Thirty years of post-eruption insect succession on the Mount St. Helens volcano: The Coleoptera (beetle) community 1980-2010

Background/Question/Methods

The Mount St. Helens volcano in Washington State erupted explosively in May, 1980, creating an outward gradient of disturbance zones more than 50 km in radius.  These zones included the blast zone near Spirit Lake, the tree blow-down zone, the tephrafall zone, and undisturbed reference forests.  In addition, the post-eruption successional patterns were influenced by the pre-eruption site conditions of forest versus clearcut.  Beetles were important components of the post-eruption environment on Mount St. Helens because of the roles they played in pollination, herbivory, granivory, predator-prey interactions, decomposition and nutrient cycling, and soil disturbances.  We sampled the ground-dwelling beetle assemblages using pitfall traps at paired sites (pre-eruption forests and clearcuts) across the disturbance gradient.  Our goals were to (1) describe the beetle assemblages in terms of taxonomic and trophic composition for sites previously supporting forested and clearcut habitats, (2) compare these assemblages across the disturbance gradient from the highly-disturbed zone near Spirit Lake to the undisturbed sites well-outside the eruption zone, (3) examine the temporal changes (1980-2010) in species composition and abundances to ascertain the rate and extent of species turnover (“relay” succession) on different sites, and (4) evaluate the Intermediate Disturbance Hypothesis with respect to the beetle assemblages.  

Results/Conclusions

During the study, we collected over 35,000 Coleoptera specimens belonging to 295 species and 39 families.  Beetle assemblages differed substantially among forest and clearcut sites, and along the disturbance gradient.  Beetle taxa of undisturbed forests and clearcuts were dominated by fungivores, omnivores, herbivores and predators. In disturbed zones, early successional assemblages were composed of detritivore, scavenger and predator species (Carabidae, Agyrtidae, Tenebrionidae, Staphylinidae), which took advantage of dead plant materials and the “rain” of airborne arthropods falling into the highly-disturbed blast zone and tree blow-down zone.   As vegetation began to recover in these areas, herbivorous beetle species (Curculionidae, Chrysomelidae) and fungivores (Leiodidae) became more common.  As resources changed through time, beetle taxa responded accordingly; for example, lupine populations became spatially extensive and dense on in the blast zone during the 1990s, and noctuid moth populations increased dramatically as their caterpillar larvae fed on these plants.  Subsequently, the carabid beetle, Calosoma tepidum (the “caterpillar hunter”), colonized the area, and exhibited a tremendous population surge by 2005, driving down the noctuid prey populations.  Many other beetle taxa demonstrated patterns of population boom and bust dynamics.  However, little evidence was observed to support the Intermediate Disturbance Hypothesis across this volcanic disturbance gradient.