Pioneer Aeolian Community Development on Pyroclastic Flows after the Eruption of Mount St. Helens, Washington, U.S.A.

Abstract

The eruption of Mount St Helens in 1980 eliminated all biota from an area of ca. 80 km2 to the north of the crater. The recolonization of the mineral surface of the pyroclastic flows by arthropods was observed from 1981 to 1985. Diverse dispersing arthropods reached the site immediately after the eruption. Representatives of at least 150 families from 18 orders arrived in the devastated zone during the first 3 yr following the eruption. Among these immigrants 19 species showed evidence of established breeding populations on the primary mineral surfaces by 1985. Several carabid beetles in the genus Bembidion, notably B. planatum, B. improvidens, and B. obscurellum were the first arthropods known to breed on the pyroclastic flow; this occurred in spring 1981. Beetles predominated among the pioneers with 15 species of Carabidae, and one each of Agyrtidae, and Tenebrionidae. The hemipteran families Lygaeidae and Saldidae were also early pioneers. Pedestrian taxa (Grylloblattidae and Gryllacrididae) arrived on the pyroclastic flow after 3 yr. Primary succession at Mount St. Helens began with the development of an aeolian community of arthropod predators and scavengers, all of which were supported by an allochthonous resource of arthropod fallout.