Community change in time -- Ecological Succession



Community change in time -- Ecological Succession

T

he grad___, ord____ changes in the composition of communities towards a st____, climax type is called succession.

The seq______ of communities occurring in each stage is called seral stages, and the type of species that make up most of a seral stage is called the dom____ species.

The first to become established are the pio____. The final type of community is called the c______ community. The most important factor that determine the climax community is the c____________ of the particular locality.

Ecological succession can result in a progression from imm_____, rapidly changing, unst____ communities to more mature, self-sust_____ communities provided that the process is not disrupted ---- e.g. by large-scale natural events or human interference.

I) Succession of Plants

A) Primary succession

|If succession begins on an area |[pic] |

|which has not been previously | |

|occ______ by community (such as | |

|newly exposed rock, sand surface| |

|), the process is termed | |

|pri_____ succession. | |

What Causes such Community Changes ?

Succession is due to the fact that the action of an community on a habitat might tend to make the area m___ favourable for other communities, until the cl____ state is reached.

A typical primary succession starting from bare rocks might go through the following seral stages:

Pioneer plants moss herbs shrubs trees / forest (climax)

1. Pioneer plants

- the first plants that invade an area and start the series of succession are called pioneer plants.

- for bare rocks only lichens can grow on them. Their growth gradually create a thin layer of soil on the surface of the rock :

1. as lichens cling to the rocks, they hold w____,

2. chemicals from the atmosphere such as carbon dioxide, dust particles tend to accumulate.

3. lichens might secrete 'lichen acids' and corrode the rocks.

4. soil particles are formed as a results of we________ by physical, chemical and biological means.

5. the dead bodies of the lichens enrich the soil with or______ matter.

- all these prepare a s___ layer, covered with organic matter, for the plants of the next community. Algae and moss might also be the pioneer species in situations where there is abundant moisture.

2. Moss stage

- as the lichens have established the soil, the next type of plants that may arise are the mosses.

- the moss may com____ with the lichens and gradually elim____ the lichens in the area.

- the moss continues to b____ up deposits of organic matter.

- As time goes by, more soil particles are formed from the parent rocks and a th_____ soil layer builds up.

3. Herb stage

- the type of plants that may compete with the mosses are the herbs such as ferns and grasses.

- herbs may eventually rep____ the mosses and become the dominant plants.

4. Shrub stage

- the herbs further enrich the soil and they also furnish sh___ and act as a wind_____.

- as the shrub grow taller, they become the domi nant plants and shade the herbs. This means some of the herbaceous plants will not be able to survive.

5. Tree stage

- as the soil become sufficiently thick and fer___, seedlings of tree can become established.

- as the trees grow, they gradually form a dense vegetation that shade out the sh____.

- at this stage, a climax community (forest) is said to be reached.

- it will remain at a stage of equilibrium as long as the environment does not change.

Secondary Succession

This is the more common form type of succession. It proceeds from an area where the natural v________ has been removed or destroyed but where the s___ or bottom sed_____ has not been covered or removed.

|[pic] Deforested |[pic] recovery over 3 years |

|[pic] |

|Secondary succession is usually more |[pic] |

|r____ because the soil or sediment has | |

|already been established by previous | |

|community. | |

| | |

|Examples of areas that can undergo | |

|secondary succession are abandoned | |

|farmlands, burned or cut forests, heavily| |

|polluted streams, flooded areas. | |

II) Succession of animals

The succession of animals is less defined than plants, and the appearance of a species usually does not wipe out the existing species.

- At the moss stage, insects and small invertebrates may be attracted and find shelter there.

- At the herb stage, there will be more insects, especially the grasshoppers, beetles, bugs, bees and ants. Attracted by the insects are the insectivorous mammals. e.g. shrews and moles.

- At the shrub stage, larger mammals make their appearance, these include deer, foxes, rabbits, etc. Birds also come to seek out the insects and the fruits of the plants.

- At the tree stage (climax), the area has similar fauna--animal species, as found in the shrub stage.

III) Speed of succession

The speed of succession varies greatly. Cli_____ conditions play an important role in determining the speed. On lava flows, it takes hundreds to thousands of years for the production of s___ and the establishment of lichens, algae and mosses (primary succession).

It has been reported that in an abandoned farm field, herbs and shrubs become established in nine years, and the tree species were approaching the climax after 150 years (secondary succession).

|[pic] |Sometimes a community |

| |might be prevented from |

| |reaching its |

| |due to recurring natural|

| |or human disturbance. |

| | |

| |eg fire, weeding, |

| |logging, agricultural |

| |practice, etc |

.[pic]

IV) Comparison of immature and mature ecosystems

Immature communities at the early stages of succession have only a few species (low species diversity, fairly simple food webs, with relatively few decomposers.)

Most of the plants of a pioneer community are small annuals that grow close to the ground. They use most of their energy to produce large number of small seeds for reproduction rather than to develop large roots, stem and leaf systems (opportunistic).

In contrast, the community of a mature ecosystem has high sp_____ div_____, relatively st____ populations, and comp___ food webs dominated by decomposers.

Most plants in mature ecosystems are large herbs and trees that produce a small number of large seeds. They use most of their energy and matter resources to maintain their large root, trunk and leaf systems rather than to produce large numbers of new plants.

| Characteristic |Immature ecosystem |Mature ecosystem |

|Ecosystem Structure | | |

|plant size |Small / large |Small / large |

|species diversity |Low / high |Low / high |

|trophic structure |mostly producers, few decomposers |mixture of pro_____, |

| | |con_____ and de_________ |

|ecological niche |few, mostly gen_______ |many, mostly sp________ |

|community organization |High / low |High / low |

|(no. of interconnecting links) | | |

|Ecosystem Functions | | |

|food chains and webs |simple, mostly plants and herbivores, few |complex, dominated by decomposers |

| |decomposers | |

|efficiency of nutrient recycling |High / low |High / low |

|efficiency of energy use |High / low |High / low |

V) Simplification of Ecosystem by Human Activities

In modifying ecosystems for our use, we sim___ them. For example, we plow grasslands and cl___ forests. Then we replace thousands of interrelated plants and animal species in those ecosystem with greatly simplified, single crop ecosystems or mono_____. A monoculture of plants is an unstable and vul_______ system that lacks the checks and bal____ of a natural diverse ecosystem.

Modern agriculture is based on the practice of deliberately keeping ecosystems in early stages of su________. in which the biomass prod_______ of one or a few plant species is high. Such simplified ecosystems are highly vulnerable.

A serious problem is the continual invasion of crop fields by unwanted pi______ species, which we called w___ if they are plants, p___ if they are insects or other animals, and pathogens if they are harmful microorganisms such as bacteria, fungi or viruses. Weeds, pests, or pathogens can wipe out an entire monoculture crop unless it is artificially prot____ with pesticides and herbicides or by some form of biological control.

Pesticides are related to problems such as toxicity to human and other wild animals, ecological disruption that leads to resurgence of pests, emergence of new forms of pest species, and emergence of resistant species. When rapidly breeding insects species develop genetic res_______ to certain chemicals in pesticides, farmers must use ever-stronger doses or switch to a new product. This increases the rate of natural selection of the pests to the point that eventually the chemicals become inef_______. This illustrates one important law of ecology : we can never do merely one thing. Any intrusion into nature has numerous effects, many of which are unpredictable.

Cultivation is not the only way people simplify ecosystems. Animal husbandry also remove natural comp_____ for their sheep or cattle, and eradicates pre_____ that occasionally kill their livestock. Far too often, ranchers allow livestock to over____ grasslands until excessive soil erosion converts these ecosystem to simpler and less prod_____ deserts.

The cutting of vast area of diverse tropical rain forests is causing the ire__________ loss of many plants and animal species. People also tend to overfish and overhunt some species to ext_____, another way of simplifying ecosystems. Poll______ by various means also simplify ecosystems by killing trees and wild species.

It is becoming increasingly clear that the price we pay for simplifying, maintaining and protecting such artificial ecosystems is high: it includes time, money, increased use of matter and en____ resources, reduced bio_______, and loss of natural landscape. There is also a danger that, as human population grows, we will converts too many of the world's mature ecosystems to simple, young, productive but highly vulnerable forms.

The challenge is to maintain a balance between simplified, human ecosystem and the neighbouring, more complex, mature, natural ecosystems on which our simp____ systems and other forms of life depend.

[pic][pic]

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