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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.

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.

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.

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

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.