Ecosystems: Decomposers and Pop. Size

Revision cards on decomposers, recycling and population size

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  • Generally microorganisms
  • Secrete enzymes onto dead and waste material
  • Enzymes break down material
  • Molecules are absorbed into the organism
  • Once absorbed, molecules are stored to be respired in order to release energy
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The Nitrogen Cycle and bacteria's role within it

Nitrogen Fixation

  • Nitrogen is highly unreactive so needs to be fixed for plants to utilise it.
  • Bacteria such as Rhizobium live within root nodules of bean plants.
  • The bacteria supply the plant with fixed nitrogen in exchange for carbon (mutualistic relationship)


  • Occurs once chemoautotrophic bacteria absorb ammonium ions.
  • These bacteria gain energy from absorbing by oxidising ammonium ions to nitrates or nitrites to nitrates.


  • Some bacteria convert nitrates into nitrogen gas.
  • Occurs in anaerobic conditions- bacteria use nitrates as a source of oxygen, releasing nitrogen gas nitrous oxide.
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Population phases

Lag phase

  • Only a few individuals.
  • Still adjusting to a new habitat.
  • Rate of reproduction is slow.

Log phase

  • Individuals have adapted to habitat.
  • Resources are plentiful and competetion is minimal.
  • Reproduction exceeds mortality.
  • Population size is rapidly increaseing.

Stationary phase

  • Population has levelled out.
  • This is the carrying capacity.
  • The habitat can't maintain a higher population.
  • Reproduction and mortality rates are equal.
  • Population size remains fairly constant.
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Factors that determine carrying capacity

Biotic Factors

  • Factors that are related to living things such as:
  • Competition, both interdependent and intradependent
  • Predation

Abiotic Factors

  • Factors that are not related to living things:
  • Availabity of water or food
  • Disease
  • Extreme, freak weather
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Predators and Prey

Population of both predator and prey size often follows a pattern similar to:

  • Predator population increases
  • Demand for prey increases from the predator
  • Prey population decreases as a result of predation
  • Predator has less prey for food so competition increases
  • With increased competition, predator populaton decreases
  • As a result of decreased predator population, prey population increases
  • With an increase in prey the predator population increases
  • The cycle starts again
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