Explain How People Can Affect the Process of Succession in an Ecosystem.

Human Ecology - Basic Concepts for Sustainable Development

Environmental success stories from effectually the globe with their lessons on how to turn from decline to restoration and sustainability.

ecotippingpoints.org

Author: Gerald G. Marten
Publisher: Earthscan Publications
Publication Engagement: Nov 2001, 256 pp.
Paperback ISBN: 1853837148
Hardback SBN: 185383713X

Data for purchasing this volume:
United states/Canada - Stylus Publishing
Elsewhere - Earthscan Publications
Japanese version - Amazon Japan

Dorsum to Human Environmental - Table of Contents

Affiliate vi - Ecological Succession

• Ecological Succession
• Human-induced succession
• Managing succession
• Things to think about

Natural processes continually alter ecosystems. The changes can have years or even centuries, working and then slowly that they are scarcely noticed. They have a systematic pattern generated by community assembly, post-obit an orderly progression know as ecological succession, another emergent belongings of ecosystems.

Ecosystems change themselves and people change ecosystems. People change ecosystems to serve their needs. Intentional changes by people can fix in motion chains of effects that lead to further changes - man-induced succession. Sometimes changes are unintended. They tin be unwanted and they can be irreversible. This chapter will give three examples of homo-induced succession:

1. Overgrazing and pasture deposition.
2. Overfishing and replacement of commercially valuable fish by trash fish.
3. Severe forest fires when forests are protected from fires.

Since ecological succession tin can be of immense applied effect, humans accept responded past developing a variety of ways in which to integrate their employ of ecosystems with the natural processes of succession. Modern society uses intensive inputs to maintain agricultural and urban ecosystems by opposing the natural processes of ecological succession. Many traditional societies have drawn on centuries of experimentation and experience to develop strategies that have advantage of ecological succession in ways that allow them to use fewer inputs. This chapter will describe examples from traditional management of village forests and traditional agriculture.

Ecological Succession

Do places with the same physical weather condition always have exactly the same ecosystems? The answer is 'no'. Firstly, random elements in biological community assembly tin can lead to dissimilar ecosystems. Secondly, ecosystems experience slow but systematic changes every bit community assembly proceeds. A single site has different biological communities, and therefore dissimilar ecosystems, at different times. The slow but orderly sequence of different biological communities at the same site is ecological succession. Each biological community is a stage of ecological succession.

Modify from one biological customs to another tin can happen considering:

• Smaller species of plants and animals mostly grow and reproduce rapidly. Larger plants and animals take more fourth dimension to abound, and their population growth is slower. As a result, the rapidly growing plants and animals populate a site first, and the slower ones take over later. For example, if a fire or logging destroys a wood, in that location volition exist many species of grass growing on the site inside months because grasses grow rapidly. Later on, shrubs grow over the grasses, and after that trees grow over the shrubs.
• A biological community can create conditions that lead to its own destruction. For instance, every bit copse grow older, they become weak and vulnerable to destruction by insects or diseases. When this happens, a biological community 'grows old' and 'dies', and some other biological customs takes its place.
• One biological customs tin create conditions that are more suitable for some other biological community. A biological customs tin can change the physical or biological atmospheric condition of a site, making it more than favourable for some other biological community. One biological customs therefore leads to another.
• A biological community tin can be destroyed by natural or man-generated 'disturbances' and replaced by some other biological community. Fires, storms and floods are examples of natural disturbances. Human activities such as logging or immigration state to make agronomical or urban ecosystems tin can as well destroy a biological community. Activities such as excessive angling or livestock grazing can alter a biological community so much that it is replaced by a unlike community.

Earlier stages of ecological succession are known as 'immature'. They are simpler, with fewer species of plants and animals. Every bit community assembly progresses, the biological community becomes more circuitous. Information technology accumulates more species, many of them more specialized with regard to diet and the style they interact with other plants and animals in the nutrient spider web. The ecosystem consequently becomes more than 'mature'. The terminal phase of succession is a climax customs. Climax communities do not alter to another stage by themselves. The progression from immature biological communities to mature and climax communities is ecological succession.

An example of ecological succession

Ecological succession typically begins when the existing biological customs has been cleared away by human activity or natural disturbance such every bit a burn or severe storm. This may happen over a large area, but succession tin can also begin in a small patch of wood that is opened upward where an sometime tree has fallen. In western Japan, short grasses and minor annual flowering plants more often than not marker the first stage of succession (Figure vi.1A). Later a few years, they may be outgrown by taller grasses. Eventually, immature copse and shrubs grow upwards through the grasses to course a mixture of tree saplings and shrubs that is dense enough to shade out most of the grass (Figure half dozen.1B). Some of the saplings ultimately abound above the shrubs to form a forest. Some of the shrubs disappear, while others survive between the trees.

If the soil is deep, the first forest is typically a mixture of deciduous oaks with other trees and shrubs (Figure vi.1C). The oaks and about of the other trees are somewhen replaced past shii and kashi trees to class a climax wood (Effigy 6.1E). (Shii and kashi are broadleaf evergreen trees in the beech family unit.) As the plant species in the biological community change, the animal species besides change because particular species of animals use particular species of plants for food or shelter.

Figure 6.1 - Typical ecological succession on deep soil in western Japan

Why does oak forest appear first in ecological succession, and why exercise shii and kashi eventually replace it? Shii and kashi abound slowly, but they live for hundreds of years. With time they can grow to a cracking pinnacle. Oak trees grow apace if they take enough of sunlight, just they exercise not grow every bit tall. When shii, kashi and oak seedlings abound together in an open young ecosystem (Figure 6.1B), the large corporeality of sunlight favours oaks, which grow faster than the shii and kashi. The first forest in the ecological succession is therefore oak copse with shrubs and small-scale shii and kashi trees beneath them (Effigy 6.1C). Shii and kashi tin can survive in the shade of oak trees, and they slowly increase in height. Afterward nigh 50 years the forest is a mixture of oaks with shii and kashi all most the same height (Figure 6.1D). By this time some of the oak trees are former and senile and some may be covered with vines that 'smother' them. The oaks begin to decline.

Eventually the shii and kashi grow above the oaks to grade a dumbo leaf canopy that shades everything below. Oaks cannot survive in the shade of shii and kashi, and so the biological community changes to a climax wood of tall shii and kashi with a scattering of young shii and kashi and shade-tolerant shrubs beneath (Figure 6.1E). The unabridged progression from a grass ecosystem to a mature shii and kashi forest takes 150 years or more. Considering younger shii and kashi trees grow into the space that opens up when an old tree falls down, the climax forest stays more than or less the aforementioned unless it is destroyed by human action, fire or another astringent disturbance.

The ecological succession on shallow soil in western Nihon is different from the succession on deep soil (Effigy half-dozen.2). Oaks, shii and kashi require deep soil to grow tall, but pine trees exercise well fifty-fifty if the soil is shallow, provided they have plenty of sunlight. The climax ecosystem on shallow soil is pine forest. Pine saplings may also thrive in open sunny areas with deep soil, merely other trees eventually predominate on deep soil because pines cannot tolerate their shade.

Figure 6.ii - Ecological succession in western Japan Annotation: Sites with shallow soils accept a different sequence of biological communities compared to sites with deep soils.

It is evident then that a mural mosaic contains dissimilar biological communities at different places not only considering of spatial variation in physical conditions just likewise considering of ecological succession. Sites with similar physical weather have similar ecological successions, but sites with similar concrete atmospheric condition can take very different ecosystems because they are in different stages of the same succession.

Considering climax communities stay more or less the same for many years, one might expect a lot of shii and kashi forest in western Japan. The regional landscape was dominated by shii and kashi climax woods in the distant past, only people cut down about of the shii and kashi forests many centuries ago. Shii and kashi, including some very old and large trees, are scattered across the landscape today, but fully developed shii and kashii climax forests are unusual. Remnants of the climax forests remain primarily in sacred groves around temples and shrines.

Ecological succession every bit a complex system cycle

Ecological succession is cyclic (see Figure 6.iii). It follows the iv stages of the complex systems bicycle described in Chapter 4: growth; equilibrium; dissolution; and reorganization. Young biological communities such as grasses or shrubs are the growth stages of ecological succession. Because immature communities accept relatively few species, newly arriving species do non face strong competition from species already in the customs. Well-nigh new arrivals survive the community associates process, and the number of establish and animal species in the community increases chop-chop. The virtually successful species in young ecosystems are ones that can abound and reproduce chop-chop with an abundance of resources.

Effigy 6.3 - Ecological succession equally a complex system cycle

Ecosystems mature as boosted species of plants and animals become established over the years through the community assembly process. It becomes increasingly hard for newly arriving species to join the mature ecosystem because it has so many species, which already occupy all potential ecological niches. Newly arriving species can survive in the mature ecosystem simply if they can outcompete and displace species that are already in that location. Somewhen the biological community changes little. This is the climax community (equilibrium). Information technology has the largest number of species, and they are all efficient competitors, good at surviving with limited resources. A climax ecosystem may last for centuries, provided outside disturbances such as fire or astringent storms are not too damaging.

However, sooner or later, the climax community is destroyed by some kind of disturbance. This is dissolution. Most establish and animal species disappear from the site. Then comes the reorganization stage. Because many niches are empty at this time, competition is low and survival is piece of cake for newly arriving species if the site has suitable physical weather and the biological customs has an appropriate food source. The reorganization stage is a fourth dimension when the community might acquire ane group of plants and animals, or a very unlike group, depending upon which species happen to arrive at the site past chance during this critical fourth dimension. Ecological succession and then proceeds from immature to mature communities (growth) until at that place is some other disturbance or succession once once more reaches the climax community.

The disturbances that cause mature communities to be replaced by before stages of succession vary in scale. As a result, mural mosaics accept patches of many unlike sizes. For example, lightning tin can strike ane tree in a forest. The tree dies and falls over, opening up a small gap in the wood, which is occupied by early successional species. At the other farthermost, a severe typhoon or burn, or large-calibration logging, can tear downward hundreds of square kilometres of forest.

Interaction of positive and negative feedback in ecological succession

This section examines the tension betwixt positive and negative feedback in ecological succession. Negative feedback tends to keep ecosystems the same (ecosystem homeostasis) but they change from one stage to another as positive feedback takes effect.

We will await once more at the succession of an ecosystem from grass to shrub community, beginning with an ecosystem in which the ground is carpeted with grasses (see Figure six.4A). Shrubs may be present, simply they are young and scattered. The ecosystem may stay this way for five to ten years, or fifty-fifty longer, considering shrub seedlings grow very slowly. They grow slowly because grass roots are located in the top soil, while most of the shrub roots are lower down. Grasses intercept well-nigh of the rainwater before it reaches the roots of the shrubs. Because the grasses limit the supply of h2o to the shrub seedlings, they maintain the integrity of the ecosystem equally a grass ecosystem. At this stage, negative feedback acts to keep the biological community the same.

However, later a number of years, some of the copse and shrubs, which have been growing slowly, are finally tall enough to shade the grasses below them (see Figure half dozen.4B). The grasses then accept less sunlight for photosynthesis, and their growth is restricted. This results in more h2o for the shrubs, which grow faster and shade the grasses even more. This procedure of positive feedback allows the shrubs to take over. They now boss the available sunlight and water, and the grasses decrease dramatically.

Figure vi.4 - Competition between shrubs and grasses for sunlight and h2o

This example shows how negative feedback can keep an ecosystem in one stage of ecological succession until at that place is enough modify in some part of the ecosystem to trigger a positive feedback loop that changes the ecosystem to the next stage of succession. This example is virtually much more than grasses, shrubs and trees. The same kind of interplay between positive and negative feedback is responsible not simply for ecological succession but likewise for much of the behaviour of all complex adaptive systems. Ecosystems, social systems and other complex adaptive systems stay more or less the same for long periods because negative feedback predominates until a small change triggers a powerful positive feedback loop to change the system speedily. Negative feedback then takes over to concur the organisation in its new class.

Urban succession

Urban ecosystems and their social systems change in ways that are similar to ecological succession. As a urban center grows, every neighbourhood within it experiences changes in its social arrangement. A neighbourhood tin change drastically over a menses of 25 to 100 years. It may be primarily residential during i time and get commercial or industrial during another. Neighbourhoods feel growth, vitality and progress during sure times, and at other times they deteriorate as the focus of growth and vitality shifts to other neighbourhoods. The same is true for entire cities. Cities grow and decline as the focus of growth and vitality shifts from one city to another.

Homo-Induced Succession

Human activities tin accept a powerful upshot on ecosystems and the way they modify. This is known as human-induced succession, which can pb to changes that are often unexpected and sometimes seriously detrimental to the benefits that people derive from ecosystems. Pollution of the lagoons that surround modest South Pacific islands provides a striking example. Many Due south Pacific communities now consume imported packaged and canned foods, disposing of the empty cans and other waste in dumps. Rainwater runoff from the dumps pollutes the lagoons, reducing the quantity of fish and other seafood. With less seafood, people are forced to purchase more than and more than cheap canned food, the pollution becomes worse and the lagoon has fewer fish. This positive feedback loop changes the lagoon ecosystem while also degrading the people's diet.

Pasture degradation due to overgrazing

Another instance of human-induced succession is the effect of overgrazing on pasture ecosystems. Overgrazing occurs when a pasture ecosystem has more grazing animals such as sheep or cattle than its conveying capacity for those animals will support.

Pastures usually have a mixture of different grass species that differ in their nutritional value. Many species of grass are not nutritious as a defence confronting beingness eaten past animals. Some species of grass are even poisonous. Because grazing animals know which grasses are best to swallow and which are not, they select the nutritious species and leave the rest uneaten. Different species of grass in the same pasture compete with each other for mineral nutrients in the soil (mainly nitrogen, phosphorus and potassium), water and sunlight (see Effigy vi.v). A mixture of different grass species tin coexist in the same ecosystem as long as no species has an advantage. However, if some species have a disadvantage, they will disappear and the other species will have over.

Figure 6.5 - Competition between grasses that are nutritious or non nutritious for sunlight, water and mineral nutrients

What happens when also many cattle graze for a prolonged period of fourth dimension? Because cattle select nutritious grasses, these species have a disadvantage in their competition with grasses that are not nutritious. The population of nutritious grasses decreases, leaving more than resources for other species to grow and increase in affluence. A positive feedback loop is set in motility for grasses that are not nutritious to replace nutritious grasses. By tracing the arrows through the diagram in Figure half-dozen.5, it is possible to see that each species of grass has a positive feedback loop that passes first through its 'food' in the soil and and so through the other species of grass. This replacement process can take years, just when information technology is finished, the pasture has changed from an ecosystem with a mixture of grasses to an ecosystem dominated by grasses of depression nutritional value. As a upshot, the carrying capacity for cattle is much lower than it was before.

Desertification

Grass ecosystems are an early stage of succession in regions where the mature ecosystems are forests. All the same, grass ecosystems are climax ecosystems in grassland regions, where at that place is not plenty rainfall to back up a forest. Desert ecosystems are climax ecosystems where there is non enough rainfall even for grassland. Desertification is the change from a grassland ecosystem to a desert ecosystem in a region where the climate is suitable for grassland. At that place is plenty rainfall for grass, but overgrazing can modify the grassland to desert.

In a healthy grassland ecosystem, all of the ground is covered by grasses, which protect the soil from erosion due to wind or pelting. If there are also many cattle, grass comprehend is reduced. Fleck by bit, current of air and rain carry abroad the fertile topsoil from ground that is no longer protected by grasses. When topsoil is lost, the soil becomes less fertile and its chapters to concord water declines. Grasses then grow more slowly and are replaced by shrubs whose roots can reach water deeper in the soil. Considering the shrubs are not nutritious for cattle, the carrying capacity for cattle declines. People may then use goats instead of cattle because goats tin eat shrubs that cattle cannot. Goats can likewise eat grass, which they pull out past the roots. If there are as well many goats, they destroy the remaining grasses, and more footing is left without its protective embrace. At that place is more than erosion, and eventually the soil is and so badly degraded that grass can no longer abound at all. The grassland has changed to a desert with scattered shrubs (run across Figure 6.half dozen)

Figure 6.six - Man-induced succession from grassland to desert caused by overgrazing

These changes are slow. It can have l years or more for a grassland ecosystem to become a desert ecosystem that provides very little food for people. The unabridged ecosystem changes. Desert shrubs supercede grasses and the residuum of the biological community changes considering it depends upon the plants. Physical conditions change besides, often irreversibly. Because degraded soil cannot hold plenty water to support the growth of grasses, a desert ecosystem may not alter back to a grassland ecosystem, even if all the grazing animals are removed.

Worldwide, about fifty,000 square kilometres of grassland change to desert every year. The causes are complex and varied, simply overgrazing is often a major gene. Why do people put too many grazing animals on grasslands when the consequences are so disastrous? The main reason is homo overpopulation. The human population in many grassland areas already exceeds the carrying capacity of the local ecosystem. People apply too many grazing animals because they demand the animals to feed themselves at present, fifty-fifty if it means less food in the future. Desertification has contributed to famine in places such equally the African Sahel. This is an example of population overshoot that can cause the human population and its ecosystem to crash together.

Fisheries succession

Commercial angling tin can take far-reaching effects on fish populations in oceans and lakes. If fishermen focus heavy line-fishing on a few species of high commercial value, those species have a college death rate than other species of fish with which they compete for food resources. The populations of commercially valuable fish decline and are replaced by trash fish or other aquatic animals of little or no commercial value. This is known as fisheries succession. Information technology is basically the aforementioned ecological procedure as replacement of nutritious grasses by species that are not nutritious considering of overgrazing. During the 1940s and 1950s, the sardine population off the California coast declined and was replaced by anchovies. While recognizing that long-term climatic or biological cycles may accept a role in this story, information technology appears the change was primarily due to overfishing. In a similar mode, sardines replaced anchovetas off the coast of Peru and Chile when there was heavy fishing during the 1960s and 1970s, and like stories take occurred numerous times with other species of fish in oceans and lakes throughout the earth.

When this happens, a decline in the population of a particular fish species can gear up in movement a concatenation of effects through the aquatic ecosystem that alters the biological customs in many other ways. Concrete conditions sometimes modify besides. The fish that disappeared may not be able to return even later overfishing has ceased. When people damage role of an ecosystem, it adapts past changing to a different kind of ecosystem - one that may non serve human needs besides as before. A multitude of changes through the ecosystem have 'locked' it into a new biological community (encounter Figure 6.vii).

Figure 6.seven - Disappearance of commercial fish due to overfishing

The 'okay/not okay' principle of human-induced succession

Desertification and fisheries succession are examples of a more general emergent property of ecosystems. Human-induced succession can make ecosystems switch from a stability domain that serves human needs ('okay') to another stability domain that does not ('not okay').

Ecosystems go along to be 'okay' as long equally people do not change them as well much. If an ecosystem is altered drastically, natural and social forces tin transform it fifty-fifty more than to a different stability domain that may be okay simply frequently is non.

Ecosystems are impressively resilient in their capacity to keep operation and providing services over a range of uses and even moderate abuse. Moderate levels of angling, grazing, logging or other uses may alter the country of a natural ecosystem, only the ecosystem remains in the same stability domain and continues to provide fish, forage or wood (come across Figure 6.8). The aforementioned is true for agricultural and urban ecosystems that include a healthy natural biological community, such as animals and microorganisms that maintain soil fertility on a subcontract, or trees that remove pollution from the air in a city. However, if an ecosystem is transformed besides much, a chain of effects tin can exist set in move through ecosystems and social systems that changes the ecosystem even more. Fish, fodder, forests, soil animals or urban copse tin can disappear. The ecosystem country passes from ane stability domain to another, and the new ecosystem may non serve people's needs every bit information technology did before.

Figure 6.8 - The okay/not okay principle of man-induced succession

Managing Succession

Traditional wood management in Japan

Human-induced succession is not always detrimental. People who know how to interact with ecosystems on a sustainable footing tin encourage ecosystems to alter - or not modify - in means that best serve their needs. They can utilize natural processes to alter ecosystems to a stage of ecological succession that they desire. People tin besides construction their activities in the ecosystem so that the biological customs remains at a desired phase of succession instead of developing into a phase that they do non want.

The traditional Japanese satoyama system (literally 'village/mountain') is an example of sustainable landscape management that for many centuries provided essential materials for village life. The villagers maintained young oak forests and patches of a tall perennial grass (susuki) equally a major function of their landscape because of the valuable products they provided. The long tough stems of susuki grass were used equally thatch for houses and as mulch or compost on the farmers' fields. Villagers prevented their susuki grass areas from changing to forest past setting them on burn down after cutting the grass stems for use. The fire killed young trees and shrubs merely the surreptitious roots of the grass survived to sprout soon afterwards the burn down.

Village forests were the primary source of construction materials, charcoal for cooking food and heating houses, and leaf litter for application equally mulch to agricultural fields. Oak forests were more useful than the more mature shii and kashi forests because oaks abound faster. The villagers used a very unproblematic procedure to ensure that they had enough oak woods to meet their needs. Each year they cut all the oaks in a small area, doing and then in a style that allowed new oak trees to sprout from the stumps of the cut trees. Because the new oaks could use the big root systems of the cut trees, the new oaks could grow so fast that inside 20 to 25 years they were once more ready for cut. In one case the xx - 25-year-one-time trees were cutting, the same process was repeated, with new trees sprouting from the cut stumps; more oak trees were then prepare for cutting in another xx to 25 years. Because different parts of the forest were cut at different times, the mural had a mosaic of oak forests of different ages that provided a variety of woods products and a variety of habitat for many species of plants, insects, birds and other animals.

Every year the villagers cut all young shii and kashi trees then that they could non grow to a higher place the oak trees. In this way they retained oak forest every bit a major function of their landscape mosaic for centuries. Information technology was essential to cut the oaks every 20 - 25 years. If they waited too long and did not remove shii and kashi, the oaks would eventually be replaced by shii and kashi. If they cutting as well presently, the oaks would never grow big enough to produce the seeds necessary for new trees. Without new copse the oak wood would somewhen disappear and be replaced by other kinds of trees or an earlier stage of succession with grasses and shrubs.

The situation is very different today. For the by twoscore years, Nippon has imported petroleum and gas instead of using charcoal. Moreover, Japan has imported large quantities of timber from other countries for construction while using less woods from its own forests. Most farmers utilize big quantities of chemical fertilizers to their fields instead of mulch from the wood. Oak forests are no longer cut on a regular basis, the trees are becoming senile, and some are starting to die. Oak forests may eventually exist replaced past shii and kashi forests.

Forest burn protection

Frequent fires - started mainly past lightning - are a natural part of many wood ecosystems. The seeds of some plants germinate only when stimulated by fire. Dead tree leaves accumulate on the ground to grade foliage litter, which provides fuel for fires that are ordinarily started by lightning. When the quantity of litter is small, there is not much fuel, fires burn slowly and are non excessively hot. Nearly of the leaf litter burns away and some of the leaves on the trees may be burned; however, few trees are killed. If fires kill whatsoever copse (unremarkably onetime trees), young copse quickly grow to fill the awning gaps.

Fires have an of import part for forests. Fallen leaves incorporate minerals such as phosphorous and potassium that the ashes from a burn return to the soil equally mineral nutrients for trees and other forest plants. Nonetheless, if a forest has too much leaf litter on the footing, a burn down tin can burn at extremely hot temperatures because the large amount of litter provides so much fuel. A fire with too much leaf litter can spread over a large area and burn down with such intensity that it destroys all of the trees and buried tree seeds in the soil. When this happens, the forest is destroyed and a grass ecosystem emerges from the ashes. Information technology can have many years before at that place is woods again, particularly if there is no longer any woodland close past to provide a seed source.

Frequent fires are a negative feedback mechanism that prevents excessive accumulation of leafage litter in forest ecosystems (see Effigy half dozen.9A). Because frequent fires seldom result in serious damage, they are nature's way of protecting forests from severe fires that could destroy them. This is 'ecosystem homeostasis'. A wood mural with frequent natural fires is a mosaic of mature forest with grass and shrub ecosystems, and less mature forest in areas where there were fires in recent years (see Figure 6.10). The kind of ecosystem in each patch depends upon how many years have passed since a fire occurred and how severe information technology was. People generally consider a varied mural, punctuated with dissimilar kinds of woods and open areas, to exist more pleasant than a mural that is solid woods.

Effigy 6.9 - Natural regulation of woods litter by burn (no fire protection) and accumulation of litter with burn down protection

Figure 6.10 - Landscape mosaic of a forest without fire protection

Effectually 1900, the United States Forest Service initiated a policy of protecting forests from fire because foresters did not understand the value of frequent forest fires. They did not desire any tree damage due to fire. For 80 years they put out all wood fires as quickly equally possible. More and more leaf litter accumulated on the basis because then much time passed without frequent pocket-size fires to get rid of the leaf litter (see Effigy 6.9B). By 1980, foliage litter had accumulated inside forests to the extent that they were increasingly susceptible to fire. New forest fires became very difficult to control, particularly in the all-encompassing dry out areas of Western Us.

The more than the forest service tried to protect forests from fires, the worse the problem became because every fire was more difficult to extinguish and could destroy such big areas of natural habitat. Woods protection became increasingly costly because it was necessary to use big numbers of fire fighters, fire trucks and airplanes to drop h2o. Despite this effort, thousands of foursquare kilometres of forest were sometimes destroyed by a single fire.

This example shows how man interference with fire every bit a natural function of 'ecosystem homeostasis' caused fires to get a disturbance that could destroy mature ecosystems and transform them to an earlier phase of succession (grass ecosystems). The solution to the problem is controlled burning to get rid of the leaf litter and selective logging to reduce the number of trees that provide fuel for a burn. This is what the forest service does now. Even a large quantity of leaf litter does not burn at loftier temperatures if it is wet, so in that location are times (for example, later rainfall) when foresters tin kickoff a burn and burn away the accumulated leaf litter without destroying the trees. These new forest management practices are working in harmony with the natural feedback loops in ecosystems instead of fighting them. However, it has not been like shooting fish in a barrel to correct the state of affairs. Many forests all the same have excessive quantities of combustible textile such every bit decaying trees, litter or shrubs, and the United States government nevertheless spends many millions of dollars combating destructive woods fires. Moreover, controlled burning occasionally escapes control, leading to serious and unexpected destruction of forests, millions of dollars worth of property harm and considerable political controversy.

The forest fire example shows how the response of ecosystems to human activities tin can be counterintuitive - the contrary of what nosotros expect. Our actions tin have not only the direct effects that we intend; they can also generate a chain of furnishings through other parts of the ecosystem that come up dorsum in unexpected ways.

Ecological succession and agriculture

Agricultural ecosystems such as farms and pastures incorporate few species of plants and animals compared to mature natural ecosystems. People make agricultural ecosystems uncomplicated because uncomplicated ecosystems channel a large percent of their biological production to human use. Agricultural ecosystems are immature ecosystems, and like all immature ecosystems they are continually field of study to natural processes of ecological succession that change them in the management of mature natural ecosystems. Weeds invade fields. Insects and other animals that consume crops join the ecosystem. The basic strategy of modern agronomics is to counteract these forces of ecological succession. Modernistic social club uses intensive homo inputs in the grade of materials, free energy and information to prevent ecological succession from altering its agricultural ecosystems.

Information technology is typical for traditional agriculture to follow a different strategy. Information technology reduces the need for intensive inputs by harmonizing agronomics with the natural cycles of ecological succession. For example, swidden agriculture, which is likewise known equally slash-and-burn agriculture or shifting cultivation, is common in tropical areas where the soil is unsuitable for permanent agriculture. Swidden agricultural is particularly useful on:

• forested hillsides that are susceptible to erosion when a forest is cleared for agriculture;
• infertile wood soils that are vulnerable to leaching of institute nutrients to soil depths beyond the reach of crop roots.

A typical swidden process is to clear a patch of woods by cutting and burning the trees and shrubs. Fire is a means that swidden farmers employ to use a large supply of natural energy to prepare their fields for crops. Fire converts copse and shrubs to ash that serves as natural fertilizer, and fires kill pests in the soil. The ash provides natural liming to ensure suitable soil pH for crops. A farmer can abound crops in the cleared patch for i or two years. Later on that, soil fertility declines and crop pests increase, so that harvests are too small to justify the effort. The farmer abandons the patch before these problems materialize, moving to some other part of the forest where he clears a new patch for crops. The abandoned patch is left in dormant for at least ten years.

Once a patch is left in fallow, numerous plants and animals invade from the surrounding forest, generating a sequence of biological communities that follows the usual progression of ecological succession from grasses and shrubs to trees. Natural vegetation and a roofing of leafage litter protect the soil from erosion. Fertility is eventually restored to the soil surface by the wood'south nutrient pump, as securely rooted trees bring constitute nutrients to their leaves and deposit the leaves on the ground. Ingather pests disappear because they cannot survive in a natural ecosystem without crops as food. The farmer can return to the same place after about x years of dormant, repeating the process of cutting and burning trees and shrubs and planting a crop. A landscape in swidden agronomics has a mosaic of patches, some of them agricultural fields with crops, but most of them different stages of ecological succession in the course of forest fallow.

Swidden agronomics is a highly efficient and ecologically sustainable way to employ frail lands when the man population is small-scale enough for farmers to leave the land in dormant for the required fourth dimension. Unfortunately, swidden does not work if the human population is besides large. When land is in short supply, farmers are compelled to clear the wood and plant crops before the fallow has had enough fourth dimension to fully restore the land. The result is a vicious cycle of soil degradation and declining harvests. Population explosion in the developing globe has changed swidden agriculture from ecologically sustainable to unsustainable in many places. One solution to the problem is agroforestry, which mixes shrub or tree crops such every bit coffee or fruit trees with conventional food crops to create an agricultural ecosystem that mimics a natural forest ecosystem.

The human population of Java in Republic of indonesia is too large for swidden agriculture with a natural wood dormant. Almost Javanese farmers are quite poor considering they must meet all of their family needs with only i or two hectares of land, but they brand the best of this difficult state of affairs with traditional agriculture that simulates the natural cycle of ecological succession. They start by planting a polyculture of crops such as sweetness potatoes, beans, corn and several dozen other food crops that grow quickly. They plant a scattering of bamboo or trees in the same field. The fast-growing crops predominate during the starting time few years (kebun agricultural ecosystem in Figure 6.xi), and the trees or bamboo take over later (talun in Effigy 6.eleven). Equally soon as the copse and bamboo are large plenty, they harvest them for utilise as construction textile and fuel, clear the field, burn down unused plant materials, and one time again constitute a polyculture of food crops and copse. Much of the Javanese mural looks like natural forest just is, in fact, advisedly cultured agroforestry - 'forest' stages of an agricultural cycle that takes total advantage of ecological succession. Each family manages a small landscape mosaic of different fields in different stages of the cycle, then they have a continuous supply of the various foods and other materials that they need.

Effigy 6.eleven - Succession of a Javanese polyculture field from domination by annual field crops to domination by tree crops Source: Christanty, Fifty, Abdoellah, O, Marten, G and Iskandar, J (1986) 'Traditional agroforestry in West Java: The pekerangan (homegarden) and kebun - talun (almanac - perennial rotation) cropping systems' in Marten, G, Traditional Agriculture in Southeast Asia: A Human Ecology Perspective, Westview, Boulder, Colorado Kebun Kebun campusan Talun

Things to Think Nigh

1. What are the typical sequences of natural succession in your region (every bit in Figures 6.i and 6.2)? Do sites with different physical conditions have different sequences? What role does chance seem to have in the sequences that really occur?
2. Accept there been examples of human-induced succession in your region? What were the human activities that made them happen? Were they reversible?
3. Talk to your grandparents or other relatives or friends that have lived in the vicinity of your family unit home for a long time. How have natural, agricultural, and urban ecosystems inverse? Make a map showing the landscape mosaic over a radius of 1 kilometre around your family unit home l years ago. Compare the map of l years agone with the map from 'Things to recall about' in Chapter 5 (ie, a map of the ecosystems today). How has the landscape mosaic inverse during the past l years? (If at that place were no houses in your area fifty years go, brand the map for a more recent fourth dimension such as 30 - twoscore years agone.) If possible, make a series of maps that show progressive change in the landscape mosaic over time.

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Source: http://gerrymarten.com/human-ecology/chapter06.html#:~:text=Human%20activities%20such%20as%20logging,replaced%20by%20a%20different%20community.

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