Ecosystem Dynamics - WildlifeCampus · 2017-04-04 · When considering the limiting factors affecting a particular organism, a few important points need to be remembered. Firstly,

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Ecosystem Dynamics © Copyright

Module # 10 – Component # 8

Ecosystem Dynamics

Functional Aspects of the Ecosystem

The functional aspects of the ecosystem deal with the energy flow within the ecosystem, limiting factors and the cycling of materials around the

ecosystem. Energy Flow in the Ecosystem

The energy that is used by the ecosystem to drive it originates from the sun.

The light energy from the sun enters the earth’s atmosphere and is converted from light energy to chemical energy by the process of photosynthesis in plants.

The chemical energy derived from photosynthesis is then utilised to supply the living organisms in the ecosystem with a source of energy that can be used for their daily requirements. This utilisation within the organism occurs during the

process of respiration when some of the energy is “burnt” to release the energy as the organism needs it. Some of the energy that is not burnt is stored in the

organism (as fat for example) or incorporated as tissue growth in the organism. Up until ± 30 years ago, it was assumed that all ecosystems and all life was

essentially driven by solar energy at some level. However marine biologists have subsequently found what has become known as Hydro-thermal vents. These are

smallish ocean floor vents that vent geothermal steam, sulphur and other minerals. Communities of animals have formed around these vents, with the ecosystem revolving around bacteria that thrive under these bizarre conditions. The vents are

situated in the aphotic zone (below 500m / 16 500 ft. where there is no light penetration) and it was assumed that any life here subsisted on detrital rain (food

particles that slowly floated down from shallower depths).



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Productivity

When a plant absorbs light and produces energy-rich chemical substances (e.g.

glucose) during the process of photosynthesis, we refer to this as gross primary productivity. Once some of this energy has been utilised for the purposes of

maintaining the plant, there will obviously be less stored energy available in the plant. This remaining energy is called net primary productivity. This is follows:

NPP = GPP - R

Gross primary productivity less respiration equals net primary productivity The net primary productivity is therefore the energy left after the plant has

utilised the portion of the available energy it requires. This energy can then be utilised by other organisms. When there is energy transferred from one

organism, such as a plant to another organism (a herbivore), we refer to this as an interchange of energy in a food chain. The equations and how to use them only become important for physiological studies and are not frequently utilised by

terrestrial ecologists.



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Food Chains and Food Webs The transfer of food energy from the source in plants through a series of

organisms with repeated eating and being eaten is referred to as a food chain. There are at most four or five links in this chain. This is due to the extensive

loss of energy at each of the interchanges, where energy is passed from one organism to the next. There is the input of energy with some energy being consumed or stored and an output that makes the remaining energy available for

consumption by the next organism.

Incoming light from the sun is absorbed by the leaves of the plant (maize) and some misses the leaf surface. Of the energy converted to chemical energy by photosynthesis, some is used during respiration. The remainder is available for

consumption by the herbivore (locust). This passes through the herbivore or primary consumer and the remaining energy (after some is used by the

herbivore) is available for use by the carnivore (shrew) or secondary consumer. From here it may be passed to a tertiary consumer (coral snake) and even on to a quaternary consumer (raptor) Each of the links in the food chain is called

a trophic level. It is important to notice that the final output from the carnivore is a fraction of the energy taken up by the plant initially.

Roughly about 90% of the original energy is lost each time it passes through a single organism.

A food web is a more complex system than a food chain. The same principles

apply in the food web and the main difference is that there are many more participants in the food web than were seen in the food chain. This is because

more than one primary consumer will feed on each producer and several secondary consumers will feed on each primary consumer.

Two types of food chains may be found. The first is the one described above which is called a grazing food chain and consists of herbivores and carnivores. The

second food chain is the detritus food chain which consists amongst others of bacteria, fungi, termites and other insects. The detritus food chain is an important food chain as it promotes the breakdown of organic substances such as dead

wood, rotting animal remains and droppings from animals.



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Limiting Factors in the Ecosystem For an organism to survive and reproduce in the ecosystem it must have the

required materials necessary for this to occur. These basic requirements will vary from one species to the next. For example, a certain species of fish requires

water of a high quality and in sufficient quantities. Should the water source dry up this would be disastrous for the fish and it would die. Therefore, the water is a limiting factor to the fish.

The fish would have a minimum water requirement for its survival (e.g. sufficient

water to prevent the fish drying out) and if the water source was reduced to below the minimum, the fish would die. However even if the water dropped lower than the ideal for the fish, but not below the minimum level, the fish would still survive

although these conditions would not be ideal conditions for the fish. Obviously, the fish would be placed under a certain amount of stress if the conditions were

not ideal. Once placed under stress the fecundity (ability to reproduce) would be affected and the rate of egg production would be reduced.

A second important principle is the tolerance limit that an organism exhibits to a certain factor that is essential for its survival. An example to illustrate this is

seen with the effect of temperature on a certain bird species. The bird species has an ideal temperature range within which it will thrive and reproduce. However, should these temperatures become too extreme (both too high or too

low) the bird would not survive. Outside of these optimum temperatures, the birds become stressed. The numbers of the species will become markedly reduced, but

some of the species will continue to survive.

These outer limits are therefore still within the limits of tolerance. However, when the limits of tolerance are reached and passed (the temperatures become too high or too low) the zone of intolerance is reached and the birds cannot survive

under these conditions and die.

When considering the limiting factors affecting a particular organism, a few important points need to be remembered.

Firstly, the organisms may have a wide tolerance for certain factors whilst they have a narrow tolerance for other factors.

Secondly those organisms with wide ranges of tolerance for many factors will

be able to withstand greater stresses and may be found more widely spread than those organisms with mostly narrow tolerance ranges.

Thirdly where one factor is creating a stressful situation for the organism, this

may also cause the tolerances of other factors to become reduced.



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Biogeochemical Cycling As previously stated, the amount of substances required by organisms is not

found in unlimited quantities in the biosphere. This means that these substances must be used repeatedly. The cycling of these substances is referred

to as the biogeochemical cycles. The name is made of three portions. Bio refers to the substances in living organisms; geo refers to substances in the lithosphere. Therefore, these chemical substances are found in living organisms, in the

lithosphere or the atmosphere.

Generally speaking, biogeochemical substances are found in a reservoir pool and in an exchange or cycling pool. These two sources of substances therefore have a great interchange of material between them. For example, the nitrogen cycle

has the atmosphere as its main reservoir pool, whilst the cycling pool is found in living organisms as proteins and in sediments in the form of nitrates.



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The Nitrogen Cycle The main source of nitrogen is the atmosphere. Here nitrogen is taken up by

bacteria and fixed to form nitrates in soil or water, which are then taken up and used in protein manufacture by plants and animals. Nitrogen is also fixed by

lightning in the atmosphere. Without these two means of fixing (i.e. incorporating nitrogen into new substances) the vast amount of atmospheric nitrogen could not be utilised. Waste nitrogenous substances from animals and

plants (such as ammonia) are broken down by other bacteria into useable nitrites and nitrates.



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Development of the Ecosystem

Introduction

The ecosystems and the communities they consist of are dynamic entities. This means that the ecosystem is not static but will change slowly over a period of

time. This is seen for example when a totally barren volcanic island becomes vegetated over many years. This process is called ecological succession.

The development of an ecosystem will follow the same process of succession. If

certain conditions should change in the ecosystem, for example if the stocking rate was to be raised on a game farm, the ecosystem would undergo a change. This could entail the loss of the more palatable grasses and the replacement of

these with less palatable grasses. Succession may take on two forms, namely primary succession and secondary succession.



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Primary Succession Primary succession occurs where no vegetation has ever grown before, for

example on a volcanic island or the retreat of a glacier. The volcanic island consists of barren rocks and no soil. In the early stages, only hardy organisms

will be able to survive on this rocky island. These could be brought onto the island by migrating birds. These hardy organisms include lichens, mosses and bacteria. The lichens start to grow on the rocks and when these die off, their

remains start to decompose and humic acid is formed which starts the breakdown of rock into soil.

This is a very slow process and many years pass before a shallow layer of soil is formed (soil forms a rate of approximately 10cm [0.4 in] per 100 years).

This soil layer forms the habitat for grass seeds and these germinate producing a covering of grass on the island. The lichens, however, are not able to live in the

soil and are displaced by the grasses. As these annual grass plants die off they replenish the humus in the soil and create a suitable environment for other plants to establish. The lichens, mosses and annual grasses mentioned above are

the first organisms to establish in the ecosystem and they are referred to as pioneer species.

The modifying of the habitat will continue over a period. Eventually plants such as shrubs and trees could become established and these tend to remain for

longer periods of time as the changes to the ecosystem become less frequent and the ecosystem tends to become more stable. This stage of the process of

succession is called the climax stage, consisting of climax plants. The stages between the pioneer stage and the climax stage are called Seral Stages.

Different ecosystems will exhibit different climax plants or climax communities. Tropical forests, mangrove swamps, a small collection of desert

plants and savanna grasslands are all climax communities. The characteristics of each specific biome determine what the particular climax community will

consist of.



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Secondary Succession The second form of succession occurs in areas where vegetation was

previously established but has been disturbed or destroyed. This is more important as it is a phenomenon that occurs in all ecosystems and communities to

some extent. An example will illustrate this concept. When a game farm is overstocked with

game, the game will tend to overgraze the available grass plants. This will mean that the biomass of grass will become diminished on the game farm and the

competitive ability of the grasses will be lost. The grasses will no longer be able to suppress the growth of tree seedlings (by shading them) and as the biomass of grass becomes reduced, the trees will start to establish

themselves. This then leads to bush encroachment. This entails the changing of the structure of the plant community with more trees growing and an even

lower biomass of grass due to the shading effect of taller trees. This means that less food is available for the grazers in the future. The numbers of grazers will then tend to become reduced as well.

Secondary succession is almost always the result of some sort of disturbance.

This area of study has evolved into its own discipline called Disturbance Ecology. The disturbance may either occur very rapidly (major fire, flood, nuclear meltdown or volcanic eruption) or gradually (over stocking a reserve, introduction

of an alien species or sustained drought.)

Secondary succession is the phenomenon whereby an ecosystem corrects an imbalance within the system. This occurs through very much the same process

as primary succession, with different colonising species being continually replaced until a stable community has established. Since the ecosystem will already be largely endowed with many minerals, nutrients and biotics the process of

secondary succession proceeds far more rapidly.



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Comparison of Pioneer and Climax Ecosystems Some important characteristics can be compared in these ecosystems. More

energy will be available for consumption in a climax community. In other words, pioneer plants tend to be less palatable than climax plants and therefore

more energy is available for consumption on a climax plant. The species diversity is much higher in a climax community than in a pioneer

community. Greater species diversity (more different species) tends to make the community more stable. Food chains tend to be more complex because of the

greater species diversity. This means that animals will have a greater variety of food sources. Ecological niches are broader in pioneer communities than in climax communities, making the pioneer organisms more adaptable to less

favourable conditions.



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Stability and Resilience of the Ecosystem The stability that occurs in the ecosystem is often brought about by a factor such

as climate. This is seen in a desert where low rainfall limits the climax vegetation to hardy shrubs and grasses or very few species of hardy desert plants. This in a

sense halts the process of succession and a climatic climax results. Resilience is the ability of an ecosystem to recover from a disturbance. In

other words, the ecosystem will tend towards stability over a period, should it be disturbed.

Documents

Ecosystem Dynamics - WildlifeCampus · 2017-04-04 · When considering the limiting factors affecting a particular organism, a few important points need to be remembered. Firstly,