On 18 May 1980, the eruption of Mount St. Helens volcano removed or leveled 350 square kilometers of old-growth and younger-aged forests and dramatically altered all types of ecosystems within this area (Fig. 1). Before the eruption, this landscape was typical of those found throughout mountainous regions of the Pacific Northwest: dense, temperate coniferous rain forests, with large areas partially modified by timber harvest activity, and sparse alpine vegetation occurring at higher elevations above treeline. Crystal clear lakes and cold, fast-flowing streams were common. In contrast, the eruption created a starkly barren landscape that bore little resemblance to preeruption conditions.



Although the eruption resulted in catastrophic loss of human life, wildlife, and forests, subsequent study of ecosystem recovery has revealed important insights regarding the role of natural disturbance in regulating the productivity and biodiversity of a variety of Northwest ecosystems. The range of disturbance caused by the eruption and the resulting spatial mosaic of disturbance types provided a unique opportunity for ecologists to study how large-scale disturbances influence natural ecosystems.

Volcanism and Western Montane Ecosystems

Within the scale of a single human lifetime, the eruption of Mount St. Helens appears to be an extraordinary and unique event, a disturbance that dramatically altered the local environment but also an event so unusual that it would seem to have little relevance to understanding the "normal" processes that generally shape ecosystems of the Pacific Northwest. Closer inspection, though, has revealed that such eruptions have greatly affected ecosystems throughout the mountainous regions of the Cascade Mountains and the Sierra Nevada. For example, Mount St. Helens has erupted more than 20 times within the last 4,500 years, an average of once every 225 years (Crandall and Mullineaux 1978). Before 1980, Mount St. Helens last erupted 123 years ago. These periods are well within the 500­600 years that it takes to produce an old-growth Douglas-fir forest (Franklin and Hemstrom 1981).

Natural Disasters, Biodiversity, and Ecosystem Recovery

The eruption of Mount St. Helens instantly created a large-scale natural experiment that ecologists could use to evaluate theoretical ideas about how entire communities recover from disturbance and the mechanisms most important in recovery. We present some examples of the lessons ecologists have learned by conducting long-term studies of both terrestrial and aquatic ecosystems after the eruption. These examples illustrate situations in which theory accurately predicted observed recovery or in which we learned something fundamentally new.

Terrestrial Vegetation

Ecologists recognize that several mechanisms may act singly or in concert to influence the development of plant assemblages. It is not always clear, though, under what conditions different processes will dominate. The eruption of Mount St. Helens had highly variable effects on vegetation. In general, both initial plant survival and rates of recovery were inversely related to disturbance intensity. It is important to recognize, however, that the dramatic visual differences between pre- and posteruption landscapes were due to the removal or leveling of a few tree species. Vegetation responses differed considerably within two distinct zones of disturbance--the blowdown zone and the pyroclastic flow zone.

In the blowdown zone, overstory trees were either blown down or snapped off, and understory species were buried under as much as a meter of ash. Wind-dispersed herbs, such as fireweeds and composites, colonized the barren surfaces of the blowdown zone during the first year following the eruption. Since then, they have spread by seeds and vegetative growth and dominated many areas within 4 to 7 years (Halpern et al. 1990). In general, this pattern fits classic ecological theory--early successional species colonize and exploit nutrient-poor, disturbed substrates that retain little water.

Not all patterns were this predictable. In several upland areas of the blowdown zone, the recovering plant assemblages are bizarre mixtures of late-successional understory and pioneering species--assemblages we never would have expected to encounter (Halpern et al. 1990; C. M. Crisafulli, U.S. Forest Service, Amboy, Washington, unpublished data). The reasons for these kinds of assemblages are related to the survival of a few late successional species and the colonization of other species. Although our initial inspection suggested that no species survived, some local patches escaped complete destruction. Four factors appeared to increase the probability that individual plants would survive in these locations: (1) patches of late-lying snow shielded some plants from the blast; (2) plants living on the lee sides of ridges were not exposed to the main force of the blast; (3) some plants survived in soils on the exposed rootwads of large blown-down trees; and (4) some plants were able to resprout from perennial root stock on steep slopes where erosion quickly cut through ash deposits (Frenzen and Crisafulli 1990; Halpern et al. 1990).

Of the individuals which survived the initial eruption, some flourished, whereas others perished quickly because of the dramatic change in conditions. Because the overstory that had formerly intercepted nearly all sunlight had been removed, surviving saplings of Pacific silver fir and mountain hemlock previously in the forest understory experienced tremendous growth and were producing cones by 1993. The survival of these few individuals will greatly accelerate the overall recovery process, because seeds will not have to arrive from distant sources beyond the disturbed area. These new conditions, though, created an intolerable stress for other survivors. Shade-adapted understory herbs, such as wintergreen and fawn lily, were unable to tolerate the posteruption conditions of increased light, temperature, and desiccating winds, and soon perished.

In contrast to the slow recovery of upland vegetation, most riparian areas recovered rapidly. Bank erosion quickly re-exposed some buried shrubs and trees such as salmonberry and willow. Fragments of some species--such as willows--were swept downstream of their original locations and then sprouted. Surviving plants quickly produced wind- and water-dispersed seeds that colonized wet shorelines.

Within the pyroclastic flow zone, no individuals survived. Considering the intensity of destruction, classical successional theory predicts a long successional recovery in which mosses, liverworts, and lichens or wind-dispersed herbs establish first, followed by shrubs and then conifers. Which species colonize and when they actually establish are theoretically governed by their dispersal abilities, subsequent alteration of the site by colonizing species, and competition among late-establishing species. Studies conducted on the pumice plain within the pyroclastic flow zone, however, show that this classical pattern of succession has not necessarily happened. Many areas within the pyroclastic flow zone remain sparsely vegetated 15 years after the eruption, and late successional species (5 species of conifers, sword ferns, and lady ferns) have colonized along with wind-dispersed herbs such as fireweed and pearly everlasting (del Moral and Wood 1993; Crisafulli, unpublished data). Still, only two main types of plant assemblages have developed here: willow­herb communities that are restricted to a few springs and seeps, and patches of lupines (C. M. Crisafulli, W. M. Childress, E. Rykiel, Jr., and J. A. MacMahon, Amboy, Washington, unpublished manuscript).



Although the prairie lupine lacks specialized structures for long-distance dispersal, this short-lived perennial herb was among the first species to arrive on the pumice plain and has profoundly influenced the first 15 years of succession (Fig. 2). A few critical attributes appear responsible for its successful establishment. First, lupine has a mutualistic relationship with a root bacteria that fixes nitrogen, and the soils of the pumice plain have extraordinarily low amounts of nitrogen (Halvorson et al. 1991). Second, because this species produces prodigious amounts of seed, populations are spreading at a rapid rate from centers of initial establishment (Crisafulli, Childress, Rykiel, and MacMahon, unpublished manuscript). When these populations are dense and growing vigorously, they inhibit colonization by other species, but once they die, they leave a nutrient-rich substrate where other species can thrive (Morris and Wood 1989).

The establishment of several species of conifers also appeared to defy conventional wisdom. Conifers are poor long-distance dispersers because they have heavy seeds, and they require the presence of symbiotic soil fungi called mycorrhizae to survive. Scientists did not believe that the barren soils of the pumice plain could support these fungi, but the fact that these species arrived and are persisting suggests that we do not fully understand either their dispersal dynamics or the conditions they require to successfully establish.

Birds

Ecologists probably know more about birds than any other group of animals, and many ecologists would predict that two factors strongly influence the development of a bird community: structural complexity of the environment should affect species diversity, and the type and abundance of resources should influence the types of birds occurring in an area. Monitoring over a 12-year period generally confirmed theoretical predictions, although there were a few species-specific surprises. When we consider the natural history of each species, even these surprises were understandable, although not necessarily predictable.



Undisturbed forests in this area have about 15 species of birds. No bird species survived in either the blowdown or pyroclastic flow zones, so recovery of the avifauna in both areas started in the complete absence of birds. The pattern of recovery differed greatly between blowdown and pyroclastic flow zones over a 13-year period (1980­1993; Crisafulli and MacMahon, unpublished data; Fig. 3), an anticipated result considering that the two zones differed greatly in structural complexity following the eruption and recovery rates of vegetation.

The physical environment of the blowdown zone following the eruption was complex but offered few food items for birds. Habitat consisted of tangled trees and their branches embedded in a deep layer of ash and pumice. Little living aboveground vegetation existed. Bird colonization in this zone occurred in two phases. Within a year of the eruption, seven species had colonized (dark-eyed junco, white-crowned sparrow, northern flicker, hairy woodpecker, mountain bluebird, American kestrel, and Vaux's swift); these species are either ground foragers that nest on the ground or in cavities, or species that fly from perches to forage. These birds occur in open landscapes with sparse vegetation, though colonization by Vaux's swift was initially surprising, because it traditionally had been thought to occur in association with mature or old-growth forests (Manuwal 1991). Its establishment suggested that what the swifts require is snag habitat, and not old-growth forest per se.

The second recovery phase occurred about 7 years after the eruption and was directly associated with the colonization and expansion of erect, woody vegetation (alder, willow, and cottonwood) along water courses. At this time, an entirely new assemblage of species colonized the blowdown zone, including yellow warblers, orange-crowned warblers, MacGillivray's warblers, willow flycatchers, and warbling vireos. The new species were added to those present rather than replacing them; all these species nest in deciduous shrubs and trees and forage either by gleaning insects from the surface of vegetation or by catching flying insects on the wing. After 15 years, the bird species richness was 70% that of undisturbed forest, but the species composition remains markedly different from the undisturbed forest.

Bird colonization in the pyroclastic flow zone, where no remnants of the preeruption landscape remained, was slower than in the blowdown zone (Fig. 3) and involved different species. This new landscape is stark and open, with undulating pumice hills and complex networks of rills and gullies; it presently supports bird assemblages with only 46% of the species richness of undisturbed forest. The assemblage that developed in this area was not initially anticipated, but its establishment makes sense in hindsight. These species comprised three subgroups, each with strong affinities for completely different habitat types. Red-winged blackbirds and savannah sparrows usually inhabit low-elevation wetlands or pastures; horned larks, rock wrens, and western meadowlarks are associated with shrub-steppe habitats; and gray-crowned rosy-finches and water pipits are normally found in high-elevation, alpine conditions. None of these species are normally found within montane coniferous forests, but the pyroclastic flow zone provided a new set of habitat conditions that mimicked conditions typically found in other locations.

Stream Ecosystems

Until recently, succession in stream ecosystems was thought to occur mainly in response to vegetation changes in the surrounding watersheds and riparian zones, which are known to influence both habitat features in streams and the abundance and type of food available to aquatic animals. Succession caused by competition andpredation was thought unimportant since annual floods disturbed streams too frequently for biotic interactions to influence long-term successional dynamics. Research at Mount St. Helens has shown that succession in streams can be a long and ecologically complex phenomenon.

Three months after the eruption, we began an annual monitoring program of several streams in which disturbance varied from complete elimination of living things to a modest reduction in their abundance and diversity (Hawkins 1988). Data from the most severely disturbed streams show that invertebrate species richness increased very rapidly over the first 5 years following the eruption and continued to increase, though at a slower rate, up to 1990, the last year for which data have been compiled (see Anderson 1992; Fig. 2). By 10 years after the eruption, these streams had recovered about 80% of the invertebrate species typically found in an undisturbed stream.



Five species of vertebrates occurred in many of our study streams before the eruption (cutthroat trout, brook trout, shorthead sculpin, tailed frog, and Pacific giant salamander). Although at least a few individuals of most of these species were observed soon after the eruption, recovery of densities varied greatly among species (Fig. 4). In many streams, all of these animals appeared to have been completely extirpated, but within 5 years of the eruption, modest to abundant populations of tailed frog tadpoles and sculpins existed even in heavily disturbed streams. We recorded the highest densities of tailed frog tadpoles and shorthead sculpins ever reported by 4 and 5 years after the eruption (Hawkins et al. 1988; C. P. Hawkins, Utah State University, Logan, unpublished data). In contrast, the recovery of trout and giant salamanders has been slow; 15 years after the eruption, their densities are only 5% to 10% of those observed in undisturbed streams (Hawkins, unpublished data).

We believe the existence of protected refugia was largely responsible for preserving a few individuals in even severely disturbed streams. These refugia appear to be the source of the populations that established later. The eruption occurred in late May when there was still snow cover on some hillslopes and ice on some lakes. At least a few trout are known to have survived in the ice-covered lakes (Crawford 1986) that served as sources of colonists for many streams. Second, we believe that a few sculpins and adult tailed frogs survived in small springs that were also probably snow-covered and topographically shielded from the full force of the eruption.

One clear lesson that emerged from these studies was that appearance of the surrounding landscape is not necessarily related to the quality of stream habitat. Although the floodplains and hillslopes surrounding many of these streams were still largely barren, conditions within streams quickly recovered sufficiently to support an abundant and diverse fauna (Hawkins 1988; Anderson 1992). In general, amphibians are thought to be highly sensitive to landscape alterations that affect either adult habitat conditions (temperature, humidity) or the availability of breeding sites. We thought amphibians would have been exterminated by the eruption, but we have found that many species survived and in some cases recovered rapidly (MacMahon 1982; Zalisko and Sites 1989; Crisafulli, Hawkins and MacMahon, unpublished manuscript; Fig. 3).

Aquatic species generally had higher survival rates than terrestrial species, and among aquatic species, pond breeders fared better than stream dwellers. At the time of the eruption, aquatic species were present as both terrestrial adults and aquatic larvae. Because ice, snow, and cold water buffered the aquatic biota in many high-elevation lakes and streams from the impact that devastated neighboring terrestrial environments, some individual animals that were in water or under snow survived (frogs and toads that were hibernating, tadpoles of the tailed frog, and larval and neotenic salamanders). We think these individuals served as a source of colonists to lakes and streams at lower elevations where aquatic biota appeared to have been completely extirpated. Dispersal of colonists therefore appears to have radiated from epicenters of survival within the blast zone rather than from distant, unaffected populations.

One of the most astonishing events that we observed was that four species of frogs and toads and one species of pond-breeding salamander had colonized all available lake habitats within 5 years of the eruption even though absolutely no dispersal corridors existed between lakes. These animals were dispersing great distances over nonforested, barren pumice substrates. Another surprise was that the eruption may have actually created more aquatic habitat than existed before the blast.

In contrast to the aquatic species, three species of salamanders in the family Plethodontidae, which is a largely terrestrial family, seem to have been eliminated from the entire disturbed landscape. The only species of this family to survive was a semiaquatic species. Because all three of the extirpated species are thought to have low mobility and require mesic forest conditions, we expect these species to be absent from this landscape for decades or centuries.

Not only has the eruption of Mount St. Helens provided many insights into the vulnerability of many types of plants, animals, and ecosystems to a catastrophic disturbance, but it has also shown us that many of our ideas about succession and the factors that influence the colonization and establishment of species need refinement. In almost every case in which we were surprised at a response, we had lacked sound information on the basic biological attri-butes of a species. If nothing else, the study of biotic recovery at Mount St. Helens has convinced us that we must continue to describe, document, and quantify the basic biological features of this nation's flora and fauna.