On May 18, 1980, the Mount St. Helens volcano in the northwestern USA underwent an energetic explosive eruption, involving a complex ensemble of disturbance processes that dramatically altered ~600 km2area. Disturbance processes included an enormous debris avalanche, a powerful lateral-blast pyroclastic density current (PDC), multiple lahars, tephra falls, pyroclastic flows, and creation of a new lava dome. The intensity of disturbance decreased with distance from the crater, creating a disturbance gradient affecting the types, numbers and distribution of plants and animals within the pre-eruption ecosystems. All flora and fauna were killed in the area of lateral blast and debris avalanche, while differential mortality extended outward along the 30 km disturbance gradient. We took advantage of the outstanding research opportunities created by the eruption by establishing a constellation of long-term permanent study plots along the disturbance gradient, and sampling them every 1 to 5 years from 1980 through 2015. We report on patterns of initial survival and longer-term rates of community assembly for small mammals, birds, amphibians, and a number of arthropod taxa, examining long-term inter-taxa differences in species recruitment, loss and turnover rates.
Species survival was influenced by disturbance intensity, species natural histories, and site conditions at the time of the eruption (season and time of day). All birds within the 600 km2 blast PDC area were instantly killed, contrasting with some small mammals, amphibians, fishes, reptiles and arthropods that survived in all but the most disturbed areas, albeit in greatly reduced numbers and confined to isolated refugia. Within minutes of the eruption, animals began to colonize the disturbed landscape; most found the area inhospitable and either perished or left. Overall, the pattern and rate of community assembly was influenced by biological legacies and plant/soil/litter development, rather than species’ dispersal capabilities. For both birds and small mammals, secondary-successional community assembly was more rapid in the blowdown zone, where there were abundant biological legacies, than in primary succession sites. The complete suite of pond-breeding amphibian species colonized primary successional ponds by the ninth post-eruption year. Some invertebrate taxa showed rapid turnover rates as soils and plant assemblages developed. Recruitment of vertebrates and some invertebrates was cumulative, steadily increasing with minimal species losses; however, complete recovery of fauna is still decades away. Mount St. Helens continues to provide exceptional research opportunities, with ~140 faunal studies published over 35 years.