Food webs and ecosystem interactions

Trophic levels can also be described as food chains or energy chains. When a food chain is taken to its natural conclusion, humans are found at the end of it, meaning that we are dependant on an efficient transfer of energy along a food chain. Every link in the chain acquires food and feeds on the link prior to it. Each link is also consumed by the link that follows it, for example, blackbirds eat green plants, but in turn are eaten by tertiary consumers such as hawks. Food chains are the process whereby energy that is trapped in carbon compounds is transferred through an ecosystem.

A simple food chain is unlikely to be found within an ecosystem and a more complex food web is more common, as shown in the diagram below:

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Food webs exist because as shown in the above diagram there is a wide variety of consumers operating at each level. In addition animals such as foxes eat plants and animals, meaning they cannot be placed solely on one category.

The transfer of energy through an ecosystem is in one direction, where light energy from the sun becomes heat energy, is fixed in plants via photosynthesis, and then lost as it moves through subsequent trophic levels. The movement of both materials (chemicals) and nutrients within an ecosystem is different.

The diagram below shows the main parts of the nitrogen cycle. Protein is synthesized from inorganic compounds found in the soil or free nitrogen in the air. This process is helped by nitrogen fixing bacteria in the soil. Certain bacteria have a symbiotic (beneficial) relationship with other plants. A smaller organism (symbionant) usually lives on the host (larger organism).

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Chemicals are an important part of an ecosystem, as they are needed to produce organic material that is moved around the ecosystem and continually recycled. Examples include both carbon and nitrogen, which are absorbed by plants as gases and salts. The gases come from the atmosphere and the salts come from the soil. At the basic level in each cycle plants take up chemical nutrients, utilize them and then forward them to herbivores and then carnivores.

Ecosystem productivity

Every ecosystem has a level of productivity, which helps discover the potential of an ecosystem for food production.

Primary productivity: Plant productivity.

Secondary productivity: Animal productivity.

Gross Primary Productivity: The measure of all photosynthesis that occurs in an ecosystem.

Net productivity: energy left after losses as a result of respiration, growth, excreta.

Net primary productivity (NPP): Amount of energy made available by plants to animals, only at the herbivore level, and is expressed as kg/m2/yr.

This means that once the rate of primary productivity in an ecosystem is established it is possible to compare ecosystems, as a figure for the potential of each ecosystem for food production can be found. The NPP of an ecosystem depends on the levels of heat, moisture, nutrients available, competition, amount of sunlight, age and health of plants. In broad terms NPP increases towards the equator and decreases away from it, towards the poles.

Examples of Plant primary productivity and biomass for various ecosystems:

Ecosystem: Area (million km2): Mean NPP g/m2/yr : World net primary productivity (billion tonnes per year): Mean biomass (kg/m2):
Tropical Rain Forest 17.0 2200 37.4 45
Savannah 15.0 900 13.5 4
Tundra and Alpine 8.0 140 1.1 0.6
Temperate Grassland 9.0 600 5.4 1.6

Reasons for decrease in energy at each level within an ecosystem:

Energy is lost at each stage in an ecosystem (at each transfer).

The following diagram shows how energy is lost within an ecosystem:

Energy loss

The temperate deciduous forest

It could be argued that the temperate deciduous forest is not a true example of a high productivity ecosystem, but in the UK it is one of the most productive. The table and map below highlights its main characteristics:


Productivity High NPP, 1.2kg/m2/year, result of high summer temps and large amounts of daylight. Large amount of biomass as a result of woody material.
Vegetation Varies with soil type acidic = birch and rowan trees, alkaline = box and maple, elm common on clay, willow on gleyed soils. Oak often dominant due to tolerance of wider Ph range. More ground vegetation beneath oak trees, as small leaves allow more light to ground.
Climate Mild / wet up to 1500mm per year, more in winter often from depressions. More precipitation than evapotranspiration. Temps above freezing in winter. Average summer temperature = 15 - 20.
Soils Fertile brown earths, with a mildly acidic mull humus. Wide range of flora and fauna in litter layer, soil mixing encouraged by earthworms. Blurred soil horizons due to worms.
Nutrient cycle Fast rates of leaching balanced by fast rates of weathering. Many nutrients in soil as a result of slow winter growth and low density of vegetation.
Animals Adaptations occur in the winter because of low temperatures. Many animals either migrate or hibernate.
Human interference Few natural areas are left, and many areas have been cleared for agriculture, or recreation.

Temperate coniferous (boreal) forest

Although found in the UK the boreal forest is not as common as the deciduous, and is more common on Canada and Eastern/Central Europe as shown on the map:


Productivity Low, NPP = 0.8kg/m2/year. High biomass from the woody material.
Vegetation Trees 20 - 30m high, mainly pine, larch and spruce. Evergreen to allow photosynthesis all year. Needle leaves reduce evapotranspiration, conical shape removes snow easily.
Climate Either cool temperate or cold continental. Small amounts of rainfall (below 500mm per year). Summer frosts, much winter snowfall, precipitation above evapotranspiration. Reduced growing season, but 16 - 20 hours of sunlight in summer increases photosynthesis.
Soils Normally podsols. Leaching is a result of snowmelt. Humus is acidic (pH 4.5 - 5.5). Iron pans may form. Few earthworms lead to distinct horizons. A thick litter layer.
Nutrient cycle Controlled by the low temperatures which limit rates of weathering in transfer, resulting in many nutrients in the litter.
Animals Sparse as little available food.
Human interference In the UK many are planted and used for forestry.

Succession in ecosystems

The major factor influencing plant communities that exist in an area is the climate, in association with rock type, landforms, water and soil. Locally climate may be modified.

Succession is the changes that occur over time as a plant community reaches a seral climax. It is influenced by - competition, number of new species and environmental stress, for example, lack or water.

An ecosystem passes through several stages before it reaches a climatic climax. These are called seres.

Particular species are associated with each sere, with certain species becoming dominant. This progression through seral stages is shown below:

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If natural conditions are not interrupted climactic climax is the final stage that seres reach. It is the natural vegetation that should be found in an area. For example, for most of the UK this would be deciduous woodland.

Plagio Climax: This is where the resultant community has been permanently influenced by humans. For example, by burning or grazing.

Sub climax: If vegetation does not reach its climax as a result of interruptions by local factors such as soil changes or differences in parent rock. The interruptions are known as arresting factors.

Types of succession

The route to climatic climax can take place in two ways as shown below:


Primary succession is found on a new land surface or in water and various seral stages are passed through before climatic climax is reached. It is an orderly sequence of events where one community is replaced by another. Biomass is created via decomposition and provides more nutrients for the soil allowing for greater and variety of plants and animals to exist at each successive seral stage:

Formation of a primary succession (lithosere):

A Bare rock colonised, by pioneer community, for example, lichens, mosses, bacteria, that can survive in hardy conditions, and need few nutrients.
B Rock slowly weathered creating thin soil.
C Plants die, creating humus, leading to a more fertile soil, grasses replace the mosses and lichens as the dominant species.
D Grasses decrease in number, quick-growing shrubs become dominant.
E Fast growing trees dominate.
F Over time slower growing trees such as oak become dominant and form the climatic climax community.

Other examples of primary succession include:

  1. Haloseres - (salt marshes/estuaries).
  2. Psammoseres (sand dunes).
  3. Hydroseres (lakes).

Primary succession

If plant succession is halted before reaching dynamic equilibrium a secondary succession occurs. Interruptions include fire, disease, climate change and deforestation. These events can also alter the final climax community that result. Both primary and secondary succession are shown below:

Secondary succession

Rooted plants increase the sedimentation and nutrients available. They alter the micro-environment so that reeds, fen, carr and oak are increasingly able to tolerate the thickening and drying soil.

Secondary succession

Not all climax communities are the same, and if physical factors such as altitude or water hinder ecosystem development a sub-climax community may result.

The Tropical Rainforest

Tropical Rainforests are the world's most productive ecosystems in terms of NPP and biomass. They are complex ecosystems with variations in climate, temperatures, and vegetation, within individual forests.

Temperatures are often thought to be permanently high, but they are highest on the forest edge where vegetation is more limited. The climate changes on a daily basis within the forest, and the idea of the forest being difficult to penetrate is only true at the edges where the sunlight allows rapid growth of vegetation.

Their development relies on:

Insolation and temperatures High temps. Allow all year growth of vegetation.
Moisture Varies throughout the year, and true TRFs are said to have rainfall in excess of 2000mm and a dry season of no longer than 2 months.
Nutrients Scarce, but are rapidly recycled and transferred. Phosphorus and nitrogen are the nutrients needed most.


The nature of a forest in terms of its ecology is due to the amount of energy from the sun that reaches plants and animals. Several vertical layers are found, with distinctive plant and animal species.

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Layer: Characteristics:
Top (Euphatic) Most productive (NPP, animal life and biomass). Tress over 25m. 25% of available energy absorbed.
Middle (Oligotrophic) Trees are 10 - 25m.
Bottom Smaller trees and some young saplings between 5 and 10m in height.
Shrub Smaller seedlings and some pygmy trees of 5 to 10m.
Herb Tree seedlings, and ferns in existence.
Top Root Roots that penetrate to a depth of 5cm (majority).
Middle root Fewer roots to a depth of 5 - 50cm.
Bottom root Minimal numbers of roots in this layer, below 50cm.

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  • Small annual temperature range (2 - 4 degrees).
  • Regular insolation, result of location near equator.
  • Daily temperature range in excess of annual range (8 - 10 degrees), with daytime temperatures up to 32 degrees.
  • High levels of convectional rainfall, often with a minimum in July/August/September.
  • Extremely high humidity levels.
  • Light winds.
  • Quick evapotranspiration.
  • Year long growing season (but not all plants in the forest grow all year).

  • Decay of leaf litter is rapid, and nutrient recycling.
  • Photosynthesis occurs all year.
  • Extremely large variety of vegetation, trees appear to be evergreen but are deciduous.
  • Vegetation adapts to the climate. For example, buttress roots to support tall trees, drip tips to help remove excess moisture.
  • Tallest trees can be over 50m high and are known as emergence.
  • Where forests are near the coast Mangrove swamps may be found.

May appear to be fertile but the tropical latosols are not. The humus layer is extremely thin, as is the amount of litter. Leaching occurs as a result of the high precipitation and is increased greatly by deforestation. Minerals such as Calcium and magnesium are lost. Bedrock is weathered quickly. The characteristic red colour of the soil is due to iron and aluminium accumulating.

Click on the arrows to move the diagram up and down:

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If the rainforest is undisturbed the nutrient cycle acts effectively to help maintain the large and varied vegetation found within them, as mentioned in the previous section on nutrient cycling.

Once the cycle is disturbed the fragile ecosystem is quickly upset as shown in the diagrams below:

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Managing the Tropical Rainforest

Pressures on Tropical rainforests are vast, mainly as they provide numerous opportunities for development, especially in LEDC'S where they are viewed as a highly profitable resource.


This results from:

  • Mineral extraction (Amazonia).
  • Logging for tropical hardwoods (world wide).
  • Energy production (dams for HEP).
  • Tourism.
  • Population pressure.
  • Wood pulp for paper mills.
  • Banana and coffee plantations (Costa Rica and South America).

Rainforest clearance leads to major changes in the ecosystem as the state of dynamic equilibrium is upset as a result of changes to inputs and outputs into the system. Impacts are felt on plants and animals, water cycle, climate, landscape.

The list below outlines concerns in more depth:

  • Greater possibility of pests and disease as monoculture provides a uniform source of food, and herbivores can easily adapt to this.
  • Vital stores of nutrients are lost as decomposers that release and fix nitrogen in the soil are removed.
  • Up to 99% of all nutrients are stored in plant material; their removal quickly diminishes rainforest fertility.
  • Less interception of rainfall increases sheet and gully erosion.
  • Silting of lakes and rivers occurs.
  • Disruption of traditional ways of life.
  • Loss of diverse flora and fauna.
  • Contribution to global warming.
  • Decrease in productivity of ecosystem.

Rainforest clearance

Nutrient Cycle Changes:

These have been outlined in the diagram under the heading Nutrient Cycle.

There are a variety of approaches, some of which are more appropriate than others. Increasingly the move has been towards sustainable approaches, where the use of resources is less than the ability of an ecosystem to replace itself, thus meaning that a state of dynamic equilibrium is maintained.

Approaches to management include:

  • Reducing amount of burning to reduce amounts of fertiliser, and maintain organic matter.
  • Stop monoculture, and encourage planting of a variety of crops, reducing nutrient depletion.
  • Limiting use of large machinery, which encourages soil compaction.
  • Limit large scale grazing, again to reduce effects of soil compaction.
  • Control of pH value of soil to limit high levels of acidity.
  • Limit the area that is being developed.
  • Production of sustainable crops and products such as, fruit trees, rubber, brazilnuts, wax and honey, ecotourism.

This is low-density tourism with a small impact on the natural environment, which takes place mainly in small groups. It is important to local communities as they have control over it and see direct benefits. Within the Amazon Basin ecotourism is in operation and today there are approximately 80 agencies offering 'eco' packages, ranging from day trips to larger packages. The Amazon State Tourist Board supports them.

The key features of the trips include:

  • Little technology is used.
  • Waste is removed from the forest.
  • Solar power is a typical form of energy.
  • Meals are prepared with local ingredients.
  • The staff are from the local area (employment opportunities).
  • Groups are limited in their number - (usually 10).
  • Accommodation is constructed from local materials to blend in with surroundings.
  • Education is a central aim of the visit.

Multiple Choice Questions

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Exam-style Questions

  1. a) Describe the type of vegetation found in the Mediterranean Woodlands.

    (3 marks)

    b) Explain how this type of vegetation has adapted to its climate.

    (4 marks)

    c) Describe the impact humans have had in Mediterranean woodlands.

    (4 marks)

    (Marks available: 11)


    Answer outline and marking scheme for question: 1

    a) Give yourself marks if you mentioned any of the following points:

    • Deciduous forest/Coniferous forests/Dense shrub land.
    • Oaks and pines/Maquis/Garrigue.

    (3 marks)

    b) Give yourself 1 - 2 marks if you mention the following points:

    • Waxy leaves/Long roots/Fire resistant
    • Give yourself 3 - 4 marks if you can expand the above points:
    • Waxy leaves prevent water loss.
    • Long roots reach water found deep below the surface.
    • Small trees and shrubs are fire resistant.
    • Rosemary rolls up its leaves to reduce moisture loss.
    • Thick bark protects against the heat.

    (4 marks)

    c) Give yourself marks if you mentioned any of the following points:

    • Much woodland has been cleared for fuel and building materials.
    • Areas have been cleared for farming.
    • Topsoil containing nutrients is washed away.
    • Grazing has caused damage to shrub land.
    • Fires have destroyed woodland.
    • Soil left infertile.

    (3 marks)

    (Marks available: 12)

  2. a) Explain why deforestation can reduce soil fertility.

    (3 marks)

    b) How can deforestation affect people who live in these areas?

    (4 marks)

    c) Describe how tropical rainforests can be managed.

    (3 marks)

    (Marks available: 10)


    Answer outline and marking scheme for question: 2

    a) Give yourself marks if you mentioned the following points:

    • Humus is not replaced.
    • Nutrients are washed away downwards.
    • Leaves do not fall to create humus.
    • The vegetation creates most of the nutrients.

    (3 marks)

    b) Give yourself 1 - 2 marks if you mentioned the following points:

    • Increased flooding/homes lost/moved to urban areas.
    • Created jobs.

    Give yourself 3 - 4 marks if you can expand any of the above points. You may also use case studies.

    • Increased surface runoff due to deforestation can cause increased flooding, which can lead to the destruction of infrastructure.
    • Homes of native groups of people living in the forests can be lost.
    • As the soil becomes less fertile farmers in North and Central Brazil have moved to shanty towns, like Roçhina, in urban areas.
    • The Carajas mining project has created 2000 jobs.

    (4 marks)

    c) Give yourself marks if you mentioned the following points:

    • Sustainable development.
    • Replacing the same amount of trees as you remove.
    • Create more National Parks where deforestation cannot occur.
    • Reduce the mass burning of trees.
    • You may also gain marks for using relevant case studies.

    (3 marks)

    (Marks available: 10)

Ecosystem Processes

The most important concept relating to ecosystems is that they are a biological community, which is self-regulating, where living things interact. Their size varies enormously from a pond to a tropical rainforest. At its largest scale, the entire globe is referred to as a global ecosystem.

Central to ecosystems is the idea that links are in existence, and a balance (dynamic equilibrium) exists between inputs and outputs that enables an ecosystem to function effectively.

The main links are between the hydrosphere, biosphere, lithosphere and atmosphere.

The biosphere is the living world, for example, any part of the earth and atmosphere that is able to support and maintain life; it includes oceans and lakes.

Ecosystems have two main elements:

Abiotic: Theses are non-living, such as air, water, heat, rock.

Biotic: These are living, such as plants, insects, and animals. They can be further sub-divided into autotrophs (producers) and heterotrophs (consumers) that include herbivores, carnivores, and omnivores, detritivores (decomposers).

All ecosystems have an organic community, which are living organisms,and an inorganic community, which are non-living environments, for example, chemical and physical.

Community: The entire variety of species that are found in an ecosystem.

Population: individual members of a certain species that are found in an ecosystem.

Below is a diagram showing a pond as an ecosystem:

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The sun is the source of all energy for all life on earth, and provides both heat and light energy. Despite this, it is kept by the biosphere for only a short time, before it is re-radiated back out to space. The initial process in energy flow is that of photosynthesis where light energy from the sun is trapped by green plants and turned into chemical energy, which can then be used for plant growth. Energy is then passed along through the ecosystem as food in a food chain or in a more complex food web.

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As energy is passed through an ecosystem several processes occur:

  1. Energy passes through different trophic levels.
  2. The amount of energy there is decreases.
  3. Fewer species and biomass exist at each level.

The diagram below that shows the idea of trophic levels in ecosystems and the process of energy loss:

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The flow of energy decreases at each successive trophic level as does the amount of biomass and the number of organisms. The reasons for this are outlined in the section relating to ecosystem productivity.

Energy found within any plant and animal material is known as biomass, and it can be measured in a variety of ways:

  1. Calories.
  2. Dry weight.
  3. Ash weight (weight after burning).

The links within an ecosystem are known as trophic levels or energy levels. Each has its own particular characteristics, which are outlined in the table below:

Trophic Level: Producer / Consumer: Name / Example: Action: Energy transfer:
Level 1 Producer. Autotrophs (self feeders). For example, green plants. Plants are capable of producing all their own food. Only one energy transfer, from sun to plants.
Level 2 Consumer. Herbivores (primary consumers). For example, caterpillars. Eat the green plants (producers). Two energy transfers have occurred.
Level 3 Consumer. Carnivore (secondary consumer). For example, blackbird. Meat eaters, feed upon the herbivores, fewer in number than primary consumers. Three energy transfers have occurred, more chance for energy to be lost via respiration, excreta.
Level 4 Consumer. Omnivores (Deversivores). For example, hawks, humans. Have two sources of food, because eat both plants and animals. Four energy transfers.

The transfer of energy is not 100% efficient, as energy is lost via respiration, dead organisms, decay, excreta, and heat given off. The result is that fewer organisms are supported at each level, but the individual size of each organism increases at each trophic level. The loss of energy through the trophic levels places a limit on the total mass of living matter (biomass) and organisms found at each level. Detritivores (bacteria and fungi, operate at all levels).

The model of the nutrient cycle was first developed in 1976, by P.F. Gersmehl, who attempted to show differences between ecosystem regards nutrients, transferred and stored between three areas. Plants take in those nutrients where they are built into new organic matter. Nutrients are taken up when animals eat plants and they returned to the soil when animals die and the body is broken down by decomposers.

In all nutrient cycles there are interactions between the atmosphere and soil and many food chains are involved. Nutrient cycles vary greatly between ecosystems, as the rate of nutrient transfer is dependent on the amount of moisture, heat, vegetation and the length of the growing season. The diagrams below show the model of nutrient cycling, and the variation between different nutrient cycles within the Taiga, Steppe and Equatorial Rain Forest.

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Litter: This is the surface layer of vegetation, which over time breaks down to become humus.

Biomass: The total mass of living organisms per unit area.

In each of the three diagrams the amount of nutrients transferred are shown by the width of each arrow, and the amount of nutrients stored in the soil, litter, or biomass is indicated by the size of the circle.

Reasons for differences between nutrient cycles:

Ecosystem: Characteristics:
Taiga Litter = the largest store, due to the slow decomposition of needle like leaves from coniferous trees. Biomass is low as a result of little undergrowth and few species of plant. Few nutrients are found in the soil, as rates of leaching are high, and the breakdown of rock is extremely slow due to low temperatures.
Steppe Biomass is small due to limited moisture, and low temperatures that limit the growing season to 6 months. Many nutrients are kept in the soil as a lack of rainfall means little leaching takes place. Nutrient transfer from biomass to litter is high as the grass dies back in winter.
Tropical Rainforest Extremely rapid rates of nutrient transfer, due to high temps, rainfall and humidity. Biomass is the largest store of nutrients due to the vast arrays of plants found in the TRF. Few nutrients are in the litter, due to their rapid decomposition as a result of high temperatures. Leaching is rapid and more so in areas of rainforest clearance.

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Non-living components of the ecosystem (water, rock, air, sediments).

Autotrophs (producers)

Organisms that can convert energy from the sun by photosynthesis.


Living components of the ecosystem.


Calcium accumulation in the soil as a result of low rainfall.


Meat eaters.


Iron and aluminium sesquioxides removed by chelating agents.

Climatic climax

Final stage in an ecosystem reached by vegetation.


All species found in an ecosystem.




Plant eaters.

Heterotrophs (consumers)

Feed on other organisms.


Material deposited in lower soil horizons.


Material that is soluble moved downwards through the soil by solution.


Removal of fine clay particles from soil.

Net Primary Productivity (NPP)

Energy made available by plants for animals.

Plagio climax

Plant community unable to reach climatic climax due to human influence.


Intensive leaching.

Primary productivity

Productivity of plants only.


Soluble salts moved upwards through the soil as a result of capillary action. Secondary Productivity - productivity of animals.


A group of species, found as an ecosystem progresses towards climatic climax.

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