1.6- Ecosystems, Nutrient Cycles and Human Impact on the Environment

1.6- Ecosystems, Nutrient Cycles and Human Impact on the Environment

1. Energy Flow in the Environment

   • Energy in Food Chains
     • Sunlight is the source of all energy on the planet. This energy passes from organism to organism by means of food chains. Plants are the first links in all food chains because they are producers- they change the energy in sunlight into stored chemical energy, although plants can only capture about 5% of the energy in sunlight. When plants are eaten by herbivores, some of the energy is passed to the next link, the consumers in the food chain. When the herbivore is eaten by a carnivore, the process of energy transfer is repeated. Energy passes in this way from carnivores to scavengers and decomposers (feed on dead organisms)
       • The energy transfer at each stage is not 100% efficient. Energy is lost in the following ways:
         • A lot of light energy misses plants completely.
         • Some of the light energy that hits a plant is reflected or goes through the leaves and is not absorbed.
         • Each stage in the food chain uses some of the energy that it takes in for its own purposes (e.g. movement, cell repair, reproduction). That energy is dispersed and cannot be passed on.
         • Organisms rarely eat every bit of another organism (e.g. herbivores don't eat the roots of the plant) so they don't take in all the energy available.
         • Even what is eaten is often not completely digested, so some energy never gets in to the feeding organism
     • As energy is lost at each stage of a food chain, the numbers of organisms that can exist at each level of a food chain goes down as the food chain progresses.

2. Trophic Levels

   • The stages in a food chain are called trophic levels, and each level is given a specific name. The following is an example of a food chain.
     • Plant plankton ---> animal plankton ---> small fish ---> larger fish ---> tuna ---> human
       • Producer
       • First stage consumer
       • Second stage consumer
       • Third stage consumer
       • Fourth stage consumer
       • Fifth stage consumer

3. Food Webs

   • In nature food chains often interlink, because most organisms eat a lot of different things and are eaten by many different animals. Interlinked food chains are called food webs, however even they are an oversimplification of all the feeding relationships that exist in the environment.

4. Ecological Pyramids

   • Feeding relationships can be illustrated as pyramids. The width of each block in the pyramid is an indication of the number (or mass) of that type of organism at that feeding level.
     • Pyramid of Numbers
       • Shows the number of organisms per unit area or volume at each feeding level.
     • Pyramid of Biomass
       • Shows the dry mass of organic material per unit area or volume at each feeding level
       • Pyramids of biomass give a more accurate picture than pyramids of numbers. Pyramids of numbers are sometimes not pyramid shaped. Sometimes the producers (e.g. trees) are much larger than the herbivores that feed on them (e.g. insects). One tree can support thousands of insects, so the bottom block is narrower than the block for the first stage consumers. A tree weighs more than all the insects feeding on it, so a pyramid of biomass remains pyramid shaped.
     • Dry mass: the mass of a materials after all the water has been removed

5. Microorganisms in Ecosystems

   • Microorganisms play a vital role in life on Earth. They dispose of animal waste and the dead animals and plants. In doing so, they return vital nutrients into the soil, which can then be used for new growth. For example, nitrates. They are formed from the breakdown of proteins in dead tissues and are used by plants to form new cells. Phosphates, which are needed for many vital functions in living bodies, are also recycled. The cycling of these nutrients ensures a balance in ecosystems, as the processes that remove materials are balanced by the processes that return them.
   • Ecosystem- the living organisms in an area together with the non-living components of their environment.

6. The Carbon Cycle

   • • The two main processes are photosynthesis and respiration. Photosynthesis takes carbon dioxide out of the air and respiration puts it back. Respiration of bacteria is important in returning the carbon locked in the dead bodies of plants and animals to the atmosphere.
     • In some conditions, dead bodies don't decay but become fossiled to form fossil fuels. When fossil fuels are burnt, the carbon in them is released as carbon dioxide, adding to the levels in the atmosphere. This has only happened over the last 200 years and it is causing an imbalance.

7. The Nitrogen Cycle

   • • Nitrogen in the air is changed by nitrogen-fixing bacteria in the soil into nitrates, which plants can absorb and use. Nitrogen-fixing bacteria are also found in the roots of one group of plants, the legumes (peas, beans and clovers), in special structures called root nodules. The nitrates absorbed by the plants are passed on to animals that eat the plants, and the nitrogen is eventually returned to the soil in urine and faeces from animals, and when dead animals decay. The nitrogen from wastes and decay is in the form of ammonia, which has to be converted into nitrates by bacteria, which is then absorbed by plants.
   • Nitrogen is returned to the air by denitrifying bacteria, which use the nitrates and release nitrogen into the air.
   • Nitrogen fixation: the conversion of nitrogen in the air into nitrates,
   • Lightning: the large amount of energy from the lightning breaks nitrogen molecules and allows them to bond with the air forming nitrogen oxides. These then dissolve in rain to form nitrates.

8. Urease

   • Animal urine contains nitrogen in the form of urea. To recycle this, it is broken down into ammonia by bacteria. The ammonia can be converted in the soil to nitrates. The enzyme that converts urea to ammonia is urease.

9. Conservation

   • Human needs sometimes conflict with conservation of the environment. Humans sometimes have to destroy habitats in order to meet essential needs, for example:
     • People need places to live and work, but building houses, roads etc. destroys the habitat.
     • Farmland completley changes the natural habitat, but people need food.
     • To meet energy needs, people have to build power stations and tidal barrages
     • To provide essential drinking water, rivers may be dammed and areas deliberately flooded.
     • The issues vary according to the situation. How intense the human need is will vary. The need to conserve the natural environment will be stronger in certain situations.
       • Habitat may contain rare or endangered species
       • Habitat may have a very large variety of species in it, so that many population will be affected if it is destroyed.
       • The habitat itself may be unusual or rare, making its conservation important.

10. Farming Practices

    • Intensive farming is an agricultural practice that involves trying to produce the maximum yield from the land available. it applies to both animals and plants, and involves using chemicals such s fertilisers and pesticides to control disease. Farm animals may be kept indoors in restricted conditions so that as many as possible can be kept in a given space. This is called battery farming.
    • Advantages are:
      • The yield is high because more livestock/plants can be kept and conditions can be controlled. This makes food cheaper to produce.
      • Food is cheaper in the shops, allowing people to choose a healthier diet.
      • Increasing the yield allows us to meet the needs of our growing population.
    • Disadvantages are:
      • The chemicals used (pesticides, antibiotics etc) could enter the food chain and get into our bodies.
      • Chemicals can cause pollution and harm the wildlife.
      • Natural environments are destroyed e.g. hedgerows are uprooted to create larger fields.
      • Causes animals distress abd discomfort, as well as poor quality of life

11. Pollution

    • Types of Pollutant
      • A pollutant is something that has been added to the environment that damages it in some way. Some common pollutants are:
        • Solid or liquid chemicals, such as oil, detergents, fertilisers, pesticides and heavy metals
        • Gaseous chemicals, such as carbon dioxide, methane, chlorofluorocarbons (CFCs), sulfur dioxide and nitrous oxides
        • Noise
        • Heat
        • Non-recyclable household waste
    • Detecting Pollution
      • Water pollution can be detected by a fall in the oxygen level or a change in pH in streams and rivers. Scientists often judge the level of pollution by using indicator species- plants and animals that have a tolerance to pollution. Some will always be absent once pollution reaches a certain level, while others will only be found when pollution exceeds a certain level.
      • There are many different indicator species which are found in fresh water and can be used to test pollution (e.g. rat tailed maggot). Lichens are a good indicator of polluted air, different species being intolerant or very tolerant to air pollution.
    • Pollutants in Food Chains
      • Some harmful chemicals cannot be broken down in the body. If they are eaten, they remain in the tissues. Any animal that eats and organism that contains such a harmful chemical will then take it in. Organisms each large numbers of prey from the trophic level below them, so they will take in a lot of the chemical. This happens at every stage in the food chain and so the higher up the food chain we go, the greater the level of the harmful chemical in the body. The level can get so high that it harms or kills the organism.

12. Eutrophication

    • Sewage and fertilisers sometimes get into streams and rivers from farmland, washed from the soil by rain. This can cause eutrophication, which happens in the following way:
      • The sewage or fertiliser causes an increase in the growth of microsopic plants.
      • These plants have short lives and soon die.
        • bacteria feed on the dead plants in large numbers,
          • The oxygen level in the water drops because of bacterial respiration
            • Fish, which need a lot of oxygen, die due to low oxygen levels.
              • In small bodies of water, such as ponds, the microscopic plants may form a blanket over the surface, blocking out sunlight and stopping the plants at the bottom of the pond from photosynthesising.

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