Robert A. Cannings. 1998. Robber Flies (Insecta: Diptera: Asilidae) in Smith, I.M., and G.G.E. Scudder, eds. Assessment of species diversity in the Montane Cordillera Ecozone. Burlington: Ecological Monitoring and Assessment Network, 1998.

ROBBER FLIES
(INSECTA: DIPTERA: ASILIDAE)

Robert A. Cannings

ANALYSIS OF TRENDS IN SPECIES OCCURENCE AND ABUNDANCE

1. Grassland destruction

The most serious historic anthropogenic stress on robber fly populations has been the destruction of grassland habitat. The elimination of grasslands, usually at low and medium elevations in the southern valleys, for agriculture and housing, has undoubtedly reduced populations of robber flies, especially Intermontane species. Only about 10 per cent of the original grasslands in the Okanagan Valley remain in a relatively natural state and many are considerably disturbed. The Antelope-brush steppe in the extreme south of the valley is one of the most endangered habitats in Canada; it contains one of the richest assemblages of southern species in the nation. Only 40 per cent of the original habitat remains, and only 9 per cent of that is undisturbed. Overgrazing by cattle, disturbance by vehicles, and introduced weeds (Cheatgrass (Bromus tectorum) and Diffuse Knapweed (Centaurea diffusa) are perhaps the most noxious) in many remaining grasslands may have an effect on populations, but no studies have been undertaken in the ecozone that show this. Some overgrazed sites have dense and healthy populations of Scleropogon neglectus, Stenopogon inquinatus, Efferia benedicti and E. harveyi.

2. Logging

The extensive logging that has occurred in the ecozone has probably reduced the populations of forest species in both the mid-elevation Montane forests and the subalpine forests at higher altitudes. The suppression of forest fires, largely because of the interests of the forest industry, perhaps has had some negative affect on species such as Andrenosoma fulvicaudum, which depends on fires to create habitat for its wood-boring beetle prey (Fisher 1986). Suppression of fire has also allowed the invasion of pines and Douglas-firs into the grasslands. In the Rocky Mountain Trench, for example, the forest has taken over about 30 per cent of the grasslands since 1960 (Cannings and Cannings 1996), perhaps altering the makeup of the robber fly assemblage there.

3. Climate change

Climate change may significantly affect present asilid distributions if grasslands increase in extent and forest types shift in altitude and coverage. Hebda (1982, 1995) revealed that grassland-steppe vegetation was much more prevalent in the southern part of the Ecozone during warmer climatic regimes in the early to mid-Holocene than today. Arid grassland asilids such as Lasiopogon albidus Cole and Wilcox and L. chaetosus Cole and Wilcox now ranging as far north as the Columbia Basin in Washington State could readily enter the Canadian Montane Cordillera if xeric habitats proliferate. Likewise, components of the rich asilid fauna of the Cascade Mountains and montane forests of the Great Basin may move northward as the climate warms.