Figure 33.-Primary succession in mountains such as the Rockies begins with bare rock.  Lichens (left foreground) may eventually create places where mosses (center foreground) can grow.  Finally, many thousands of years later, a community of ferns, forbs, and grasses may evolve (right foreground).  If conditions are very favorable, a community of shrubs and trees may eventually develop.

Figure 34. -Succession on abandoned wagon train trails in the Great Plains. The bare soil ( 1) of the most recent trails first supports scattered annual 'weeds (2), then short-lived grasses (3), and later early perennial grasses (4).  From 20 to 40 years after abandonment, the trails once more support the original dense grass climax (5).

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Succession also occurs in less visible places.  Many different communities of soil insects and microorganisms developed during the transition from bare dune sand to the moist organic matter-rich soil of the forest.  Even wounds in living trees, like bare fields, are colonized successively by communities of microorganisms; this colonization usually begins with "pioneer" bacteria, followed by staining fungi, and eventually by decay fungi.
Succession of Saprotroph decomposer communities within a fallen tree (fig. 35) begins when the dead wood is attacked by insects and microorganisms, and ends when the wood is decomposed by wood-rotting fungi, bacteria, and other Saprotrophs.

Figure 35. -Succession in a fallen pine tree.

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FACTORS AFFECTING SUCCESSION
Successions of birds and animals also occur-usually as a result of changes in the plant community.  And animals and birds can significantly affect the course of succession.  Birds and animals often introduce seed from plants of more advanced stages, and burrowing animals provide habitats of mineral soil for plants of earlier stages.  In mixed plant communities, overbrowsing of early stage species by herbivorous mammals and insects can speed up succession.  But, overbrowsing in general usually sets succession back to an earlier stage.
    Succession also can be set back by other major disturbances - both natural and human-caused.  Fires, hurricanes, and logging operations are some of these disturbances. 
But we might ask ourselves, so what?  What is actually happening ecologically when the Game of the Environment is played by nature's Rules?  Why is this important to people?  We can provide some answers to these questions by comparing the early and late stages of succession.

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SUCCESSION VERSUS PRODUCTIVITY
In the early stages of community development, materials produced by photosynthesis accumulate faster than they can be degraded through respiration.  Energy flows primarily by the grazing process in short, simple food chains. 
    As biotic development proceeds and biomass increases, respiration also increases; eventually the production of materials by photosynthesis may nearly equal that being degraded through respiration.  Energy is channeled by complex pathways through intricate food webs that mostly entail breakdown of detritus by Saprotroph decomposers.

PEOPLE AND SUCCESSION 
The characteristics and principles of ecological succession are very important to us.  In most cases, modern agriculture and forestry rely on holding natural biotic development in check, or interrupting it at some point and starting it again.  Thus, we grow simple communities-often single species or monocultures-over large areas.  Experience has shown that these systems are very vulnerable.  They can be hard hit very quickly by insects and disease or by drought or other adverse environmental factors so that we must continually subsidize them with energy in the forms of pesticides, fertilizers, and fossil fuel.  Therefore, to supply the food and fiber needs of today's population, we must break the Rules of the Game that regulate the development of natural ecosystems. 
    At the same time, we are beginning to understand that natural ecosystems also fulfill needs that are important-the modification of harsh climates: the cycling of nutrients, and the production of clean water.  And, of course, the recreational and esthetic needs of people are often best satisfied by natural ecosystems.  A logical strategy is to maintain diverse mixtures of early and later "stages" by compartmentalizing the landscape.

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---- ---- ---- ---- ---- ----

 For some natural ecosystems in your area, describe some of the steps that you can see in the biotic development (succession) of the plant communities. 
 For your area, list some plant communities whose development is regulated or held in check by people.  How and why are these communities regulated?
 Compare a natural and a people-regulated plant community in your area in terms of the numbers of different plant and animal species present, the sources of energy, and the sources of nutrients.
 Describe some ways that succession, or biotic development, has determined your lifestyle or those of members of your community.

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Figure 36. - Wildfire may be a powerful natural Foul in that it interferes with the nutrient cycling and energy flow Rules.  Some natural communities exist because fire "breaks" the Rules.

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SOME OF NATURE'S FOULS

The successional patterns of the natural ecosystems just described occur when the Game of the Environment proceeds unhindered by major upsets.  But upsets are common and natural.  Defoliation by insects in the Northeast, hurricanes in the Southeast, wildfire in the West, drought in the Plains, and volcanoes in Central and Latin America are some natural events that interfere with (break) the energy flow and nutrient cycling Rules of the Game.

Wildfire
Wildfire (fig. 36) has a powerful influence on patterns of vegetation. Fire sets back succession and influences species composition.  Some species are favored by fire; others are hindered or are eliminated. 
    Fire is a major agent in decomposing organic matter and in recycling nutrients.  Periodic fires preserved the vast prairies and oak openings of yesteryear; they are responsible for maintaining the shrubby California chaparral, the jack pine forests in the Midwest and in Canada, and the mixed pine flatlands of the Southeast.  The important role of wildfire in natural systems is now being recognized; under certain circumstances, fire in wilderness areas is permitted to run its course. 
    Fire has long been exploited by people-primarily in setting back succession to a more productive stage.  Foresters use controlled ground fire to suppress hardwoods in pine forests, to create openings and food for wildlife, and to reduce accumulated litter.  Thus the use of fire prevents catastrophic hot crown fires and perpetuates useful shrub species, such as blueberry and huckleberry, which serve as food for wildlife and people.  Fire is used to recycle nutrients in grass communities, to eliminate encroaching weed shrubs, and to reduce the number of disease organisms on stubble and other plant parts after harvesting.

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Drought (fig. 37) also has greatly influenced the development of ecosystems.  The severe drought of the 1930's caused widescale alterations in species composition, and major disruptions of communities.  As plants died, their roots were no longer able to hold the soil-the result was massive erosion of topsoil rich in nutrients and organic matter.  During this period, succession was often set back to "square one" - bare subsoil.

Figure 37 - Severe drought is a natural event that results from disruptions of the water cycle, and that, in turn, interferes with the cycle of nutrients and flow of energy in ecosystems.

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"Square one" also is the successional stage after volcanism (fig. 38).  Bare volcanic rock may suddenly appear where only minutes before grew lush tropical or subtropical vegetation; also, volcanoes are natural sources of pollution because they emit particulates, heat, and gases.

Figure 38. - A volcano may suddenly and violently set back succession to "square one, ,.. or bare subsoil.

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Powerful hurricanes, tracking north and east from their spawning grounds in the Caribbean, periodically "sweep" the forests along the southern and eastern coasts of North America (fig. 39).  Records suggest that hurricanes remove the "big trees" of the eastern forests every 100 to 150 years; when this happens in mature climax forests, the light that reaches the forest floor allows light-demanding plants to grow on mounds of soil raised by the tipped roots of the toppled giants.  In this way, hurricanes (and other wind storms) create a diversity of species by providing gaps for species that demand light.

Figure 39. - Hurricanes periodically interrupt or influence succession in North America.

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"Overgrazing" by defoliating insects also can disrupt natural forest ecosystems.  Defoliation-removing the food-making machinery-seriously interferes with energy flows.  The ingestion of leaves that otherwise would have fallen and decomposed interferes with normal nutrient cycles.  Periodic severe outbreaks by insects such as the gypsy moth (fig. 40), spruce budworm, tent caterpillar, and cankerworm can alter species composition significantly and influence succession. 
    Repeated defoliation of oaks by the gypsy moth may result in the death of significant numbers of trees; depending on the site and stage of development, defoliation also can advance or set back succession.

Figure 40. - Defoliation by insects alters energy flow and nutrient cycling.

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Figure 41.-Domestic and wild animals can overgraze their energy and nutrient supplies if they become too numerous.

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Outbreaks of gypsy moths are frequently associated with a previous disturbance in the forest caused by humans or nature. Recent evidence that low populations of this insect are strongly regulated by predatory insects and small woodland mammals (deer mice and shrews) suggests that disturbances that reduce predator populations (for example, fire or urbanization) may indirectly trigger insect outbreaks.
    Outbreaks by other grazer Consumers due to the control of predators are well documented.  The unwise destruction of Consumers at the tops of food chains - wolves, coyotes, hawks, eagles-for sport or for protection of domestic and wild animals has resulted in explosions of grazing prey populations (fig. 41).  In many instances, the outcome has been overgrazing and depletion of the range, and the starvation of these "protected" herds. 
    Similar consequences have followed the introduction - intentionally or accidentally-of herbivores into ecosystems that have no predators.  Thus populations of the introduced gypsy moth seem to be explosively higher where the insect moves into new territories than where it has been established for many years.  When plotted, such population patterns typically follow a J-shaped growth curve, the population rises exponentially until the resources of the environment suddenly become limiting.  When this point is reached, the population usually crashes to a low point.  With time, subsequent oscillations in populations of some organisms may become less violent, and population levels fluctuate near the maximum number that can be supported by the given habitat.  This population level is known as the carrying capacity.

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Carrying capacity is an important concept.  Basically, it tells us that there can be only so many Players of one kind in an Arena.  The J-shaped curve is the growth form of populations not responsive to the carrying capacity.  Often, however, increasing populations do respond to increasing environmental limits.  In these cases, the shape of the population growth curve is an S; it begins slowly, then increases rapidly, and later, as resource limitations provide negative "feedback," levels off near the carrying capacity. 
    It is important to point out that outbreaks of native insects occur periodically, and that they may contribute in major ways to the development of an ecosystem.  In the terminology of our Game, these Consumer Players, by interfering with the flow of energy and nutrient cycling (excessive grazing versus the normal slow decomposition of detritus), can alter succession.  Outbreaks of the spruce budworm, like fire-in the jack pine forests, are believed to be the primary agents responsible for periodically killing off, renewing, and thus perpetuating the northern spruce-fir "forest of catastrophe." 
    The human need for a predictable and continual supply of wood led to attempts to alter these natural outbreaks - primarily by controlling insect populations with chemicals.  But wide scale use of certain pesticides brought other adverse effects (see pages 132 and 133).

 List some other natural events (Fouls) that have interfered with energy flows or nutrient cycles in our country.

 What natural Fouls have you encountered in your community?

  In what ways have they interfered with the Rules of the Game?

 Describe the ways in which succession could have been affected by one of these events.

 How have people's activities or plans been influenced by natural events in your area?

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Like nature's Fouls, the adverse effects of human actions are primarily due to interfering with and breaking the Rules of the Game-to messing up the natural order of things.  Our general view of these environmental "problems" is intended to serve not as a warning of impending doom but as a guide for future action.  We have an unequaled capacity to wreak havoc with our environment and, therefore, to endanger ourselves.  But we also have the capacity-unique among all creatures-to evaluate and correct past mistakes.  Our future course must be guided by hindsight-and by an understanding of ecological relationships. 
    The unparalleled ability people have to "move things about" has created adverse and complex situations that must be faced by future generations.  Figures 42 through 58 summarize some of our Fouls and Penalties as we manipulate ecosystems, deplete finite resources, and pollute our environment.  That all of these actions are closely interrelated emphasizes once more the interdependence of ecosystems and of events within them.

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Figure 42. - People often move too much, too often, and in too little space. Domestic and industrial development of the flood plain and improper strip mining on the hills above can create serious flood and siltation problems.

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Manipulation of ecosystems
Disruption of flood plains

Let's consider the following scenario centered on the City and the river around which it was built (fig. 42).  Initially the source of power, transport, food, and water (upstream) and waste disposal (downstream), the river is now a "problem."  Encroachment by buildings, shopping centers, and highways onto the river's flood plain have created a flood problem; this requires the construction of a dam-the "solution."  But siltation behind the dam from upstream developments is now decreasing the effectiveness of the dam.  Downstream, the false security created by the dam's presence encourages further clearing, filling, and developing of the convenient flood plain land and the surrounding watershed.  Housing developments and industrial complexes encroach even onto the salt marshes.  Destruction of the marsh "nurseries" brings about the demise of the once thriving sport fishing of the harbor.  And runoff from nonabsorptive surfaces and erosion accelerates silting of the harbor-a problem that is "solved" by dredging.

Dredging and dumping
But a new problem arises: what to do with the dredged spoils?  Previous solutions, returning the spoils to land to fill "waste spaces" such as wetlands or dumping them at sea, are now questioned because of their adverse environmental impact.

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Figure 43. - Outbreaks of diseases and insects (group of yellow trees on hillside, left center) can occur when trees are rendered susceptible to them by factors such as pollution.  This can be especially serious when too much of the same crop is grown in one area.

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Growing monocultures
In trouble now are the farmlands and the forest.  Agriculture is relegated to less fertile land above the flood plain; farmers and foresters are forced to focus only on a few productive species.  The growing of a single species to the exclusion of others on a large area is known as a monoculture.
    There are outbreaks of bark beetles (fig. 43) in the pine forest monocultures weakened and predisposed to insect attack by air pollution, especially that from automobile exhausts.  Harvests are now being conducted to salvage the dead and dying trees.  Improperly installed logging roads used in this operation augment erosion and silting of the harbor.  Air pollution also affects the agricultural food crops "downwind" of the city.  Impacts on the city's food source will be great if the monocultures are susceptible to the high levels of oxidents reached during "episodes" of inversion (air trapped under a warm upper layer that prevents the pollutants from rising). (See page 141.)

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Strip mining
When the city's demand for energy exceeds the river's capacity to produce it, the search for other sources leads to the coal in the hills to the west (fig. 44).  Now the unreclaimed spoils disgorge their acids into streams and the river, eliminating them as sources of potable water and desirable fish species.  
Excessive predator control 
On the hills to the east, sheep that were "protected' when their predator-the coyote-was poisoned now must compete with high populations of rodents (fig. 45).  (Today, poisoning of coyotes is strictly controlled and limited primarily to offending, individual animals.)  The consequent overgrazing and deterioration of the range leads to the elimination of plant cover and to erosion.
    Thus by merely moving things about and by "shaping" nature to suit our immediate needs in what seemingly is the most practical manner, we have often created problems-and the solutions, in turn, have created additional problems.  As a powerful geophysical agent, we can counter natural succession, block critical steps in important food chains, and modify natural habitats.

Figure 44. - Unreclaimed strip mines -a blight on the landscape.

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 What "manipulation" Fouls have been committed in your community?

  Describe. some consequences of these Fouls.

  How could they have been avoided?

 How can they be corrected?

  How would the "corrected" Fouls change lifestyles in the community?

 How would your family's activities be affected?

Figure 45. - "Protecting" herds of grazing animals by removing their "predators" can backfire when competition by other grazers results in destruction of the natural range.

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