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Define the term ecosystem.

All living organisms and all the non-living components in a specific habitat, and their interactions. 

Ecology is the study of the interactions between organisms and their environment

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Dynamic systems

Ecosystems are dynamic systems. 

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Define biotic factor and abiotic factor

Biotic factor: how living organisms affect each other, food supply, predation, disease.

Abiotic factor: the effect of non-living components of the ecosystem, pH, temperature, soil type.

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Define these terms.

Producer: An organism that converts simple inorganic compounds into complex organic compounds.

Consumer: An organism that gains energy from complex organic matter.

Decomposer: An organism that feeds on waste from other organisms, or dead organisms.

Trophic level: Each feeding level in a food chain.

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How energy is transferred through ecosystems

Energy is transferred by organisms consuming each other. This is shown in a food web, with the arrows representing the flow of energy between organisms.

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Measuring energy transfers between trophic levels

The energy content of samples of organisms from each trophic level is measured. 

  • Each sample is dried in an oven
  • The samples are weighed
  • The samples are burned in a bomb calorimeter in oxygen
  • The energy produced passes to a known mass of water and the temperature rise of the water is measured
  • How much energy is released per gram is calculated and converted to kJ
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Efficiency of energy transfers between trophic lev

Energy is lost between trophic levels because animals never eat all of the available food and cannot digest all of the food they eat.

They also use energy to respire, lose heat energy to the surroundings and lose energy in urine and faeces.

Net primary productivity is different in different ecosystems because tropical ecosystems compared to temperate ones have higher temperatures and more sunlight (days are longer) so they photosynthesise faster and form more biomass and therefore energy. Woodland or rainforest ecosystems compared to grassland have a greater complexity with greater biodiversity and so more niches. This means competition for space is less limiting. 

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Human manipulation of energy flow

Increasing productivity of producers through:

  • Soil improvement
  • Irrigation
  • Fertilisers
  • Removal of competing weeds, damaging pathogens and pests

Increasing productivity of producers and consumers through selective breeding or genetic engineering.

Sheltering organisms from damaging environmental factors.

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Primary succession and climax community

  • Sand dunes:
  • Pioneer plants such as sea rocket colonise the sand just above the high water mark. These can tolerate salt water spray, lack of fresh water and unstable sand.
  • Wind-blown sand builds up around the base of these plants, forming a 'mini' snad dune. As the plants die and decay, nutrients accumulate in this mini dune. As the dune gets bigger, plants like sea couch grass colonise it, which has underground stems to help stabilise the sand.
  • With more stability and accumulation of more nutrients, plants like marram grass start to grow. Marram grass shoots trap windblown sand and, as the sand accumulates, the shoots grow taller to stay above the growing dune, thus trapping more sand.
  • As the sand dune and nutrients build up, other plants colonise the sand. Many are members of the bean family, which have nodules in their roots which contain bacteria which convert nitrogen to nitrates. With more nitrates available, more species colonise the dunes, stabilising them further.
  • Rock:
  • Algae and lichens begin to live on the bare rock
  • Erosion of the rock and build up of dead and rotting organisms produce enough soil for larger plants, such as mosses and ferns to grow
  • Larger plants succeed the small plants until the climax community is reached
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Measuring distribution and abundance

Line transects: On a line across the habitat and record every species touching the line, and their position. Species are identified using a key and recorded in a pre-prepared table.

Belt transects: Quadrats are placed sequentially along a line transect.

Quadrats: A square fram is placed at random in the habitat. Each species present is identified, and the percentage cover of ACFOR is recorded for each species (abundant, common, frequent, occasional, rare).

Point quadrats: Frames with long pins. Lowered vertically at random. Each species which touches a pin is recorded, along with the number of times it is touched.

Ladybirds would be sampled using a sweep net or beating trees and bushes or using a pooter.

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Role of decomposers in decomposition

Decomposers are organisms that return inorganic minerals from the bodied of dead organisms to the abiotic environment.

Decomposers feed on waste from other organisms. They recycle materials such as carbon and nitrogen. If they did not break down dead organisms, energy and valuable nutrients would remain in the dead organism.

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How microorganisms recycle nitrogen

Plant protein is converted to animal protein when animals eat plants. The animal digests (hydrolyses) the protein to amino acid. These amino acids move into the blood and then cells. They are used in the synthesis of proteins (translation). Plant and animal protein becomes humus in the soil when they die or leaves are lost. They then decay. Animals also excrete urine and defaecate. 

Nitrogen gas converted to ammonium compounds. Nitrogen is fixed by bacteria such as Rhizobium that live in root nodules of legumes (peas, beans). They have a mutalistic relationship with the plant - they fix the nitrogen for the plant and the plant provides the bacteria with carbon compounds (glucose). There are proteins which absorb oxygen, keeping conditions anaerobic, so that nitrogen reductase can reduce nitrogen gas to ammonium ions.

Ammonium compounds converted to nitrite ions. Ammonium ions are released by bacteria in the putrefaction of proteins found in dead or waste organic matter. Nitrosomonas bacteria obtain their energy by oxidising ammonium ions to nitrites under aerobic conditions.

Nitrite ions to nitrate ionsNitrobacter obtain their energy by oxidising nitrites to nitrates under aerobic conditions. Nitrosomonas and Nitrobacter are nitrifying bacteria. Plants absorb the nitrates from the soil to make amino acids, proteins, enzymes, DNA, RNA and chlorophyll.                Nitrate ions converted to nitrogen gas. 

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