visitation rates that span more than one season, and that could be easily replicated. This is not meant to be a comprehensive list of replicable studies.
Boyle and Philogène (1983) studied the variations in abundance and significance of native pollinators in an apple orchard in Ontario for three years. Apple blossoms were monitored for insect visitors throughout the blooming period each year. Weather conditions and numbers of apple blossoms were also recorded. Insects collected were noted to species, but insects visiting flowers were grouped into 20 categories that included seven groups of flies. 11,401 insects were observed in the three-year survey.
McCall and Primack (1992) compared insect visitation patterns to flowers in three different communities. Two of the three study sites were in North America: tundra habitat of White Mountain National Forest, New Hampshire at two different elevations (46° 16’N, 71° 18’W; 1664 m and 1539 m; 320 observation periods; 35 plant species; three growing seasons); Rock Meadow Conservation Area, Belmont, Massachusetts (42° 23’N; 71° 23’W; 2 years; 649 observation periods in 1982; 124 plant species, 392 observation periods in 1983, 108 plant species). Visitation was measured using the standardized 10-min visitation periods introduced by Arroyo et al. (1982). They counted any insects touching reproductive parts of flowers, and classified them to order in the field. Representative specimens were archived at Boston University. Pla