Mount St. Helens Revisited #1

Food and shelter are still not abundant, and the volcano continues to rumble, but many kinds of animals -- both survivors of the eruption and recent immigrants -- are making efforts to repopulate the mountain.


by James A. MacMahon




Reprinted from Natural History, Vol. 91 (May 1982)



For many years to come and certainly long after the second anniversary of its May 18, 1980, eruption, Mount St. Helens will continue to provide scientists with an incomparable opportunity to observe natural repair processes at work.  Yet the task of deciphering the story of life on the volcano has turned out to be more difficult than many scientists ever envisioned.  Instead of an affected area of about 160 square miles that could be subdivided into three or four neat devastation categories, researchers have found that nearly every patch of ground must be sleuthed to reconstruct the causes of its fate since the eruption.  Was the site forested, clear-cut or above timberline before the big event?  Was it snow covered on the morning of the May 18 eruption?  Did it receive only a shower of pumaceous material or was it subjected to the full fury of the mixture of hot gas and rock material known as pyroclastics?



Adding to the difficulties of reconstructing the past are the problems of predicting the future.  It is impossible to tell what the future activity of the volcano will be, and a sizable eruption would probably destroy many of the present scientific study sites.  Even as we go to press, the mountain is rumbling, and scientists disagree as to whether the volcano's dome is about to erupt or whether Mount St. Helens is in for a period of intensive dome building.  One thing is sure, however.  If the volcano erupts now or at some future time, plants and animals -- and scientists -- will undoubtedly return to the mountain and start their work again.  Certainly all three groups have been busy since the mountain blew two years ago.



Survival of plant and animal life in the areas devastated by the May 18 blast was often due to a chance set of circumstances, which no one could have modeled mathematically and which were independent of the myriad adaptations organisms possess as a result of thousands of generations of evolution.  A few hundred feet above Spirit Lake, for example, the presence of a single rock outcrop in the middle of the most ravaged lands seems to have permitted the survival, virtually unchanged, of an oasis containing the normal mix of plants and animals.  The isolation of these islands of survivors may lead to genetic differences between populations derived from the survivors and more widespread populations of the same species.  Such areas might also form a source from which organisms can recolonize the landscape faster than if recolonization occurred only from the perimeter of the large devastated area.



Being alive on May 19, 1980, however, was no guarantee of continued survival.  Today, two types of organisms occupy the affected areas of Mount St. Helens:  survivors (or their offspring) and colonizers.  Those that lived through the original catastrophe may have been protected by the deep snow that was still present on some sites, or they may have been dormant in belowground burrows or safely ensconced at the bottom of a montane lake.  Continued survival depended on the nature of the changes in their environment and their ability to cope with those changes.  Many of the animals seen on the earliest reconnaissance missions in the summer of 1980 were not present in the spring of 1981.  Some of those that have persisted have gone on to recolonize other parts of the volcano.  Colonizers have also come from outside the regions affected by the eruption.



Because chance may spare a site within a devastated area and because survivors at one site may be colonizers in another, neither the types of organisms nor the categories of devastation are completely clear.  Nevertheless, these categories and types can help clarify what happened during --  and since --  the eruption.



As Mount St. Helens erupted, its north slope broke up.  Great chunks of the mountain and glacial ice flowed northward and westward in landslides and a debris flow that eventually turned into mud flows.  A major tongue of this flow was directed into Spirit Lake, causing it to slosh up the side of its basin and scour the hills down to the bedrock in some areas.  A more westerly directed arm followed the valley up the North Fork of the Toutle River.  This landform contains blocks of material standing more than 90 feet above the surrounding area.  The flow, as thick as 300 feet in some areas, averages more than 120 feet thick over a total area of about 23 square miles.  Apparently no organisms were able to survive the mechanical damage caused by an overburden of this magnitude.



Damage caused by the blast itself depended on the distance from the volcano.  The affected areas are characterized, in a very general sense, as the blast, or near, zone where trees were completely removed; the blowdown, or intermediate, zone where trees were leveled; and a scorch, or outer, zone where trees still stand but were scorched.  Close to the volcano, many areas were also subjected to the high temperatures of pyroclastic flows and surges (hot mixtures of gases, ash, and up to boulder-size pumice and rock).  Temperatures beneath the surface of materials deposited by these flows often exceeded 480 degrees F --  precluding the survival of organisms and creating some of the most desolate vistas on the volcano.



The volcano also deposited ash and other airborne materials. Beyond the scorch zone this is the only effect of the eruption.  In such areas, including places on the relatively unaffected south slope of the volcano itself, the list of plant and animal species remains nearly the same as during preeruption times, although each species is often represented by fewer individuals.  For example, at Butte Camp, on the south slope, Roger del Moral, Larry Bliss, and others at the University of Washington have compiled a plant species list that is virtually identical to a typical list for alpine areas in that section of the Cascade Range.  Notable animals at Butte Camp are a black bear that has persisted in the area since the time of the blast and pocket gophers that have reproduced within the last twelve months.



This is not to say that the ashfall had This is not to say that the ashfall had no consequences:  John Edwards and Lawrence Schwartz of the University of Washington have demonstrated in the laboratory that ash, by causing physical scarification of insect cuticle and subsequent desiccation, can kill individuals much as some commercial silica-dust insecticides do.  Ash certainly killed many insects, but after the eruption, numerous sites in the high-ashfall zones close to the blast area contained up to seven species of ants, a dozen spider species, and more than fifteen beetle species, all survivors.  At one of the oases above Spirit Lake, where ash is more than six inches deep, I have seen ground-dwelling spiders, two millipede species, a centipede, three ant species, and several beetles.



(Bees, whose dense body hairs act as a trap for dust, were among the insects hardest hit by the abrasive ash.  Populations of these beneficial insects, however, are recovering.  Ladybird beetles probably did not survive the May 1980 eruption, but ladybugs were back on the mountain as early as the summer of 1980.  Aphids, feeding on surviving fireweed, provided the beetles with food.)



Evidence for the effect of ash on vertebrates is also somewhat debatable.  As far from the volcano as eastern Washington, David Pyke of Washington Sate University has found deer mice (Peromyscus maniculatus) with signs of eye swelling that may have been induced by ash.  In contrast, Doug Andersen of Purdue University --  my collaborator on Mount St. Helens -- and I have found no eye swelling in deer mice from our Spirit Lake blast site or from any of our high-ashfall areas fifteen to twenty miles northeast of the crater.  Furthermore, we found no lung damage in any of the dead animals we examined microscopically.  Interestingly, in 1945, less than four inches of ash near Iliamna Lake, Alaska, reportedly led to the death of certain rodents, blindness in rabbits, and illness in reindeer.



Richard Mack of Washington State University has documented ashfall effects on vascular plants in Washington and Idaho more than ninety miles from Mount St. Helens.  Ash deposits mechanically overloaded several species, killing some and burying the fruits and flowers of others.  The ash may also have dusted the leaves of some plants in ways that altered the energy balance, causing the leaves to overheat and die.  Mack believes that the most important effects of the ash may become more obvious in the future if compacted ash alters the nature of seed residence sites and the availability of seeds to seed predators.



The kind of disturbance experienced by an area influenced not only what survived but also what can now successfully colonize the site.  Potential colonizers, however, must first get there.  The animals and plants that have moved into affected areas of Mount St. Helens, whether from oases on the volcano or from outside, must have good dispersal mechanisms.  The cysts and spores of myriad microorganisms, carried along by winds, fall on the volcano constantly.  Some plant seeds and spiders are also passively dispersed by winds.  Immature spiders of many species spin threads of silk that remain attached to their spinnerets.  These parachutelike apparatuses carry the spiderlings "on gossamer wings"  and are responsible for the presence of young orb-web weaving spiders on the debris flow, an area that still supports very little life.  Highly mobile animals, such as birds and some large mammals, were able to move easily into many devastated areas within hours or days after the eruption.



For all these migrants, getting on the volcano is just the beginning of their problems.  For most species, suitable home sites and food sources are not available.  Wood-boring insects would appear to be an exception.  The abundance of downed timber led to some anticipation of a boom of bark beetles (Dendroctonus sp.) and an infestation of nearby valuable timberlands, but to date this has not occurred.



Not surprisingly, colonizers of all kinds are scarce in the blast area.  There are few food resources:  few plants or plant remains and thus few animals.  My impression is that many of the invading invertebrates are predatory forms, an observation that agrees with findings at other sites highly altered by such agents as fire.  Colonizers often include a high proportion of carnivores because primary plant productivity is lower than in better developed ecosystems.  Collecting insects and spiders in the blast zone with any quantitative certainty, however, has been difficult since there is so little life of any sort.  The exceptional places, such as the series of protected oases near Spirit Lake, are so small that I do not want to alter them by intensive study techniques.



(Many factors, including chance, influenced the survival of plants and animals after the May 1980 eruption.  In most of the blast zone, for example, devastation was nearly total, but the arms of Spirit Lake are dotted by a dozen or so oases -- patches of vegetation a few yards across -- that survived intact and now contain a normal mix of organisms.  In other areas, only some species survived, and the composition of the community changed.  Around Meta Lake, for instance, gophers survived the eruption but most plant life did not; unable to find food, the gophers had disappeared by the summer 1981.)



In some parts of the blowdown zone, moving among the tangled trunks is so hard that collecting, observing, or even laying out study plots is a challenge.  Deer and elk, however, were seen in blowdown areas within days of the May 18 eruption, and they continue to make forays through the area.  These large mammals have also been sighted in mountain basins just north of Spirit Lake, more than five miles from normal vegetation.  When they find no food, they simply leave.  If they do come across a patch of vegetation, the damage they cause by browsing can be traumatic for the plants and the creatures dependent on them.



America's cosmopolitan deer mouse occurs in a few blowdown sites.  In some, where the Soil Conservation Service has attempted to stabilize the soil by seeding with grasses and forbs, high population densities have developed.  Gophers, on the other hand, are on the decline in many places.  These small mammals feed on bulbs and other belowground plant parts.  The roots of such plants as bracken fern and fireweed, often the most conspicuous survivors, do not supply the necessary nutrition.  Other, more suitable plants have not yet returned in great enough numbers to support gophers.



In the high-ashfall sites, insects and spiders are abundant and seem to represent a normal admixture, except for ground dwelling forms.  Small wolf spiders (Lycosidae) are common in high-ashfall; other ground dwellers have not fared so well.  Although in reduced numbers, the same species of small mammals, as well as plants and many other animal groups, occur in heavily ashed areas today as during preeruption times.  Our pocket gopher data, for example, suggest population densities lower than at other places we have studied, and our marked population, lacking adequate food here as in blast and blowdown sites, may be declining slightly.



Birds are so mobile that their potential for reestablishment on the volcano is nearly unlimited.  Some food exists, in the form of seeds and insects.  In some areas, the reseeding projects have produced a considerable food supply for birds that can utilize grass seeds.  Many of the bird species inhabiting Mount St. Helens' forests before the eruption, however, were conifer-seed eaters or insectivorous.  For these birds, food is less abundant.  Nevertheless, on a given day, a few birds can be seen flying overhead almost anywhere on the volcano.  In the crater itself, helicopter pilots have reported that hummingbirds have dive-bombed their orange flight suits and the ubiquitous red plastic markers of geologists and biologists.



Establishing that birds are breeding on a site is more difficult than just compiling a list of sightings.  Hole-nesting species, which require tree snags, might be expected to do well in the blowdown zone -- if they could find food.  Similarly, ground-nesting species might not find the ash too much of a problem.  We have noted refous hummingbirds, a raven, and a sparrow hawk in our plots in the blowdown area, but the birds we have seen most often on our early morning censuses of these sites are Oregon juncos (ground nesters) and mountain bluebirds (hole nesters.)  In the high-ashfall area twenty miles northeast of the crater, we have found nests and eggs of the junco.  For one such site, we estimated, in midsummer 1981, more than sixty birds per twenty-five acres, mostly Oregon juncos and robins but also orange-crowned warbiers, pine siskins, blue grouse, three species of woodpecker, varied thrush, and Townsend's solitaire.  The site had been clear-cut some time ago and planted with conifers, which are now about six feet tall and growing out of six inches of ash.  A mature forest adjacent to the clear-cut area, also heavily ash covered, contained eleven species and about thirty birds per twenty-five acres.  These species lists and densities are not out of line with expectation for similar sites unaffected by the volcano.



I have not spent much time studying aquatic habitats on the mountain.  In September of 1980, I visited temporary ponds in Mosquito Meadow in the high-ashfall area.  They contained salamander and toad larva that were about to transform, as well as a garter snake.  In May of 1981 , these ponds were full of breeding salamanders of two species, the long-toed salamander (Ambystoma macrodactylum) and the northwestern salamander (Ambystoma gracile.)  Jim Seddell of the U.S. Forest Service Laboratory at Corvallis, Oregon, has been studying streams in the blast area and has, for some sites, lists of species similar to normal situations.  Among the species are insects, fishes, the tailed frog (Ascaphus truei), and the Pacific giant salamander (Dicamptodon ensatus.)  Muskrats, trout, mink, and newts (Taricha sp.) have persisted in some lakes -- for example, Ryan Lake, also in the blast zone.



In addition to determining what animals now reside on the mountain, and in what numbers, Doug Andersen and I are attempting to test some hypotheses about the role of animals in the process of ecosystem succession.  Working in a clear-cut, high-ashfall site, we are particularly interested in the impact of small mammals and ants on plants and soil.  Studies of succession in the subalpine habitats of Utah have generated some specific and some general hypotheses about the sequence of events following disturbance to the landscape.  For example, animals, as dispersers of seeds and fungal spores, may influence the presence or absence of some plant species.  Currently, I am also testing these hypotheses on sites altered during the process of strip mining.  My assumption is that ecosystem recovery processes should be similar regardless of the disturbing agent -- fire, grazing, clear-cutting, or mining -- and that observing the processes in both managed and unmanaged situations should increase the ability to repair ecosystems effectively.  When Mount St. Helens erupted, we recognized in the volcano a natural laboratory that might allow us to test some of our ideas, on a scale that no federal or private funding could underwrite and in areas much like the sites I was studying in Utah.



A final report of our finds would be premature, but the pocket gopher, our favorite beast, seems to be living up to our expectations.  Gophers consume the underground portions of plants:  bulbs, corns, roots, and rhizomes.  In the winter, gophers fill snow tunnels with soil, and melting snowpack discloses ribbons of disturbed soil on the ground and a disk of freshly overturned soil around the burrow openings.  These very characteristics were the basis for our postulation that gophers might have a positive role in the reestablishment of ecosystems on Mount St. Helens.  First, gophers were good candidates for being survivors.  Assuming that, even in the blast area, heat from deposited materials was not too great, some of these fossorial animals might persist in otherwise  barren landscapes, at least until they exhausted their belowground food resources.  This has been the case, and we have found gophers in areas representing most types of damage, including blast, blowdown, and high-ashfall areas.



Second, we thought the soil-turning proclivities of gophers might be beneficial.  If well-developed soil was trapped under volcanic material, the gophers, just like a plow, might turn over the fresh and inert ash  material and mix it with the old soil.  This is occurring, but at a slow rate because so many gophers have died since the eruption for lack of the right plants to eat.  Where mixing is taking place, our prediction seems to be borne out.  In the summer of 1981, soil turned over by gophers had more seedlings growing on it than did adjacent volcanic material not mixed by the animals.



There are many reasons why this might be the case.  The buried soil contains more organic matter and various chemical nutrients than does the new material deposited by the eruption.  The tilling activities of gophers might also make possible an important association that many higher plants form with a group of fungi known as mycorrhizae.  This association may increase the plants' ability to get nutrients and water from the soil and may be critical to the survival of some species.  Mycorrhizal spores are not as easily disseminated as those of other fungi.  Several studies implicate small mammals, such as rodents and rabbits, and insects, such as grasshoppers, as dispersing agents.



Most well-developed soils contain the desirable mycorrhizae, but covered by ash, these soils might be ineffective.  In an analogous situation -- where topsoil stripped from coal mine sites is stored in large piles until mining is completed -- mycorrhizae have been shown to decline with the time stored in the pile.  The stored soil thus becomes an even poorer potential growth medium.  On Mount St. Helens, the gophers' digging activities appear to rescue spores from the old soil and mix them into the ash material, increasing the ability of the mixture to act as a growth medium.  In tests, samples of soil processed by gophers contain forty to eighty spores per grams of dry soil, while the adjacent volcanic material had an average of less than one spore per gram of dry soil.



I do not mean to imply that gophers will be the main factor in the regeneration of the whole volcanically altered landscape, only that like all organisms, they have an influence, no matter how small.  Mount St. Helens, recently reduced to a science fiction caricature of the surface of a planet, is very much a functioning, natural ecosystem in which the rules of the biosphere operate.  Careful study of the varying abilities of different plants and animals to exist on the new Mount St. Helens is leading to insights on the varying roles these organisms have in the recovery of the volcano's ecosystems.  Some of these insights.  especially when combined with studies of undisturbed ecosystems, may help humans, users of various landscapes, approach management problems.


James A. MacMahon is a professor of biology and a member of the Ecology Center at Utah State University.






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