iGCSE Biology Section D

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ecosystem - self supporting system of organisms
population - organisms of a paticular species living in an ecosystem at a particular time
community - all populations of living organisms living in an ecosystem at a particular time
habitat - the plave where organisms live
quadrats - used to sample the distribution of organisms in their habitats

Plants are producers
Animals are consumers
Bacteria and fungi are decomposers

Food chains are simplified. In real ecosystems, real food chains link together to form food webs. However, food webs are also simplified as they don't tell you numbers or mass of the organisms involved and they do not show the role of decomposers.

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Ecological pyramids

Pyramids of numbers: numbers of organisms at each trophic level.
Pyramids of biomass: biomass of organisms at each trophic level.(http://lgfl.skoool.co.uk/uploadedImages/12.2_energy_food_chains_pob.gif)

Pyramids of biomass gets less at each level because:

Some of the organism isn't eaten and not ll can be digested / absorbed.
Some of the food is excreted or used for respiration
Only a small part of the food is used to produce growth and new body mass.

Only about 10% of the energy is passed on at each level of a food chain.

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Carbon cycle


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Nitrogen cycle

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Water cycle


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Human influences on the evironment

sulfur dioxide causes acid rain
eutrophication - lack of oxygen
increased nitrates in water from pesticides > more algal growth > blocks light > plants die > algae die as they run out of nitrates > decomposers break down dead things using oxygen > oxygen levels go down > animals can't live there.

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