Controlling Population Growth

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Population Growth

  • The usual pattern of bacterial growth constists of 4 phases:
    -The lag phase: this may last anywhere between a few minutes and a few days; the time where there is an adaptation or preparation with intense metabolic activity. Then slow growth.
    -The exponential phase: without limitation the numbers increase rapidly, dividing at a large rate but it cannot be maintained.
    -The stationary phase: population growth enters this phase when birth rate = death rate; certain factors limit any further growth; and this is called the carrying capacity of a population as it is the maximum number of bacteria an area can support.
    -The death phase: when death rate > birth rate, may occur when food/nutrients are used up.
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Population Growth (Cont.)

  • Limiting factors include:
    -Nutrients will be used up.
    -Toxic waste will build up.
    -Space may become limiting.
    -Disease may occur due to overcrowding.
  • A term used to describe all the factors that may limit the growth is called environmental resistence. 
  • Biotic limiting factors:
    -Competition from other species
  • Abiotic limiting factors:
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Factors that regulate population increase

  • Some factors are density dependent, thus their effect increases as the density of the population increases:
    -Accumulation of toxic waste
    -Depletion of food
  • Some factors are density independent, and their effect is not affected by the density of the population; usually due to a violent change in an abiotic factor:
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Population Fluctuation

  • The general size of the population is determined by birth and death rate, and these fluctuate around a certain point; thus meaning the population fluctuates around a certain amount that the equilibrium can support.
  • Size may vary on a constant basis and the oscillations are regulated by negative feedback:
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  • Two types of competition:
    -Intraspecific competition occurs between individuals of the same species, this is density dependent because it is affected by the number of individuals fighting for survival; those best adapted are those that survive.
    -Interspecific competition occurs between different species, where there are two species fighting for the same biological niche. The role and space can only be occupied by one, and the most adapted is the one that survives or does best. 
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Pest Control

  • Pests cause damage by:
    -Feeding on crops and animals
    -Competing with crop organisms
    -Directly causing disease
    -Making infection pathogens more likely
    -Spoiling food when transported
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Chemical Control

  • This involves using pesiticides; be it fungicides or insecticides etc.
  • They should be:
    -Non-accumulating along a food chain
    (DDT accumulated and was persistent and so a resistence was built up)
  • Advantages:
    -Very effective means of control
    -Pests are eradicated quickly and cheaply
    -Chemicals can be applied on a small scale
    -Application does not require a high level of skill
  • Disadvantages:
    -Not specific and can eradicate beneficial insects
    -Pests may become resistent
    -May kill animals by infecting their food/water sources
    -Long-term use can harm humans 
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Biological Control

  • Specific kind of interspecific competition evolving between predator and prey.
  • The whole process would maintain itself via negative feedback.
  • Advantages:
    -Highly specific to one pest, targeting the pest only
    -Long-term control if population equilibrium is established
    -Initial research may be costly, but cheap in the long run
    -There is no environmental contamination
    -Can be used in a glasshouse situation
  • Disadvantages:
    -Biological control agents are slow to build up in number
    -Succesful examples are relatively low in number
    -Detailed knowledge of the life cycle is required, requiring high level of skill
    -There is the potential for the release of a harmful species
    -Frequent input is needed to attain a population balance
    -Large-scale operations are very unlikely to be succesful 
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Carbon Cycle

  • Level of C in the atmosphere has been maintained for millions of years, but over the last few hundred years the level has increased due to:
    -Burning of fossil fuels
    -Deforestation, so less CO2 is being removed from the atmosphere
  • The basis of the C-Cycle includes:
    -CO2 is added to the air by the respiration of animals, plants and micro-organisms and by combustion
    -Photosynthesis takes place on a scale where the CO2 is used as much as it is released
    -Saprobionts ultimately release CO2 from dead and decaying matter
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Carbon Cycle Diagram


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Nitrogen Cycle

  • The flow of organic and inorganic N within an ecosystem.
  • Ammonification:
    -Bacteria and fungi are called decomposers and result in the decay of dead plants and animals, into ammonium ions; also called putrefaction. (Proteins etc. -> Ammonium ions)
  • Nitrification:
    -Ammmonia formed is converted by nitrification via nitrites to nitrates; these processes are undertaken by the bacteria Nitrosomonas and Nitrobacter under aerobic conditions
  • Denitrification:
    -N is lost from ecosystems by denitrification, occuring when under anaerobic condtions Pseudomonas can reduce nitrates and ammonium ions into N.
    -Human activities can improve the circulation of N:
       -Large amounts of animal waste is used as manure
       -Sewage disposal boosts organic N supplies
       -Micro-organisms for making compost/silage
       -Farming practices such as draining land, ploughing and planting legumes.
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Nitrogen Cycle (Cont.)

  • Nitrogen Fixing:
    -Atmospheric N can be converted into ammonia by nitrogen-fixing bacteria such as Azobacter; this species is free-living and reduces N to ammonia and then into amino acids
    -Atmospheric N is reduced to ammonia by Rhizobium, using nitrogenase - the root nodule of the legume is pink due to the haemoglobin inside of it, which is used to protect the enzyme from O2; as it inhibits it.
    -The nodules and bacteria within the legumes allow the plant to grow succesfully even with a lack of nitrates, on the death of the plant the N containing substances go back into the soil.
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