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The 1980 eruption of Mount St. Helens created an outstanding opportunity to investigate mammal community assembly during primary succession. From 1983 through 2015, we documented the arrival of 34 of the 45 mammals in the regional species pool and the successful establishment of 25 species. The majority of small mammals that established were likely derived from source populations that survived in isolated refugia in adjacent areas that were less disturbed during the eruption, requiring dispersal distances of a few to several kilometers. In contrast, large mammals arrived from more distant source populations—tens of km away. The next important transition in mammal community assembly will likely occur in three or four decades, as shrub cover and coniferous tree density increases, leading to development of open-forest conditions that provide habitat for forest-associated species not yet established and concomitant decline of early-seral mammal species.

Spirit Lake was grossly altered by the 1980 eruption of Mount St. Helens and all air-breathing life perished. A rainbow trout (Oncorhynchus mykiss) population was established circa 1991 and monitored from 2000 to 2014. During that time, the trout population increased, and individual fish size significantly decreased (both length and mass). Fish diets were primarily composed of small prey items, mostly aquatic insects and aquatic snails, and piscivory was not observed. The relatively recent (2011) observation of juvenile fish occupying streams draining into Spirit Lake suggests an increased suitability of stream habitat for both spawning and rearing, which may account for the changing population dynamics and annual variation.

A review of published studies of terrestrial and freshwater ecosystem responses to disturbance by volcanic processes reveals some unifying themes: most eruption events leave biological legacies of the pre-disturbance ecosystems, and the course of post-disturbance succession involves the protracted interplay of these legacies with immigrating species, biotic interactions, site amelioration, and secondary biological and geophysical disturbance processes. Research at Mount St. Helens has been a major contributor to this body of work dating from 1883 when the eruption of Krakatau marked the beginning of ecological studies of recent eruptions.

Lichens are symbiotic composite organisms made of a fungus (ascomycete or rarely basidiomycete), plus an alga or a cyanobacterium, or occasionally all three. They can be important components of post-eruption vegetation recovery because of their ability to fix nitrogen, which is absent or at low levels in new volcanic deposits. Patterns of macrolichen species richness, abundance, and community composition are described and interpreted for different volcanic disturbances created by the 1980 eruption of Mount St. Helens and compared to nearby control areas not affected by the eruption. We found variable responses of different groups of lichens to a variety of volcanic disturbances produced by a single eruption.

This book synthesizes understanding of ecological change in the complex and changing environment around the Mount St. Helens volcano, which is unique in the study of volcanic eruptions and subsequent ecological recovery. The variety and large spatial extent of the primary disturbance and subsequent ecological and geophysical processes that continue to unfold create rich opportunities for addressing numerous ecological questions at the volcano. Details of the 1980 eruption along with key concepts and lessons learned over the first decades of study at Mount St. Helens are presented, and key findings are discussed.

The numerous ponds formed in the weeks following the 1980 eruption of Mount St. Helens provided a unique opportunity to investigate macroinvertebrate communities in young (23- to 25-year-old) fishless ponds following a large natural disturbance. Over a 3-year period, Large-bodied macroinvertebrates were sampled from 97 ponds with a range of hydroperiods and biophysical characteristics. Macroinvertebrate communities from these ponds were spatially and temporally variable in spite of their close geographical proximity, shared regional species pool, and similar creation history. We identified 110 taxa, many of which were observed at only one pond or in only one year. Community differences appeared to be influenced more by pond size and riparian tree development rather than by hydroperiod alone, and stochastic colonization events were likely an important factor determining community composition.

Volcanic eruptions are major agents of change to earth's ecological systems. Lava flows, tephra deposition, lahars, and other volcanic processes function as ecological disturbance agents that vary in their intensity, spatial extent, and impacts to ecosystems. Typically, in the aftermath of volcanic disturbance legacies of organisms and other organic matter survive from the pre-disturbance ecosystem and these can profoundly influence post-eruption ecosystem assembly. In cases such as volcanoes emerging from beneath the sea or where there are thick pumice deposits or lava flows, primary ecological succession occurs on entirely new substrates. The course of ecosystem response is regulated by nutrient limitations, available colonists, climate, biotic interactions, and other factors. Of the 404 volcanoes with reported eruptions since the famous 1883 eruption of Krakatau, only 11% have published ecological studies, and of those only three iconic volcanoes – Krakatau, Surtsey and Mount St. Helens – have been subjects of long-term, multi-taxa ecological studies.

Spirit Lake, Washington was radically altered limnologically by the May 1980 eruption of Mount St. Helens. The eruption provided a rare opportunity to study lake response and recovery in the wake of volcanic disturbance. During the eruption, and for several months thereafter, phytoplankton and zooplankton populations were subjected to extremely deleterious conditions. Consequently, these populations were virtually eliminated except for remnant organisms that somehow survived. During the next two years, the phytoplankton community and presumably the zooplankton community were comprised of only a few opportunistic species whose combined abundance was low. By 1983, however, phytoplankton abundance and species diversity had greatly increased due to increased lake-water transparency and increased availability of inorganic nitrogen. The reestablishment of the zooplankton community was also well underway by 1983, as indicated by the abundance of some species and the presence of most taxa that existed pr...

We explored the association between Van Dyke's Salamander (Plethodon vandykei) and hydrologic condition, geomorphology, and vegetation structure in headwall seeps in the Cascade Range of Washington State. After conducting salamander surveys and measuring habitat characteristics at 40 seep sites from July to August of 2002, we modeled occurrence of P. vandykei at three site scales: between seeps, within seeps, and between microhabitat sites. We ranked a priori models using Bayesian Information Criterion (BIC). Using logistic regression, with presence and absence as the response, we found best approximating models for the occurrence of P. vandykei at the three site scales is predicted by hydrological and geological habitat characteristics. Between seeps, the probability of the occurrence of P. vandykei increased with increasing proportions of seep face having both dry and sheeting hydrology, and increasing proportions of seep face > 5 m high. Within seeps, the probability of the occurrence of P. vandykei was negatively associated with seeps where total overhead cover was > 25%. Between microhabitat sites, the probability of the occurrence of P. vandykei was positively associated with increases in the percent cover of small cobble, small gravel, and bedrock. P. vandykei appears to be associated with habitats that maintain cool thermal and hydric conditions favorable for a species that is sensitive to heat and desiccation due to physiological constraints.

The 2008-2009 eruption of Chaiten Volcano (Chile) involved a variety of volcanic and associated hydro- logic processes that damaged nearby forests. These processes included coarse (gravel) and fine (silt to sand) tephra fall, a laterally directed blast, fluvial deposition of remobilized tephra, a variety of low-temperature mass-movement processes, and a pyroclastic flow. Each of these geophysical processes constitutes a type of ecosystem disturbance which involves a distinctive suite of disturbance mechanisms, namely burial by tephra and sediment, heating, abrasion, impact force, and canopy loading (accumulation of tephra in tree crowns). Each process affected specific areas, and created patches and disturbance gradients in the forest landscape. Coarse tephra ('gravel rain', >5 cm depth) abraded foliage from tree canopies over an area of approximately 50 km 2 north-northeast of the vent. Fine tephra (>10 cm depth) accumulated in tree crowns and led to breakage of branches in old forest and bowing of flexible, young trees over an area of about 480 km 2 . A directed blast down the north flank of the volcano damaged forest over an area of 4 km 2 . This blast zone included an area of tree removal near the crater rim, toppled forest farther down the slope, and standing, scorched forest around the blast perimeter. Fluvial deposition of >100 cm of remobilized tephra, beginning about 10 days after initiation of the erup- tion, buried floodplain forest in distinct, elongate streamside patches covering 5 km 2 of the lower 19 km of the Rayas River and several km 2 of the lower Chaiten River. Across this array of disturbance processes the fate of affected trees varied from complete mortality in the tree removal and pyroclastic flow areas, to no mortality in areas of thin tephra fall deposits. Tree damage included defoliation, loss of branches, snapping of tree trunks, abrasion of bark and ephiphytes, and uprooting. Damaged trees sprouted from epicormic buds located in trunks and branches, but sprouting varied over time among disturbance mechanisms and species. Although some effects of the Chaiten eruption are very similar to those from the 1980 eruption of Mount St. Helens (USA), interactions between biota and geophysical processes at Chaiten produced some unique effects. Examination of vegetation response helps interpret geophysical processes, and disturbance mechanisms influence early stages of biotic response to an eruption.

Catastrophic disturbances often provide "natural laboratories" that allow for greater understanding of ecological processes and response of natural populations. The 1980 eruption of the Mount St. Helens volcano in Washington, USA, provided a unique opportunity to test biotic effects of a large-scale stochastic disturbance, as well as the influence of post-disturbance management. Despite severe alteration of nearly 600 km2 of habitat, coastal tailed frogs (Ascaphus truei) were found within a portion of the blast area five years after eruption. We investigated the genetic source of recolonization within the blast area and tested whether post-eruption salvage logging and subsequent tree planting influenced tailed frog movement patterns. Our results support widespread recolonization across the blast area from multiple sources, as all sites are grouped into one genetic cluster. Landscape genetic models suggest that gene flow through the unmanaged portion of the blast area is influenced only by distance between sites and the frost-free period (r2 = 0.74). In contrast, gene flow pathways within the blast area where salvage logging and replanting occurred post-eruption are strongly limited (r2 = 0.83) by the physiologically important variables of heat load and precipitation. These data suggest that the lack of understory and coarse wood (downed and standing dead tree boles) refugia in salvaged areas may leave frogs more susceptible to desiccation and mortality than those frogs moving through the naturally regenerated area. Simulated populations based on the landscape genetic models show an increase in the inbreeding coefficient in the managed area relative to the unmanaged blast area. In sum, we show surprising resilience of an amphibian species to a catastrophic disturbance, and we suggest that, at least for this species, naturally regenerating habitat may better maintain long-term genetic diversity of populations than actively managed habitat.

The sensational 1980 eruption ofMount St.Helens and the subsequent ecological responses are the most thoroughly studied volcanic eruption in theworld. The posteruption landscapewas remarkable, and nearly a quarter century of study has provided awealth of information and insight on a broad spectrum of ecological and physical responses to disturbance. The eruption and its effects on ecological and geophysical systems have many dimensions: a complex eruption affected an intricate landscape containing forests, meadows, lakes, and streams populated by diverse fauna and flora. This complexity created a rich environment and an exemplary living laboratory for study. Because the volcano is in close proximity to major metropolitan areas, scientists were able to perform reconnaissance trips and establish a network of permanent plots within days to months of the eruption. These early observations enabled scientists to assess the initial impacts of the eruption, which was important in understanding the subsequent quarter century of invasion and succession. Suddenly, and almost beyond comprehension, at 8:32 a.m. on May 18, 1980, and lasting for little more than 12 hours, the eruption of Mount St. Helens transformed more than 600 km2 of lush, green forest and meadows and clear, cold lakes and streams to a stark gray, ashand pumice-covered landscape (see Figure 1.1; Swanson et al., Chapter 2, this volume; Swanson and Major, Chapter 3, this volume). The area influenced by the eruptive events will respond to them for hundreds or even thousands of years. However, even within the 24 years since the eruption, substantial change took place as hill slopes gradually turned from gray to green, opaque lakes cleared, and streams flushed sediment from their channels. Some of the initial ecological responses are well advanced; others have been set back by secondary disturbances; and yet others, such as soil development, will respond to the eruption over millennia. The major 1980 eruption created distinctive disturbance zones that differed in the types and magnitudes of impacts on terrestrial and aquatic systems, including the types and amounts of surviving organisms and other legacies of the preeruption ecological systems (Figure 20.1). Thereafter, the natural system consisting of surviving and colonizing plants, fungi, animals, andmicrobes began responding to the new conditions. During the subsequent decades, species diversity, plant cover, and vegetation structure (the size and shape of plants) developed rapidly. Vegetation in 2005 ranged from herbs and scattered shrub cover in the severely disturbed pyroclastic-flow zone to the continuous canopy of young forest in tree plantations around the perimeter of the blast area. The story of this collective ecological response to the 1980 eruption of Mount St. Helens involves both successional change over time at individual sites and development of landscape patterns. The 1980 eruption provided a special opportunity for scientists from a variety of disciplines to study ecological survival and establishment after a large disturbance, but several caveats challenge efforts to integrate this information. Developing a synthesis of ecological responses to the eruption is complicated by three factors

Disturbance, Survival, and Succession: Understanding Ecological Responses to the 1980 Eruption of Mount St. Helens.- Geological and Ecological Settings of Mount St. Helens Before May 18, 1980.- Physical Events, Environments, and Geological-Ecological Interactions at Mount St. Helens: March 1980-2004.- Survival and Establishment of Plant Communities.- Plant Responses in Forests of the Tephra-Fall Zone.- Plant Succession on the Mount St. Helens Debris-Avalanche Deposit.- Geomorphic Change and Vegetation Development on the Muddy River Mudflow Deposit.- Proximity, Microsites, and Biotic Interactions During Early Succession.- Remote Sensing of Vegetation Responses During the First 20 Years Following the 1980 Eruption of Mount St. Helens: A Spatially and Temporally Stratified Analysis.- Survival and Establishment of Animal Communities.- Arthropods as Pioneers in the Regeneration of Life on the Pyroclastic-Flow Deposits of Mount St. Helens.- Posteruption Arthropod Succession on the Mount St. Helens Volcano: The Ground-Dwelling Beetle Fauna (Coleoptera).- Causes and Consequences of Herbivory on Prairie Lupine (Lupinus lepidus) in Early Primary Succession.- Responses of Fish to the 1980 Eruption of Mount St. Helens.- Amphibian Responses to the 1980 Eruption of Mount St. Helens.- Small-Mammal Survival and Colonization on the Mount St. Helens Volcano: 1980-2002.- Responses of Ecosystem Processes.- Mycorrhizae and Mount St. Helens:Story of a Symbiosis.- Patterns of Decomposition and Nutrient Cycling Across a Volcanic Disturbance Gradient: A Case Study Using Rodent Carcasses.- Lupine Effects on Soil Development and Function During Early Primary Succession at Mount St. Helens.- Response and Recovery of Lakes.- Lessons Learned.- Ecological Perspectives on Management of the Mount St. Helens Landscape.- Overview of Ecological Responses to the Eruption of Mount St. Helens: 1980-2005.

In this paper we summarize small mammal studies on Mount St. Helens since its catastrophic eruption in May 1980. Species surviving the initial eruption ("residuals") have been described by Andersen and MacMahon (1985a). Herein we document recolonization of small mammals [Rodentia, Insectivora, Lagomorpha and Carnivora (Mustelidae)] into representative sites of four broadly defined montane habitats (forests, clearcuts, subalpine meadows and riparian ravines) that were subjected to increasing degrees of volcanic disturbance (undisturbed, tephrafall, mud flow, tree blowdown and pyroclastic/ debris flow). We live-trapped small mammals on 19 sites (with extensive population studies on 12 sites) from 1980-1987. By 1987, of a hypothetical list of 32 resident small mammal species, six species were captured or observed in the pyroclastic/debris flow zone, 15 species in the tree blowdown zone, and 22 species in the tephrafall zone. Small mammal communities in forest and clear-cut habitats, and across different disturbance zones, exhibited little species similarity among sites, even after 7 years of posteruption recovery. Small mammal species composition in several subalpine meadows converged through time to that found on a clearcut site in a tephrafall zone. Pre-eruption conditions, such as the patchiness of habitats and small mammal populations, and stochastic events, such as (1) the initial survival of small "islands" of vegetation and resident mammals; (2) localized habitat alteration by posteruption erosional processes, and (3) an unusually wet summer (1983) followed by 3 yr of summer drought (1984-1986), had a major influence on small mammal recolonization in this complex mosaic of habitats. We argue that observed differences in species recolonization and establishment success were more attributable to the availability of requisite food and shelter resources than to differences in species-specific dispersal capabilities. INTRODUCTION The explosive eruption of Mount St. Helens on 18 May 1980 resulted in a large-scale destruction of montane/subalpine forest and meadow habitats and caused the local extirpation of many small mammal species (Franklin et al., 1985). At least 14 species of small mammals survived the initial eruption and concomitant habitat alterations (Andersen, 1982; Andersen and MacMahon, 1985a). Individuals of many "residual" species survived the volcanic blast by virtue of their fossorial habits; being in underground burrows at the time of the early morning (0832 PDT) eruption shielded them from the rapidly moving hot gasses and tephra. Although the blast virtually annihilated all exposed plants and arthropods in nearby areas N of the volcano, the existence of underground plant parts and invertebrates, combined with a limited "rain" of recolonizing arthropods onto the disturbed areas (Edwards et al., 1986), provided an adequate food supply for some of the surviving mammals. Localized 'Present address: Department of Biology, University of New Mexico, Albuquerque 87131 2 Present address: World Wildlife Fund, 1250 24th St. NW, Washington, D.C. 20037. 3 Present address: Mount St. Helens National Volcanic Monument Headquarters, Route 1, Box 369, Amboy, Washington 98601.