Search

Airborne volcanic ejecta (tephra) can strongly influence forest ecosystems through initial disturbance processes and subsequent ecological response. Within a tephra-disturbed forest, large trees may promote plant growth and create favorable sites for colonization. Three primary ways trees can influence posteruption vegetation response include: (1) amelioration of volcanic substrates, (2) providing source propagules from the tree or from associated epiphytes, and (3) sheltering understory vegetation, thereby increasing the rate of recovery near tree bases. Here, we evaluate Valdivian temperate rainforest understory vegetation responses in close proximity to large trees that survived the 2015 eruption of Calbuco Volcano. Understory vegetative cover was higher near the base of trees for mosses, many epiphytes, and some herbaceous, shrub, and trees species. However, significant interactions with year of measurement, and individualistic responses by many species made generalizations more difficult. Shrubs and trees in particular demonstrated patterns of recovery that were frequently independent of distance. In some cases, percent cover of colonizing vegetation actually increased away from trees by 2019. The soil surface was similarly variable where bare soil cover was associated with locations proximal to tree bases, but material shed from living and dead standing vegetation increased wood and litter abundances on the soil surface away from the base of trees. Soils near trees had lower pH, elevated organic matter, and higher nitrogen and carbon. Our results support the assertion that in this temperate rainforest ecosystem large trees may provide important early refugia for vegetative regrowth following a tephra-fall event with altered edaphic conditions. Nevertheless, individualistic dynamics of different species and growth forms suggest the influence of large trees on nearby understory plants is more complex than a simple facilitative model might suggest.

Four decades of research into biophysical responses to the 1980 eruption of Mount St. Helens have vastly improved our understanding of how landscapes react to cataclysmic disturbances.

Los grandes disturbios tienen un rol fundamental en la dinamica de los bosques de Nothofagus a traves de la generacion de grandes aperturas del dosel que crean condiciones ambientales espacialmente heterogeneas para el posterior establecimiento de la regeneracion (Gonzalez et al. 2014). En el caso del vulcanismo, uno de estos agentes de grandes disturbios, si bien se reconoce su importancia en la dinamica de los bosques andinos (Veblen, 1985), el papel de la ceniza volcanica como impulsor de establecimiento y agente de mortalidad en estos bosques ha sido escasamente estudiado (e.g. Vogel, 1996).

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.

Spirit Lake was changed biologically, chemically, and physically by the 1980 eruption of Mount St. Helens and the concurrent landslide that essentially buried the old lake and created a new one in its place. Recent changes to the surrounding watershed, spurred by ever-increasing regrowth of terrestrial vegetation, continue to alter the biogeochemistry of Spirit Lake. Here the authors outline over a decade of work developing a hydrologic and nutrient model for the lake and discuss preliminary work to elucidate the ecological role of the floating logs in Spirit Lake.

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.

We documented the recruitment, loss, and turnover of 51 species of ground beetles during 30 years of post-eruption primary succession on the Pumice Plain near Spirit Lake at Mount St. Helens. We observed high species turnover, with many species colonizing, expanding their populations, and then declining to local extinction over periods of 10–15 years. Species patterns were related to soil and vegetation development, accrual of prey species, and natural-history characteristics of the resident species (e.g., trophic role, dispersal traits (flight/ambulatory), microhabitat preferences, seasonal/daily activity times). Overall, species successional turnover rates were substantially higher than plants and vertebrate wildlife species.

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.

Mount St. Helens and the surrounding landscape were dramatically altered during and after the 18 May 1980 eruption, creating large expanses of disturbed land that could be susceptible to exotic plant invasion. We studied spatial patterns of exotic plant richness and abundance along a road and pedestrian trail that traverse a volcanic disturbance gradient with varying management prescriptions. In blast areas, exotic plant richness and abundance increased near the road, demonstrating the role that both roads and canopy cover play in the dispersal and establishment of exotic species. However, the overall richness and abundance of exotic plants were relatively low, and the pyroclastic-flow zone vegetation was dominated by native plants.

Formation of stable soil organic matter is typically the result of a relatively slow series of decomposition processes that can be constrained in early successional sites. Alternatively, compounds such as glomalin, a glycoprotein produced by arbuscular mycorrhizal fungi, may form relatively early during soil development and improve aggregate stabilization, water infiltration, and carbon and nitrogen storage. After 31 years of development, significant amounts of soil C, N, and BRSP, an indicator of glomalin, had accrued in pyroclastic deposits, in patterns affected by both plant community type and soil depth. Mycorrhizal fungi are important, but incompletely understood, drivers of pedogenic processes during primary succession and may exert disproportionate effects on soil processes and plant development prior to the accumulation of humified soil organic matter.

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.

Mycorrhizae are symbiotic mutualisms between plants and fungi, in which carbon is exchanged for nutrients. The eruption of Mount St. Helens was a large event that covered a topographically complex land area with disturbances of varying intensity that altered survival of soil organisms. Animals from rodents to elk fed upon and transported mycorrhizal fungi through the different disturbance zones, facilitating succession and altering the newly forming soils and vegetation. Mechanisms of interaction among individual combinations of mycorrhizal fungi, plants, and animals were predictable, but the different species combinations resulting from initial survival legacies created the dynamic and complex landscape we observe today.

Early-successional forest ecosystems that develop after stand-replacing or partial disturbances are diverse in species, processes, and structure. Post-disturbance ecosystems are also often rich in biological legacies, including surviving organisms and organically derived structures, such as woody debris. These legacies and post-disturbance plant communities provide resources that attract and sustain high species diversity, including numerous early-successional obligates, such as certain woodpeckers and arthropods. Early succession is the only period when tree canopies do not dominate the forest site, and so this stage can be characterized by high productivity of plant species (including herbs and shrubs), complex food webs, large nutrient fluxes, and high structural and spatial complexity. Different disturbances contrast markedly in terms of biological legacies, and this will influence the resultant physical and biological conditions, thus affecting successional pathways. Management activities, such as post-disturbance logging and dense tree planting, can reduce the richness within and the duration of early-successional ecosystems. Where maintenance of biodiversity is an objective, the importance and value of these natural early-successional ecosystems are underappreciated.

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 Larch Mountain salamander (Plethodon larselli Burns, 1954) is an endemic species in the Pacific northwestern United States facing threats related to habitat destruction. To facilitate development of conservation strategies, we used DNA sequences and RAPDs (random amplified polymorphic DNA) to examine differences among populations of this species. Phylogenetic analyses of cytochrome b revealed a clade of haplotypes from populations north of the Columbia River derived from a clade containing haplotypes from the river's southwestern region. Haplotypes from southeastern populations formed a separate clade. Nucleotide diversity was reduced in northern populations relative to southern populations. These results were corroborated by analyses of RAPD loci, which revealed similar patterns of clustering and diversity. Network analyses suggested that northern populations were colonized following a range expansion mediated by individuals from populations located southwest of the river. Changes in the Columbia River's location during the Pliocene and Pleistocene likely released distributional constraints on this species, permitting their northern range expansion. Based on the barrier presented by the Columbia River's present location and differences in haplotype diversity and population structure observed between northern and southern populations, we suggest that designation of separate management units encompassing each region may assist with mitigating different threats to this species.

Fossorial mammals may affect nutrient dynamics and vegetation in recently initiated primary successional ecosystems differently than in more developed systems because of strong C and N limitation to primary productivity and microbial communities. We investigated northern pocket gopher (Thomomys talpoides) effects on soil nutrient dynamics, soil physical properties, and plant communities on surfaces created by Mount St. Helens' 1980 eruption. For comparison to later successional systems, we summarized published studies on gopher effects on soil C and N and plant communities. In 2010, 18 years after gopher colonization, we found that gophers were active in ~2.5% of the study area and formed ~328 mounds ha(-1). Mounds exhibited decreased species density compared to undisturbed areas, while plant abundance on mound margins increased 77%. Plant burial increased total soil carbon (TC) by 13% and nitrogen (TN) by 11%, compared to undisturbed soils. Mound crusts decreased water infiltration, likely explaining the lack of detectable increases in rates of NO3-N, NH4-N or PO4-P leaching out of the rooting zone or in CO2 flux rates. We concluded that plant burial and reduced infiltration on gopher mounds may accelerate soil carbon accumulation, facilitate vegetation development at mound edges through resource concentration and competitive release, and increase small-scale heterogeneity of soils and communities across substantial sections of the primary successional landscape. Our review indicated that increases in TC, TN and plant density at mound margins contrasted with later successional systems, likely due to differences in physical effects and microbial resources between primary successional and older systems.

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 three major types of disturbed habitats resulting from the eruption of Mount St Helens were a sterile pyroclastic flow site, a blast zone with all of the vegetation destroyed and tephra deposited on the surface, and a high ashfall region with deposited tephra over surviving plants. We present data assessing the fate of the invading mycorrhizal fungi and their associated plants by characterizing vesicular-arbuscular and ectomycorrhizal fungi from soil and root samples from sites within the three disturbance zones over the decade following the 1980 eruption. We also assess the importance of gopher-dispersed old soil containing VA mycorrhizal inoculum. Both ecto- and VA mycorrhizas re-established in disturbed areas primarily through inoculation from the buried old soils, a process probably facilitated by gophers. Mycorrhizas always formed at a quicker rate in the less disturbed areas of the eruption site. Detailed observations on the sterilized pumice plain indicated that, through 1990, the ectomycorrhizas were poorly developed and no fruiting structures were seen. Three possible hypotheses are proposed for the slow development of ectomycorrhizas despite their being wind-dispersed on to the site, and may be related to either slow soil development and/or compatibility restrictions. These results indicate that the magnitude of the disturbance regulates the rate of recovery not only of the vegetation, but also of both the VA and ectomycorrhizal fungi.

Arthropods are important components of ecosystems because of the roles they play in pollination, herbivory, granivory, predator–prey interactions, decomposition and nutrient cycling, and soil disturbances. Many species are critical to the structure and functioning of their ecosystem, although some (particularly insects) are considered pests in farmlands and forests because of their detrimental effects from feeding on foliage and transferring pathogens to trees and crops. Arthropods also constitute a high-protein prey resource for vertebrate wildlife (especially small mammals, birds, reptiles, and amphibians), thus contributing to the existence and stability of thesewildlife species. As such, studies of arthropod population dynamics and changes in species assemblages following natural disturbances are important for understanding ecosystem responses. In the case of the Mount St. Helens volcanic eruption, studies of arthropods not only can provide information on natural history and ecology of many different species but also are relevant for evaluating theories of disturbance ecology and postdisturbance successional processes. The 1980 eruption ofMount St. Helens provided researchers with an opportunity to test a wide range of theories concerning the structure and functioning of ecosystems. In particular, the existence of a continuum of disturbance intensity across a large landscape made possible a suite of comparative studies that evaluated the influence of different levels of volcanic disturbance on the survival and initial recolonization patterns of plants and animals. For example, researchers to date have documented the survival and reestablishment of a number of plant species and described the patterns and rates of vegetation successional processes of the disturbed ecosystems (see Lawrence, Chapter 8, this volume;Antos andZobel, Chapter 4, this volume; Dale et al., Chapter 5, this volume; del Moral et al., Chapter 7, this volume; and references therein). In addition, numerous faunal studies havequantified the eruption’s impacts on survival and subsequent short-term responses of small mammals (Andersen 1982;Andersen andMacMahon 1985a,b; Adams et al. 1986a; Johnson 1986; MacMahon et al. 1989; Crisafulli et al., Chapter 14, this volume), birds (Andersen andMacMahon 1986), amphibians (Karlstrom 1986; Hawkins et al. 1988; Crisafulli and Hawkins 1998; Crisafulli et al., Chapter 13, this volume), and arthropods (Edwards et al. 1986; Sugg 1989; Edwards and Sugg 1993; Crawford et al. 1995; Sugg and Edwards 1998; Edwards and Sugg, Chapter 9, this volume). Information collected on the fauna and flora of Mount St. Helens during the past 20 years facilitates the analysis of recolonization patterns in the context of two ecological theories: relay successional processes (MacMahon 1981) and the intermediate-disturbance hypothesis (Connell 1978). The term relay succession refers to the sequential replacement of species (plant and animal) in an ecosystem recovering from some form of disturbance. This process typically begins with species that either survived the disturbance or immigrated to the site shortly thereafter. Some of these species are well adapted to the disturbed conditions of the site and can greatly increase in abundance, whereas others are poorly adapted and become locally extinct. Biotic interactions (competition, predation, herbivory, and parasitic and disease infections), coupled with abiotic factors (extremes of temperature or moisture), often determine the success or failure of each species survival. Through time, as different species colonize the site, they alter the environment’s characteristics (e.g., plant regrowth provides shade, cools soil surface temperatures, increases soil moisture and organic matter, and provides substrate for fungi and vegetation for herbivores). As the environmental conditions change, new opportunities are created for additional species to colonize and dominate, eventually replacing established species that have become competitively inferior in the altered environment; hence, the “relay” of species during postdisturbance succession. This process applies to both plant and animal species assemblages and inherently involves complex interactions among plants and animals (MacMahon 1981). The second theory, the intermediate-disturbance hypothesis (Connell 1978), addresses the patterns of species richness

Primary succession, the formation and change of ecological communities in locations initially lacking organisms or other biological materials, has been an important research focus for at least a century (Cowles 1899; Griggs 1933; Eggler 1941; Crocker and Major 1955; Eggler 1959; Miles and Walton 1993; Walker and del Moral 2003). At approximately 60 km2, primary successional surfaces at Mount St. Helens occupy a minor proportion of the blast zone, yet they are arguably the most compelling. The cataclysmic genesis of this landscape, its utter sterilization, and the drama of its reclamation by living organisms stimulate the imagination of scientists and nonscientists alike. These primary successional surfaces are the most intensively monitored areas at Mount St. Helens because of what they may teach us about the fundamental mechanisms governing the formation and function of biological communities. At a practical level, understanding successional processes provides a conceptual basis for the restoration of devastated landscapes (Bradshaw 1993; Franklin and MacMahon 2000; Walker and del Moral 2003). Succession is a fundamentally multitrophic process. It involves not only plants but also herbivores, predators, and decomposers. Yet, the important effects of these other trophic levels are sometimes ignored. Study of trophic interactions (i.e., interactions between consumers and resources) in primary succession can provide new insights into mechanisms of primary succession and can inform the debates surrounding what controls the level of herbivory in terrestrial ecosystems. In this chapter, we describe often-devastating attacks by insect herbivores on the prairie lupine, Lupinus lepidus var. lobbii (Dougl.), and how they have affected the (spatial) spread of this little plant, generally considered the most important colonist during the first two decades of primary succession on the Mount St. Helens Pumice Plain. We also discuss a surprising spatial pattern in the intensity of lupine herbivory on the Pumice Plain and outline two hypotheses for this pattern. One hypothesis is based on gradients in plant quality; the other is based on gradients in the density and diversity of the herbivores’ natural enemies. Both hypotheses involve processes that are inherent to primary succession and that are likely relevant to systems beyond Mount St. Helens.

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.