Volcanic eruptions create exemplary arenas for studying reassembly of biological communities. On May 18, 1980 Mount St. Helens (USA) underwent a violent explosive eruption that dramatically altered a >600km2 area containing forest, meadow, and riverine habitats. In the aftermath, a complex disturbance gradient developed. It extended ~40km north of the edifice, and included several structurally distinct zones defined by the initial types and intensities of geophysical forces and the carryover of residual biological and physical components of the pre-eruption systems. These zones included, from most to least disturbed: the pyroclastic flow zone, the tree blowdown zone, the tephrafall zone, and undisturbed reference forests. Arachnids are important components of ecological systems because of the role they play as primary consumers of insects, thus influencing pollination, herbivory, granivory, nutrient cycling, and soil development. Here we report results from sampling ground-dwelling arachnids using pitfall traps across the disturbance gradient during 1990, 2000, and 2010. Our goals are to: (1) describe arachnid community structure, (2) compare these assemblages across the disturbance gradient, and (3) examine the temporal changes (1990-2010) in species composition and abundances to ascertain the rate and extent of assemblage similarity and stability.
During our study, we collected over 16,700 arachnid specimens belonging to 109 species and 38 families. Species richness in the tephrafall zone followed by the tree blowdown zone - both sites of intermediate disturbances - reported the highest richness values, indicating support for the Intermediate Disturbance Hypothesis. Comparisons of the similarity of species composition within sites and across time showed that with an increased level of disturbance, species composition was less similar throughout time. When comparing similarities of species composition between sites during the same sampling period, we observed that arachnid assemblages in the tephrafall zone became more similar to those of the undisturbed reference forest, while pyroclastic flow (most disturbed) assemblages became less similar to the reference forest over time. Furthermore, we observed boom and bust patterns in both early (e.g. Micaria constricta) and late colonizers (e.g. Phalangium opilio). Highly disturbed sites remain taxonomically distinct from reference sites 30 years post-eruption, and the extent to which they will converge remains unclear.