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Published by EMPORIA STATE UNIVERSITY
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Editor: JOHN RICHARD SCHROCK
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Cover: Nutrients tied up in dead leaves and other plant tissues are well protected by the plant cuticle. Succession requires that plant nutrients be returned to soil and be made available to future plants Shredders are important invertebrate animals that mechanically shred dead leaves, exposing the plant nutrients and dispersing the bacterial and fungal spores that can decompose the dead plants and build soil.
Many internal line drawings are by Robert Clarke.
The Kansas School Naturalist is indexed in Wildlife Review/Fisheries Review.
by Thomas Eddy and John Richard Schrock
WHAT IS "SUCCESSION"?
Succession is an orderly process of change in living communities over time and was first described for the Plains by Frederick Clements of Nebraska in 1893.
From its earliest use, succession has been viewed as a set of consequences, where climate and soil determine the plant communities and the plant communities determine the animals that can exist. Many modern biology textbooks perpetuate this story today. However, even from the earliest research, it is obvious that animals are a driving force in succession by recycling nutrients by: shredding plant litter; building up, turning over, or damaging soil; dispersing seeds; selectively grazing, and changing plant populations in other ways. Simply, take away the animal life and the succession of communities is soon altered, sometimes dramatically.
When succession begins with hard bedrock or other soil-less strata, it can take hundreds of thousands of years to build up the soil layers to support the climax vegetation that can grow in the local climate.
When soil layers are already present, farming, fires, tornados, floods, erosion, and other factors can push the community back to an earlier seral stage. Untended, the community will undergo rapid succession, usually in decades, back to the potential natural community since soil layers to not have to be built from bedrock.
The first organisms to establish themselves on rock and other barren strata are pioneer species. Lichens and mosses are commonly described as pioneer species that begin contributing to soil buildup, and the animals of these barrens are pioneer species as well.
Any gardener who abandons his garden knows that the grasses are soon shaded out by tall weeds, which in several years are overshadowed by cedars and shrubs, before eventually becoming forest if there is enough rainfall. These intermediate stages are "seral stages" and many hundreds of sequences have been described for the many different climates and soils of the world. Animals are important actors as grazers, seed dispersers, and in other ways to drive succession along different pathways.
Anyone who has walked through the few remaining U.S. virgin forests, or across virgin grasslands, realizes these magnificent communities of plants and animals are what widely existed in the absence of human's extensive farming, industrial and settlement activities. In the absence of human disturbance, we assume that disturbed communities would undergo secondary succession back to climax communities of grassland or forests. This is not always the case.
A climax community perpetuates itself and is not followed by a different subsequent community. Unless there are local natural disturbances or new glacial ages, we expect dry prairies to remain prairies and forests to remain forests.
Climax communities were once considered "stable" or unchanging in their composition, but more recent research shows the climax plant and animal community is in a constant state of flux.
Because human disturbances are now everywhere, the term "potential natural community" can more accurately replace "climax."
FREDERICK CLEMENTS - FATHER OF THE SUCCESSION CONCEPT
Born in the Great Plains in 1874, Frederick Clements grew up in prairies dominated by shortgrass or tallgrass hills and cottonwood valleys. The plow, fires, tornadoes, bison herds and cart tracks could tear away at the landscape. And gradually the ruts would fill in, weeds would fill the gaps and soon the prairie would return. The young Clements saw what everyone saw, but thought what no one before had imagined. This was a grand design in nature and he called it "succession."
Frederick attended the University of Nebraska and, at the age of 23, completed his thesis on "The Phytogeography of Nebraska." He had driven a mule train across the state, and his careful and descriptions of pioneer plants grasslands offered a convincing explanation of successional change.
However, Clements came to view plant communities as super organisms that invaded as a team: "As an organism, the formation arises, grows, matures, and dies."
He believed that the whole group of plants cooperated in invading and forming the next stage: "Furthermore, each climax formation is able to reproduce itself, repeating with essential fidelity the stages of its development. The life history of a formation is a complex but definite process, comparable in its chief features with the life-history of an individual plant."
But do plants move as communities to invade a disturbed area? For decades, botanists argued whether Clement's plant communities or individual plants competed for space. The issue was mostly settled when Robert Whittaker found that individual species of plants-not communities-gradually changed as conditions changed. Today, most ecologists recognize that competition among individual plants is the major force that drives succession, not between plant communities. The "invading-community" concept of Clements was proved wrong, but his central idea of succession found great use in forestry, agriculture, and reclamation.
In 1913, the zoologist Victor Shelford of University of Chicago Communities in Temperate America. Dr. H. C. Cowles had earlier described the succession of plants along the dunes lakeshore of Lake Michigan, to the east of Chicago. Shelford went into far greater detail describing the successional stages as associations and formations, and utilizing the far greater diversity of animal life to distinguish the communities.
For instance, the Carolina grasshopper was associated with the bare clay through sweet clover stages, while the yellow-margined millipede is found in shrub through forest stages.
While his study was exhaustive in description, he mainly examined how light and temperature and glacial history led to the plants that in turn supported the animal life. This remains the view reported in many textbooks today. He did not consider the possibility that the animals were also major causative agents in succession.
POTENTIAL NATURAL COMMUNITIES
In major portions of China, India, Europe and the U.S., it is difficult to find "virgin" land undisturbed by human history.
The potential natural community (or "PNC") is what we suspect would be established under present conditions if succession occurred without further human interference.1 But present conditions may include the heat of downtown buildings or soils compressed by farm machinery. Thus a vacant lot or farm field may not return to forest in Europe, China or eastern U.S. Nor may a wheat field return to natural tallgrass prairie under existing site conditions in Russia or western Kansas.
Pollution, soil damage or other conditions may pass a threshold where natural succession back to the historical climax community is no longer possible. In this case, an "altered PNC" replaces the natural climax.
HOW DO WE KNOW IF A COMMUNITY IS A SERAL STAGE OR A CLIMAX?
Over a period of time, ecologists can detect changes in plant and animal species. This directly shows a community is moving through seral stages.
However, careful examination of a community at one point in time may show that there are only old members of some tree species while young saplings are a new or invading species. Such a community is obviously still changing-a seral stage.
If this was a climax community, the distribution of young and old plant and animal species would not show a directional change. Species of animals and plants would drift slightly but randomly.
SUCCESSIONAL INDICATOR SPECIES
When a species is only common during one successional stage, it can serve as an indicator species. At first, only plant species were used as indicators. Today, we use insects, mites, birds and other animals to assess the stage of succession.
Birds and rodents are major actors in a back-and-forth succession of creosote bush and cactus in Texas. The creosote bush, Larrea tridentata, spreads its seeds by wind and colonizes open spaces. Birds roost and rodents shelter under the bush, depositing seeds of the cactus, Opuntia leptocaulis. The cactus grows under the creosote bush. Its roots outcompete the creosote bush roots and eventually the creosote bush dies. Standing alone, the cactus soon dies as rodents, wind and water expose its shallow roots. This again leaves a barren area that can be invaded by windblown creosote bush seeds.
THE LACK OF DOMINANCE IN TROPICAL CLIMAX FORESTS
In temperate climates, climax forests usually consist of a few dominant species, such as beech maple or oak-hickory, that make fairly uniform forests best adapted for the wetter or drier conditions. But in the tropics, no tree dominates. Instead, a dense climax rainforest may be made up of hundreds of different species. Not only is there no dominant species in the tropics, but two members of the same species are usually quite far apart in a tropical forest. Why?
Entomologist Dan Jantzen discovered the answer: insects! Trees rely on seeds to produce the next generation. In the temperate zones, winter stops the insects and slows down rodents that destroy these seeds. But in the tropics, there is nothing to stop these seed predators from cleaning up every last fruit and seed. Jantzen scattered seeds under parent trees in Costa Rica and measured survivorship; the attack on tree seeds approached 100 percent. With seed predators concentrated under parent trees, a sapling can only survive at great distances away. Climax tropical forests do not contain dominant trees because tropical forests do not have winters to control the insects!
In 1962, D.A. Crossley and Mary P. Hoglund conducted the first litter bag study of small arthropods that help change leaf litter into rich soil. Additional researchers confirmed their simple study in far more detail.
They plucked leaves from trees at the time of leaf fall and placed a measured amount inside bags. The bags varied in mesh size-some had small holes and others merely contained the leaves while most arthropods could "walk on through."
The bags were left on the forest floor under natural climatic conditions, and examined regularly.
The results were clear. Leaves that were sealed off from larger arthropods decomposed at a slower rate. The larger the holes, the more rapid the breakdown. The study of "shredder biology" had begun.
Soil organisms were actually broken into two groups. Some arthropods were true soil organisms that did not enter the organic leaf litter. But many organisms lived to shred plant leaves and stems and roots.
Springtails were well represented. Many springtails are today known to be responsible for mechanically breaking apart plant tissue. Also, spores of some species of fungus must pass through the springtail gut before it will "germinate" and grow on the shredded plant tissue. Since the fungus cannot invade and decompose the plant cell walls if they are intact, the fungus "needs" the springtail to shred the plant so it can finish the job of decomposition. The breakdown of plant litter, and mixing it with the mineral part of soil to produce humus, is called humification.
Other important shredders include mites, pillbugs, millipedes, and many insect larvae. Many of the arthropods you can collect in leaf litter through a Berlese funnel are shredders.
Ever wonder what happens to all the dead tree leaves that fall into streams in the fall? Do they eventually all wash into the ocean?
Of course, the huge amount of plant tissue including leaves that wash into streams is shredded within weeks to months, just like leaves on a forest floor. These shredders and the microbes in their gut, play the key role in converting the coarse particulate matter into a fine soup of nutrients. Once released, the nutrients then provide for additional plant growth.
Most important of the aquatic shredders are the amphipods (freshwater shrimp) and isopods (sowbugs), stoneflies, caddisflies, and some mayfly and fly larvae. Without them, the clear streams of spring would still be choked with the dead leaves from fall.
ANTS REVEGETATE THE OUTBACK!
Much of our aluminum comes from bauxite mined in Western Australia. These more modern operations were careful to scrape off and store the topsoil before removing the subsoil and extracting the ore. When mining was complete, the subsoil was returned and the rich topsoil was layered on top. Then native grass seed was scattered across the surface and...nothing grew!
Since they used fresh native grass seed taken from adjacent land, this greatly puzzled scientists. Then they examined what occurred in the unlined grassland. In undisturbed land, any grass seed that lands on the ground is rapidly grabbed by ants and carried underground. On the barren mine topsoil that lacked ants, the seeds "cooked" in minutes. To revegetate the Australian bauxite stripmines, they had to add colonies of ants!
"BUGS" RECLAIM STRIPMINES!
Eastern Europe has a tremendous amount of stripmined land, with huge areas of toxic spoilbanks where subsoils lay on the surface and the topsoils were buried deep. Succession helps reclaim the land, but with the topsoil missing, the process is slow, being halfway between primary and secondary succession.
Ulrich Neumann, a German scientist studying these reforestation efforts, found millipedes and isopods increased with canopy closure. For the first time, a scientist recommended importing soil rich in these organisms in order to develop additional soil at the earliest possible moment.2
2 Neuman, U. 1973. Succession of soil fauna in afforested spoil banks of the brown-coal mining district of Cologne. In Hutnick, R. J. ad G. Davis (eds.) Ecology and Reclamation of Devastated Land. Gordon and Breach, New York.
BIRDS AND SUCCESSION
Have you ever wondered why berries or poison ivy plants are so much more common at fencerows or along the edges of forests under the shorter trees?
Early ecologists described the successional sequences of birds found from grasslands to deep forests. It was obvious that the birds depended on the food resources, nest building materials and architecture provided by the plants. But some birds also "return the favor" and are responsible for the distribution of plant species.
DEAD DODO, DEAD TREE!
On Mauritius, an Island in the middle of the Indian Ocean, the dodo bird was the only means by which the Calvaria tree dispersed. When the dodo was driven extinct, the Calvaria seeds were not scarified by passage through the bird's digestive tract, and no new seeds sprouted. Luckily this was discovered before the aging Calvaria trees died, and foresters now artificially scratch the seeds.
ANTS AROUND A PRAIRIE ANTHILL
Western harvester ants accelerate secondary succession in the shortgrass region of western North America. Seeds gathered by worker ants from surrounding areas are carried into the food storage chambers of the nest where they are eventually eaten or rejected as food. Rejected seeds are carried to the surface and deposited at the edge of the clearing around the cone-shaped gravel and earthen mound. New nests are established in old fields or other disturbed sites adjacent to native vegetation. Seeds carried from the native vegetation may survive to establish native species in the old field around nest sites. Dispersal of progeny from these plants by wind, water, and ants from newly established nests then continues the successional process.
HOW PLANTS BRIBE ANTS TO DISPERSE THEIR SEEDS
Many plants produce seeds with oil bodies, called elaiosomes. These elaiosomes are eaten by ants, although the seed itself is not eaten, and may be carried about and stored. For many plants, this is a great advantage because dropping seeds alongside the parent plant places the offspring in competition
with the parent plant that may already have exploited the local nutrients. Wild violets are one case of such dispersal, and this accounts for why they do not occur in large clumps on forest floors but are spread thinly through a forest.
Humans are not the only organisms that disturb the land and return it to an earlier stage of succession. Prairie dogs used to exist in huge "towns" that extended several hundred miles across western prairies.
Similar to ants and earthworms, prairie dogs bring up substantial amounts of soil and have thereby "plowed" much prairie grassland. Ecologists estimate about 200-225 kilograms (nearly 500 pounds) of soil is brought up per burrow with 50 to 300 burrow entrances per hectare. This plowing of the soil alters its properties for thousands of years to come.3
But more drastically, the prairie dogs also crop the vegetation around their dens, pushing back the thick grassland and leaving a barren zone. While this might suggest the creatures utilize the nearest resources for food, they also clip stems they do not eat. This clearly serves to provide a zone of safety. Snakes can easily corner a prairie dog in its burrow, but in the open the praire dogs can get away. The barren zone provides the prairie "watchdogs" with plenty of early warning.
ANIMALS THAT "PLOW"OUR SOIL
No less than Charles Darwin took an interest in the extent earthworms "churn" the soil. This is a particularly important function in establishing and maintaining rich soils. The holes allow roots to grow easily and "breathe" compared with soils compacted by hooves or tires. Water easily drains down and away, and shredders move more readily through the soil, recycling the nutrients.
Like a pipe pushing through the soil, an earthworm also ingests and partly digests plant debris, and leaves behind casts that make the soil richer.
Charles Darwin published his research titled "The formation of vegetable mould through the action of worms." Through careful calculations, Darwin figured that earthworms may turn over two inches of soil per decade in a rich garden plot. Such natural plowing of the soil by earthworms, ants and prairie dogs. is called "pedoturbation."
Large herbivores, including the North American bison, and native grassland plants co-evolved, to the benefit of both.
However, overgrazing by confined livestock reduces the density and vigor of desirable forage plants and can expose soil to erosion. Productivity of the grassland then decreases as the vegetative composition changes and less desirable plants invade. The system reverts to an earlier successional stage.
If livestock are kept in balance with the available forage, plant vigor and density will return.
MANURE, AND DUNG BEETLES
The Great Plains is the breadbasket to the world-the most productive land on earth. Perhaps you might intuitively think a forest would be more productive, but the world's major crops of rice and wheat and maize and sugar cane are all annuals. That means that the plant nutrients must recycle through the soil year after-year. The deepest, richest topsoils are grassland soils. And these soils absolutely require grazers and dung beetles!
Crops and other grasses are adapted to be eaten by grazers. Instead of bearing their growing meristem tissues on the tips of stems, as do shrubs, the meristem is at the base of the plant lower than where a grazer munches. The ends of the blades that are eaten are soon fermented, partly digested, and redeposited as manure for fertilizing the soil.
However, the dung pads from cattle and similar grazers becomes a problem when they dry into hard pads. A cow can average twelve pads a day and blanket up to an acre of land a year. This is precisely what happened when cows were introduced to Australia, but without dung beetles. The 30 million cattle, producing over 300 million dung pads a day, were "paving over" as much as six million acres of pasture per year.
This was not a problem in Africa, where nearly 2000 species of dung beetles are available to bury all the varieties of dung each day. In less than 24 hours, 90 percent of a dung pad can be mixed and buried in the soil. By importing African dung beetles, Australia solved its pavement problems.
The critical base for many successional events is the "seed bank" or seed reservoir. A seed bank is the collection of ungerminated seed that is available in the ground to replace adult plants. Seeds that are too deep for possible germination, or that are otherwise not viable, are not part of the seed bank.
MAMMALS AS DISPERSAL AGENTS
The disperal of seeds by grazing animals contributes to succession. It is suggested that a plant's luscious foliage performs the ecological role of attracting grazing animals. Seeds eaten are then voided with the droppings at a considerable distance from the parent plants.
4 Heinrich B. and G. A. Bartholomew. November 1979. The Ecology of the African Dung Beetle Scientific American 241(5): 146-156.
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