d 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. Plant species were observed in proportion to their abundance in the habitat. Flies made roughly 10% of the visits to flowers at the woodland site, and over 90% of the visits at the tundra site.
Catherine Reed (1995a, b) monitored species richness of insects on prairie flowers in southeastern Minnesota for three years. She worked in eight separate prairie remnants or prairie reconstruction sites, hand-collecting all insects during 15-min sampling sessions on each plant species that had at least 100 open flowers (59 plant species). The focus of the study was to compare diversity in reconstructed sites with diversity in remnant prairie patches. Collections were made from late May through late September on sunny days when temperatures were between 20° and 35°C. Insects were identified to species. Sixty fly species were collected, some in reconstructions, others in remnants, some in both. Reed revisited the sites in the summer of 1999, and says the study is “quite repeatable,” although some of the sites have changed. Native prairie sites are now managed more intensely with brush-cutting, burning, and planting of seeds (C. Reed, personal communication).
Kearns (1990) and Kearns and Inouye (1994) monitored visitation rates to Linum lewisii (Linaceae) for three summers at three different elevations in the area of Rocky Mountain Biological Laboratory, Gothic, Colorado (38.5°N, 106.5° W; 2780 m, 2886 m, and 3475 m). Visitation data were recorded for bees and fly families. Fly visitation rates differed significantly among sites, but did not differ significantly among years. However, additional visitation data collected at the 2886m site in the two proceeding years showed that annual variation in visitation within a site can be large, in this case, as a result of an Entomophora outbreak in 1986 that reduced fly numbers. Visitation rates to Linum are highly correlated with Malaise trap fly samples from the same site (Kearns and Inouye 1994).
An attempt to document the conservation status of fly pollinators in North America revea