Ballooning Spiders as a Component of Arthropod Fallout on Snowfields of Mount Rainier, Washington, U.S.A.

Abstract

In 1975 and 1976, fallout (aerially dispersing arthropods trapped on snow) was collected from large permanent alpine snowfields on Mount Rainier (4392 m), Cascade Mountains. Spiders comprised 5.5% of all specimens collected, being fifth in abundance after Homoptera, Heteroptera, Hymenoptera, and Diptera. Overall, spider fallout density decreased steadily with elevation, probably reflecting a decrease of spider density in the atmosphere with altitude. Spiders from distant sources reached higher elevations. From April to October, spider fallout density (and thus spider dispersal in this area) showed a moderate peak in May/June, a minimum in September, and a high peak in October, similar to patterns known elsewhere. Only 5 of 23 spider species found in fallout were previously known as ballooners. Species ballooning as adults belonged to the families Araneidae, Theridiidae, Salticidae, and Thomisidae, not just Linyphiidae as previously reported. Analysis of probable origins of fallout species shows the importance of major landforms and weather patterns to success of spider aerial dispersal. Compared with embarkment and flight of ballooners, their immigration has been little studied; alpine fallout is an important potential source of such data.