Mount St. Helens: 30 years later -- what a comeback!
The story of how plants and animals on Mount St. Helens have rebounded after its cataclysmic eruption 30 years ago shows how ecosystems respond to major disturbances.
The cataclysmic eruption of Mount St. Helens 30 years ago today devastated the surrounding landscape, with the hot gas and debris killing countless animals and damaging or destroying large swaths of forest. But life did not entirely end then and there. Among the reasons the ecology rebounded are some surprising factors, including the early morning timing of the eruption, the fact that spring had been late to arrive that year, and the amazing ability of insects to parachute in once a recovery was underway.
Some species managed to survive amid the the volcano's eruption on May 18, 1980. Others scraped by at the edges of the devastation and literally crawled back. Together they sowed the seeds of a comeback that progressed in fits and starts and continues today.
Ecologists have been watching the process from the very beginning, noting what species were wiped out from the area and which still had a few representatives; which returned to the area and when; and what parts of the damaged landscape were the first to see regrowth.
The recovery of the Mount St. Helens area was "a wonderful living laboratory" to investigate how ecosystems and species respond to and recover from major disturbances, said Charlie Crisafulli, a research ecologist with the Pacific Northwest Research Station in Amboy, Wash.
This natural experiment gave scientists like Crisafulli plenty of surprises and has revealed some important factors that influence how an ecosystem recovers from such widespread devastation, which they have used to study other areas impacted by volcanic eruptions.
One key factor that influenced the recovery of different areas around the volcano was the variety of ways they were impacted by the explosion:
- Nearest the volcano, the explosion completely toppled trees, an area called the blowdown zone that covered about 143 square miles (370 square kilometers). The blowdown zone was also covered in layers of ash of varying depths. Along the fringes of this zone, trees remained standing, but were scorched and killed by the hot volcanic gases and rock fragments that rushed laterally from the explosion. The scorch zone covered about 42 square miles (109 square km).
- The pyroclastic flow raged out of the volcano’s mouth at speeds of up to 125 mph (200 kph) and reached temperatures of up to 1,200 degrees Fahrenheit (650 degrees Celsius). It created a pumice rock plane of about 6 square miles (15.5 square km) just to the north of the volcano. In this barren area where the pumice reached up to 131 feet (40 meters) thick, no remnants of the former forest remained.
- Mudflows, also known as lahars, scoured and buried much of the landscape, killing most of the plant and wildlife in their path, though some survived along the edges of these flows.
- Ash rained down on the landscape for hundreds of miles away from the volcano, carried by the prevailing winds, coating trees and other plants and accumulating in deposits along the ground.
These varying effects created by the explosions established different landscapes in the area that suited some species better than others and set in motion different types of recovery at varying rates.
Timing was key
One critical factor that influenced what species were impacted was timing – both the time of day and the season.
Because the major explosion occurred at 8:32 a.m. local time, many nocturnal animals were already bedded down for the day and so were more likely to have been protected in burrows and to have survived the explosion than their neighbors up and about during the daylight.
"You just don't think about that; that's a chance event," Crisafulli told LiveScience.
The seasonal timing was also key – spring was late in coming to Mount St. Helens that year, and so there were still drifts of snow covering the understory of many sections of the forest, protecting the plant and animal species buried beneath them. If the explosion had occurred two months later, when summer would have already begun, that snow would have been melted away and more plants and wildlife would likely have been wiped out, Crisafulli said. Instead, many of these snow-protected species survived and were the basis for the recovery of those areas.
Similarly, lakes still covered in ice that did not thaw until several weeks after the eruption survived intact, which likely would not have been the case if the eruption were later in the year.
"The seasonal effect was pretty readily apparent," Crisafulli said.
The fact that the eruption occurred early in the spring season for the area also meant that many migratory species – both various bird species and salmon – had not yet returned from their wintering grounds and so their populations were spared.
"Those animals essentially avoided it by being away," Crisafulli said.
Once the volcano’s rumblings had ceased and the ash had fallen from the air, life could begin to reclaim the areas impacted by the eruption.
When ecologists ventured out into the Mount St. Helens area, they expected the various ecosystems that were hit to have to start from scratch, with plants and animals re-colonizing after arriving from surrounding forests. While some areas around the volcano, particularly the pumice plains created by the eruption’s pyroclastic flow, were indeed left without any seeds of life to regrow the forest, many of the impacted areas unexpectedly still had some slivers of life – what ecologists called "biological legacies."
These areas included places where some species had been shielded from the worst impacts of the explosion by ridges and snowdrifts, allowing them to start the recovery process earlier, because they didn’t have to wait for out-of-town colonizers, and recover at a faster rate than other areas.
The spots that were left virtually barren had to overcome a certain amount of "biological inertia," Crisafulli said, with little regrowth in the first few years after the eruption.
"Conditions were just harsh," Crisafulli said.
But gradually, plants and insects colonized these areas, providing food for small animals, which came next and in turn were a food source for larger animals. Ecosystems gradually gained momentum as more and more species were added and ecological spots were filled in.
"Now it's really progressing at year 30," Crisafulli said. "It's a very productive system."
Crisafulli says that most species that were wiped out by the eruption have returned to the Mount St. Helens area; and not only are they back, they are reproducing, he said.
The going hasn’t been smooth sailing though, as animals and plants would establish themselves, only to disappear locally again a few years later, before once again settling back in. The recovery "is in fits and starts," Crisafulli said.
Much of the recovery was a trial-and-error process, with seeds blown in on the wind and animals traveling to islands of surviving plants. The environment determined what thrived and what didn't, and this process has gradually built up the species now back in the area.
Colonizing populations go through these "boom and bust" cycles, because at first they have nothing putting pressure on them — no predators, pathogens or parasites — and so their populations flourish. Once those "three P's," as Crisafulli calls them, emerge, the colonizer populations can crash. Eventually though, as the recovery progresses and diversity returns to the ecosystems, the swings of these cycles become less wild and more species begin to emerge with more stable populations.
The recovery of the forests that had once surrounded Mount St. Helens depended partly on the neighboring ecosystems.
For example, Roger del Moral, a biologist at the University of Washington, and his colleagues watched the recovery of two areas covered by lahars. One lahar had cut through a forest, so it was surrounded by existing vegetation and recovered relatively quickly. The other was bounded by ravines and so didn't have any trees and other plants around it that could easily recolonize the area. While the two areas started out looking almost alike, now, there are striking differences — the forest-surrounded lahar has recovered much faster and has pines and firs atop it, while the more isolated lahar is still mostly covered by grasses, early-stage colonizers.
Elevation also affected the rate of forest recovery: At colder, higher elevations, the growing season is shorter, so plants there have less of an opportunity to regrow and recolonize each year, so higher areas have had a slower rate of recovery that those lower down the mountain.
Snowmelt also protected many of the trees and other plants that typically dominate the understory of the forest, particularly on the north side of the mountain. These saved species provided spots of green even right after the eruption when the snow melted and they emerged — larger trees were blown over or snapped by the force of the eruption. This selection of species also changed the look of these areas of the forest, with more shade-tolerant, understory trees (such as Mountain hemlock) dominating the landscape, whereas before the eruption, Douglas firs would have made up a large chunk of the forest.
Snow also helped save some trees with bendier branches, because the weight of the snow caused the branches to bend and dump the snow — along with the ash that had fallen on them — keeping them from the damage that the ash caused, said Tom Hinckley, a professor of forest resources of the University of Washington.
The ash that coated the leaves and needles of trees in the volcano's vicinity was dangerous not because it smothered the trees or introduced harsh chemicals, but because the ash was heated by the sun, stressing the plants and making them experience drought-like conditions.
This effect was particularly seen in Pacific silver firs, which began to die or die back about five years after the eruption, surprising ecologists. The die-off was also seen to affect a greater number older trees than younger ones, Hinckley said. He explained that this had to do with the rate of needle replacement on old versus young trees, with the latter replacing many more needles per year, and so getting rid of the ash-covered ones faster.
Hinckley said that the lack of resilience on the part of the old trees was surprising to ecologists.
One group of plants that particularly thrived after the eruption — and helped make the landscape more suitable for other plants — were the lupins. These purple- and blue-flowered legumes were some of the only species that could grow on the large swaths of pumice around the volcano. This rock is low in some essential nutrients, and so is ill-suited to most kinds of plants; lupins though, can make these nutrients themselves, and so can grow in these areas, while they gradually add nutrients to the soil that makes the area more suitable for other plant species.
Conifers, which are prevalent elsewhere in the Cascades Range, have been slow to return to Mount St. Helens. These trees are very susceptible to drought and need a certain type of fungi at their roots to help them grow. The habitat around much of the mountain isn't yet able to support large numbers of these iconic trees.
"It's a tough environment for conifers," del Moral said. It will be "a very long time before you can say there's a forest there."
Insect 'parachute troops'
Insects were some of the smallest creatures affected by the massive explosion, with the blast and its subsequent ash fall killing off countless spiders, beetles, grasshoppers and other insects, which are a critical component to many ecosystems.
Insects were vulnerable to the ash because it could destroy their protective waterproofing, making them prone to desiccation.
"Insects are prone to be dried out simply because of their small size," explained John Edwards, a Professor Emeritus at the University of Washington in Seattle. Because of this tendency, insects evolved a cuticle that holds their moisture in, Edwards said.But volcanic ash is very abrasive — you can essentially "think of the ash as powdered glass," Edwards said — and it can scratch and damage the protective cuticle, and as a result the insects "lose water and they're dead."
The ash was destructive even to insects far from the blast area, as it fell for hundreds of miles away, Edwards told LiveScience.
"The insect populations were heavily impacted," he said.
But once plants began to return to the areas affected by the eruption, insects soon followed — the fact that insect species are very mobile let them recolonize the area relatively quickly after the blast, Edwards said.
One particular area where insect colonizers played a key role in revamping the ecosystem was in the higher elevations of the volcano slopes — not typically where insects would be thought to dwell, in the cold and snow. But certain species of beetles and spider thrive there. There is virtually no plant life or other insects for them to eat, so these adventurous insects "make their living on what blows in on the wind," Edwards said.
Many tons of dead or moribund insects blow onto the mountaintops during the course of a year, which the beetles and spiders that brave the elements eat for breakfast.
While the original populations of these insects would have been wiped out by the explosion, many of these species thrive in disturbed habitats and can be blown in on the wind themselves to recolonize the harsh landscape. Edwards and his colleagues observed the return of insects to Mount St. Helens and found that ballooning spiders and other insects that could fly on the winds were the first to arrive — what Edwards calls "the parachute troops" — preying on other insect detritus blown in on the wind, followed by non-flying insects ("the infantry") about four to five years after the eruption, a fairly speedy pace for insects arriving on foot.
"It was quite impressive how quickly they got there," Edwards said.
In some of these areas where insects were first to arrive, their corpses and other debris served as fodder for plant seeds, allowing vegetation and then small animals to return — "and then the whole thing just takes off," Edwards said.
Survival of the small
Within the immediate blast zone of the eruption, "all large mammals perished" because they couldn't outrun the rapid pyroclastic flows and were too big to hide behind rocks or other types of shelter.
The large mammals common to the Mount St. Helens area included the majestic elk (Cervus elaphus), black-tailed deer (Odocoileus hemionus columbianus), mountain goat (Oreamnos americanus), American black bear (Ursus americanus), and cougar (Puma concolor).
Elk carcasses were found in the "blowdown zone" — the area where the forest was knocked over by the volcano's blast, Crisafulli said.
But these mammals eventually did return, migrating in from less affected areas around more distant from the volcano.
"All five of those species are now back at Mount St. Helens," Crisafulli said.
Birds, too, mostly succumbed to the eruption, with the exception of those that were away at their wintering grounds. In the most devastated areas, the only birds that could initially return were those that made their nests on the ground, such as the American pipit (Anthus rubescens) and horned lark (Eremophila alpestris).
But as plant species and the homes they provided to birds returned, so did the bird species. Some bird species new to the area were even attracted with the formation of wetlands in rolling terrain that hadn't existed before.
In one area of the blast zone, there is now actually "an absolutely bizarre assemblage of birds" that wouldn't have been what scientists predicted would be there, Crisafulli said. In this spot, there are various birds suited to completely different habitats all in the same area – "I don't think you could go anywhere in the Pacific Northwest" and see all of these species of birds together, Crisafulli said.
Small mammals — such as shrews, deer mice and chipmunks — fared better than their larger brethren, as their size enabled them to better find shelter and escape the destructive forces of the volcano, Crisafulli said. "A great number of those had survived, albeit in greatly reduced numbers."
Importantly, Crisafulli said, the small mammals that survived represented many different parts of the food web of the forest — herbivores, carnivores, insectivores — and that diversity helped enable the recovery of the ecosystem.
Scientists were surprised by how quickly the areas impacted by the eruption were recolonized, even in places where nothing had survived the blast. Today,satellite imagery shows signs of biology across almost the entire blast zone.
The relatively rapid return indicates that even the small mammals were able to traverse large, barren areas to get to the small pockets or islands where plants survived and recovered more quickly, Crisafulli said. "These animals turn out to be incredibly mobile."
One species that conspicuously hasn't returned is the northern flying squirrel. This species requires mature forests, which likely won't develop in the Mount St. Helens area for some time, Crisafulli said. "It's going to be a protracted process."
Like many of the small mammal species, amphibians actually fared surprisingly well after the eruption. Scientists had expected them to be wiped out from the eruption, because these animals tend to be particularly sensitive to environmental changes. But when scientists visited the area after the blast, they found that most of the 15 endemic species of frogs, toads, salamanders and newts had amazingly survived in much of the blast area.
The key to the survival of these species was that they spent at least one portion of their life cycle in the water — so eggs and tadpoles that sat beneath the frozen surface of ponds were protected from the explosion and could develop later in the season. Species that lived only on land, however, were indeed wiped out in the eruption.
The fate of the area's fish also varied, as some lakes were highly affected by the eruption and others were barely at all. Fish in many small lakes were spared because the lakes were still frozen. When ecologists surveyed the ruined landscape in the summer of 1980, brook trout (Salvelinus fontinalis) was the most frequently found.
The fish in Spirit Lake, north of Mount St. Helens, all perished during the eruption, and so much volcanic debris slid into the lake that its bottom was raised 200 feet (60 m). But just six years later, the lake had cleared enough once again to support fish, which were finally spotted in the lake in the early 1990s.
Observing the recovery from the eruption has been a series of surprises for ecologists, who expected the revival of the forest to progress much more slowly than it did.
The 30-year-long natural experiment has also been — and will continue to be in the coming decades — an unprecedented learning experience that shows how ecosystems respond to such a major disturbance.
Ecologists can use this knowledge to better understand both past eruptions and the ecological responses to them and eruptions today. Crisafulli has spent time observing the aftermath and initial stages of recovery at Chile’s Chaitén Volcano, which erupted on May 2, 2008, and Alaska’s Kasatochi Volcano, which erupted on Aug. 7, 2008, and using the lessons learned from Mount St. Helens to see what factors might affect the recovery of the ecosystems around these volcanoes.
"There's nothing to substitute natural history," Crisafulli said. And what better way to learn than by observing what he calls the "granddaddy of disturbances?"
- Gallery: The Incredible Eruption of Mount St. Helens
- Mount St. Helens Remains a Mystery 30 Years Later
- Why Was Mount St. Helens so Destructive?