SPIDER ARRIVAL AND PRIMARY ESTABLISHMENT ON TERRAIN DEPOPULATED BY VOLCANIC-ERUPTION AT MOUNT ST-HELENS, WASHINGTON

The 1980 eruption of Mount St. Helens created a large area (the Pumice Plain) which initially lacked any resident arthropods. From 1981-1986, ballooning spiders made up over 23% of windblown arthropod fallout and contributed 105 individuals per m(2) (91 mg dry biomass) over 1 summer. The family Lycosidae included 49% of all specimens but only 9% of all species; Linyphiidae included 34% of specimens and 50% of species. The 125 spider species taken varied widely in ballooning phenology. By 1986, six species (the lycosids Pardosa wyuta and P. lowriei and the linyphiids Erigone dentosa, E. aletris, E. capra and Walckenaeria pellax) had established reproducing populations on the Pumice Plain, almost entirely at sites already colonized by vegetation. Successful colonists showed distinct phenological patterns (compared to subpopulations composed entirely of immigrants) and, in Pardosa, direct evidence of reproduction (egg sacs and progeny). Most incoming spiders were apparently unable to reproduce on the Pumice Plain, which thus became a reproductive sink for their taxa. Aerial dispersal is of prime importance in recolonization of devastated terrain, and its extent in other situations has been underestimated.