1:Read this short article
on succession on Mt. Helen after the volcano blew at the end of the notes. In
what ways does the study confirm or refute the notes on page 2 of autogenic
succession? What new perspective(s) does the article indicate?
2. In the serpentine barrens, would degradation succession be as important as autogenic? explain what each type of succession refers to and how you would measure it in this system and its impact.
Succession: is a pattern of colonization and extinction of (species) populations on a local site over time... the pattern of changes are
1. A species will be part of the process given:
i. it is capable of reaching the location: how can species assure this?
ii. the appropriate conditions and resources exist there. Many seeds reach a site earlier but will not germinates till required conditions are met ( i.e.. oak seeds, etc.)
iii. competitors and predators do not preclude it.
2. Ecologists recognize at least 3 to 4 types of succession: degradative, allogenic and autogenic.
Allogenic succession is characterized by the serial replacements occurring as a result of changing external geophysical forces. This is illustrated in the diagram below with the conversion of salt marshes to low woods through silt carried by the river. The silt accumulation is defined by the black markings
Degradative succession is characterized by the dominance of heterotrophic organisms, maximum energy available at the start and a steady decline of energy as the succession proceeds.
An example of litter succession is illustrated by the diagram below:
Although you probably won't recognize all the genera below (perhaps Penicillium?), you can easily see how the species change over time. The real question is what motivates these changes?
Overall, though this degradative succession may not appear to exciting, how critical is it with respect to ecosystem functioning? Recall that the majority of biomass energetically goes through this 'dead' chain, not the live chain.
Autogenic succession can occur on newly exposed landforms and in the absence of changing abiotic influences.
1. The three types of relationships ( facilitation, tolerance and inhibition ) which drive autogenic succession are defined in the chart below:
2. An example of facilitation - in which species of early successional stages alter conditions or availability of resources so entry of new species is made possible is shown below: Location is in the cold intersection of Alaska and Canada. Here the pioneer stage dominated by the mosses and prostrate willows which modify the environment adding valuable organic material which then holds moisture and nutrients, forming a low lying protective barrier ( wind/temperature) for newly emerging alders and spruce and so on.The mineral soil is replaced with an organic layer containing increasing amounts of available nitrogen in about 5 decades as the trees (alder ) which support nitrogen-fixing microbes are replaced by the spruce.
Autogenic mechanisms 2: tolerance and inhibition
Tolerance implies that later successional species are not inhibited or aided by the species that came before them. They succeed because they have a lower requirement of a limited resource than an earlier species. Thus in the example below, trees which can tolerate less direct light can survive when the canopy fills over.
Inhibition - the species that takes hold in a site and can prevent others from taking over wins - that is out competes any potential successor. An example of this can be found below in the competition between the algal species.... Gigartina can not take over unless ulva ( the limp green sea lettuce) is manually removed first. This indicates that ulva is inhibiting other algal species. Inhibition can take on devious routes as with toxins/alleochemicals, or whipping branches.
|General Trends in succession include;|
Explain why these changes occur as listed!
Recovery Rate from Resource Limitation Fast Slow
Biogeochemical Cycling Open and Rapid Closed and Slow
Rate of Net Primary Productivity High ------> Low
Community Site Characteristics Extreme Moderate (Mesic)
Ecosystem Stability Low High
Species Diversity Low -------> High? actually not so... see the notes on biodiversity for pattern
Life-History Type r K
Seed Longevity Long ---------> Short Why?
succession - is a succession where the
community becomes simpler and containing fewer species and less biomass
over time. Some retrogressive successions are allogenic in nature. As
an example, the introduction of grazing animals result in degraded rangeland.
Retrogressive succession - is a succession where the community becomes simpler and containing fewer species and less biomass over time. Some retrogressive successions are allogenic in nature. As an example, the introduction of grazing animals result in degraded rangeland.
Mount St. Helens
Disobeying the Rules of Recovery
by Sharon Levy
On the morning of May 18, 1980, Mount St. Helens erupted, belching ash into the air and sending a blast of hot gas, rock and mud down the side of the mountain. Shortly after the eruption, scientists arrived to study the recovery of life on the volcano - and have found surprise after surprise.
Between the lip of the Mount St. Helens caldera and the blue edge of Spirit Lake lies an expanse of barren pumice. Twenty-one years ago, this mountainside in the Cascade Range of southern Washington was cloaked in old-growth forest, which enfolded the lake. Now the landscape has been blown wide open, and thousands of tree carcasses float on the water's surface, bleached by the sun, snow, and wind.
On the morning of May 18, 1980, St. Helens erupted, belching ash into the air and sending a blast of hot gas, rock, and mud down the side of the mountain. The blast broke five-hundred-year-old trees as if they were matchsticks. Six hundred square kilometers of forest were devastated; even where the trees survived, the ground was coated with a thick layer of ash.
When ecologists arrived to study the recovery of life on the volcano, they expected to find a moonscape where nothing had survived. But the mountain didn't meet expectations
. "Every time you turned around, from the first hour of our observations, there was some new stratagem by which nature had persisted," recalls Jerry Franklin, a forest ecologist at the University of Washington at Seattle. "We were repeatedly struck in the face by the realization that hey, this place is alive."
St. Helens was alive in ways that stood scientific dogma on its head.
The textbook version of primary ecological succession held that simple autotrophs, such as lichens, mosses, and bacteria, should be the first colonists in a devastated area. These would prepare the ground for the first vascular plants to move in. Herbivores and then predators would show up only after the plant community was well established, in a slow, orderly process. "That's the way it was supposed to work," says Franklin. "There's supposed to be rules these organisms and successional processes follow. But there really are none."
Accidents of timing, weather, and topography had powerful effects on ecosystem recovery. Plants and animals that survived the blast while sheltered under spring snowbanks were alive and functioning immediately after the eruption. Wind-blown seeds that happened to lodge in cracks in the volcanic rock germinated and grew - including some species that had not been expected to return for years. The chance whims of the wind governed many of the early events in the recovery.
Long before plants could begin to recolonize the pumice plain above Spirit Lake, live insects and spiders rained down from the sky. John Edwards, a zoologist at the University of Washington at Seattle, found that representatives of 70 different insect families and 43 species of spider blew in on the wind during the first years after the eruption. The young of these species disperse by taking to the air and letting the wind determine their fate. Many land in inhospitable places such as alpine snowfields, or the blasted landscape of Mount St. Helens. Most of these unlucky animals die, but they don't go to waste. Within eight weeks of the eruption, Edwards saw carpenter ants and spiders preying on hapless insects that had fallen onto the mountain.
Lupines were the dominant plant pioneers on the pumice plain; mosses and lichens couldn't survive the heat or the drying effects of volcanic ash. Because they are able to fix atmospheric nitrogen, lupines were expected to act as nurse plants, creating soil that could sustain other species. This did happen eventually, but early on, the fallout of insects and spiders brought far more nutrients to the blast zone.
"At first, fallout was definitely the most important source of nutrients" says John Bishop, a botanist at Washington State University at Vancouver. "The same insect rain continues today, but its importance has diminished." As the ecosystem develops, plants grow up and create habitat for insects that live and breed on the mountain. The nutrient input from fallout, which seemed dramatic when the community was starting from zero, is now dwarfed by the production of living things native to the blast zone.
Bishop, who first came to Mount St. Helens ten years after the eruption, studies the ecology of the lupine patches. On the pumice plain, the gray of barren rock is now interrupted by lush carpets of lupine. On a windy summer day, the same breeze that blows puffs of ash out of the caldera carries the sweet smell of thousands of purple blossoms.
Bishop has found that the lupines form a stage whereon dramas of competition and survival play out. Lupines flourished in the blast zone for the first few years after the eruption, outcompeting most other plants and unfettered by pressure from herbivores. The first lupine seed germinated on the pumice plain soon after the eruption, but the insects that feed on lupines didn't arrive for about six years.** see below short article
When the herbivores did arrive - principally caterpillars that eat lupine leaves, roots, or seed pods - their populations exploded. "The caterpillars were probably escaping their predators for quite a while," says Bishop. "So the caterpillar population would explode to the point where they'd suppress the lupines, and the lupine population would start to crash." Lupines were in decline when they were rescued by the arrival of predatory spiders and parasitoid flies, which lay their eggs on caterpillars, leaving their young to hatch out and devour their herbivorous host. Now that caterpillar populations are controlled by predators, lupines are going strong again.
"When we showed that herbivores were limiting lupine growth, that was a very novel result," says Bishop. "There's been skepticism that insects may actually regulate plant populations. In primary successional systems, no one thought that herbivores could be that important."
Farther down the mountain, below the pumice plain, the 1980 blast flattened trees or left them standing dead, and the soil was buried under a thick layer of ash. In this area, called the blow-down zone, some unexpected survivors have been critical to the recovery.
Charlie Crisafulli, an ecologist with the U.S. Forest Service who has been working on Mount St. Helens since 1981, has learned a new way of looking at the animals he studies. "St. Helens was surprise after surprise," he says. "What the mountain has told us in many ways is that we lack a basic understanding of many species' natural histories, their tolerance limits, and their dispersal capabilities."
At first, the most obvious survivor was the pocket gopher, many of which weathered the cataclysm while safe in their underground burrows. "Gophers were enormously conspicuous," remembers Crisafulli. "You'd have this uniform gray ash that had been deposited over the landscape. The gophers were burrowing down in the old, rich dark forest soil, and they'd kick that up to the surface. So it was very obvious - in fact, many of the sites where gophers had survived we located from low-flying aircraft."
Before the mountain exploded, gophers had been confined to small openings in the forest. Afterwards, much of the blow-down zone was transformed into a giant meadow. The gopher population boomed, and as more and more gophers tunneled underground, they helped spread important soil microorganisms. "We've documented that gophers do transport the spores of mycorrhizal fungi and lead to the inoculation of plants," says Crisafulli. Many forest plants rely on a symbiotic relationship with these fungi to help them fix nitrogen.
The gophers have also helped other survivors - the twelve species of salamanders, frogs, and toads that made it through the eruption. Most of the amphibians were adapted to life in the cool, damp duff of St. Helens' old-growth forests. Twenty-one years after the eruption, the ground surface in summer remains far too hot and dry for these creatures, but hundreds of miles of gopher tunnels provide them with shelter from the heat.
The larval forms of many of these animals were protected under the frozen surfaces of lakes and ponds when the volcano blew. "Had the eruption occurred in August," says Crisafulli, "the consequences would have been entirely different." Three species of strictly land-living salamander were wiped out in the blast; the amphibians that still flourish on St. Helens have both aquatic and terrestrial life stages. Northwestern salamanders, for example, usually spend the first 15 months of their lives as larvae, living in and breathing water. Then they metamorphose, lose their gills, and take to the land, returning to their natal ponds only to breed.
In every population of northwestern salamanders, however, there are a few individuals that grow up and become sexually mature, but never leave the water. These eccentric individuals are called neotenes: giant-size larvae that are capable of breeding. In the aftermath of the St. Helens eruption, neotenes came to rule the waters of the blast zone. Most salamanders that metamorphosed were doomed. "They'd disperse away from the ponds looking for forest, and there was no forest out there, so they'd succumb," explains Crisafulli.
Today, the northwestern salamanders that populate the ponds and wetlands of the blast zone are 98 percent percent neotenic. For now, the ability to live permanently in water is key to their survival. A few of the terrestrial form of the salamander persist, holing up in the extensive network of gopher burrows. Over time, as the forest returns, the gopher population will dwindle and many of the burrows will disappear. But the terrestrial salamanders will make a comeback when they can once again find shelter in the cool of the woods.
The trees of St. Helens have taught scientists some profound lessons - even in death. At first it was feared that thousands of dead old-growth Douglas firs, which lay scattered everywhere in the blow-down zone, would impede the forest's recovery. This was one reason why the U.S. Forest Service had some areas salvage logged after the eruption.
That proved to be another false assumption. The dead wood has nourished new plant growth and provided habitat for insects, small mammals, and birds, speeding ecosystem recovery. The insights Jerry Franklin and other forest ecologists have gained on Mount St. Helens have changed the way forestry is practiced in the Pacific Northwest. Clear-cut logging was once the standard practice. Now, on federal lands, 15 percent of the timber in a harvest area must be left behind - dead or alive.
Today the blow-down zone on St. Helens is bursting with life. Hairy woodpeckers and northern flickers fly among the snags, feasting on insects. Pacific silver fir and western hemlock, some of which had spent decades growing slowly in the shadow of the great Douglas-fir forest, were shielded by snow on that day in May 1980. In the open sun among the fallen old-growth trees, they now shoot towards the sky: many that were inches high at the time of the blast stand 20 feet tall. If the mountain stays quiet for a couple of hundred years, a new forest, different but as grand as the one the volcano blasted away, will rise here.
Mount St. Helens yielded many secrets in the first years after the eruption. Funding and active research waned during the 1990s. But Bishop and Crisafulli are still finding new twists as the complex renewal of life on the mountain continues. Much remains to be learned. Nobody has yet tried to dissect the impact of returning elk on the lupine patches where they love to snooze, or quantified the differences between parts of the blow-down zone that were cleared of dead wood and those that were left alone. "There is an opportunity for new studies here," says Bishop. "I'm hoping we can bring in a new generation of researchers."