Population factors

Population: An interbreeding group of organisms of the same species in a particular habitat. 

Carrying capacity: Maximum number around which a population fluctautes in an enviornment. 

Population number will fluctuate depending on various factors: Birthrate, death rate, immigration and emigration 

Populations will increase in size when:

Births + immigrants > deaths + emigrants 

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Standard growth curves

Lag Phase: 

  • Cells adjust to new conditions 
  • Takes a wile for enzyme production 
  • The bacteria adjust to new conditions, synthesising carriers to absorb nutreints and enzymes to digest.
  • May involve switching on genes 

Log (exponential) phase:

  • As numbers increase, as long as there is not factors limiting growth, more individuals become availble for reproduction. Bacterial cells divide at a constant rate and the population doubles per unit time. The cell numbers increases logarithmically and so the exponential phase is also called the log phase. 

Stationary phase:

  • Occurs when birth rate is equal to the death rate. The population has reached its maximum size, which is the carrying capacity for that particular enviornment
  • The actual number depends on the resources available 

The death phase:

  • Death rate is greater than birth rate
  • Waste products such as ethanol kills cells 
  • Net decrease of population 
  • Nutreints exhausted and waste too toxic 
  • Some dead will act as a nutrient source for others 
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Environmental resistance

Environmental resistance 

Environmental resistance relates to any factor that slows down population growth and can cause it to crash 


  • Less food available
  • Build up of waste products 
  • Not enough space 

For bacteria:

  • available food 
  • Over crowding 
  • Competition 
  • Buildup of waste 


  • Predation 
  • Parasitism and disease 
  • Competition 


  • temperature 
  • Light intensity 
  • pH
  • rainfall
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Density- dependent or independent

Biotic factors are density dependent - when some factors play a more significant role in population size if the population is higher, they are often biotic factors

Density independent: Abiotic factors effect the population regardless of the population density.

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