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  • This technique is used to measure abundance of an organism.
  • it relies on a number of assumptions:
  • proportion of unmarked individuals in 2nd sample is same proportion of individuals as a whole
  • marked individuals in 1st sample distribute evenly within the population.
  • no immergration or emmergration within the population.
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3 main factors to consider:

  • The size of the quadrat to use: This will depend upon the size of the organism being counted and how they are distributed throughout the area. A large number of small quadrats wll give more valid than a small number of larger ones.
  • The number if sample quadrats to record within the study area: The greater the number of quadrats the more reliable the results.
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  • A population is the number of interbreeding organisms of the same species in a specific area.
  • Eg within a woodland you get populations of woodpeckers, beatles and earthworms
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  • A community is all the organisms of all the populations of a spesific area living and interacting at the same time.  
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  • A habitat is the place were a community of organisms live and interact
  • within an ecosystem there are many habitats and microhabitats.
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Measuring abundance

  • Abundance is the numberf of individuals of a species within a given space. 2 main measurments:
  • frequency: The likelyhood of a species occuring in a quadrat. eg species occurs 15 out of 30, frequency is 50%. Provides the species present and general distribution but not density and detailed distribution
  • percentage cover:an estimate of the area that the species covers within the quadrat. good is the species is difficult to count eg plants. Bad if the organisms overlap  
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Quadrats 2

The position of each quadrat within a study area: To produce statistically significant results random sampaling must be used.

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Random sampling

  • This is used to aviod any bias when collecting data.
  • How to set this out:

1) lay out 2 long tapes at right angles, along 2 sides of the study area.

2) obtain a series of co-ordinates by using a random number generator

3) place a quadrat at the intersection of each pair of co-ordinates ans record the species within it.

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Systematic Sampling along a transect

  • sometimes it is better to measure abundance + distribution of a species in a systematic way
  • This is important where some form of transition takes place.
  • Lay a line of tape on the ground
  • measure any organism which the line passes.
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interspesfic competition

  • This is competition between members of different species.
  • They fight for food, water, shelter and light.
  • when 2 species initially occupy the same niche, one will normally have an advantage over the other.
  • one population would diminish while the other increases
  • if this stays the same, one population will die (competative exclusion principle)
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Ecological Niche

  • A niche describes how an organism fits in with its environment,
  • ie where an organism lives and what it does there.
  • it includes all the biotic and abiotic conditions for an organism to survive, reproduce and maintain a viable population.
  • No two niches occupy the exact same niche.
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predation- predator prey relationship on populatio

  • when one organism is consumed by another.
  • predators eat there prey- reducing there prey population
  • with fewer prey available- there is greater competition with predator.
  • the predator population decreases.
  • fewer predator so prey increases in population.
  • with more prey there is an increase in predator population.
  • when population decreases selection pressure occurs where the fittest survive.
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variation in population size- population growth cu

This is the usal pattern of growth for a natural population.

1) slow growth as the initially small number of individiuals reproduce to slowly build up there numbers.

2) rapid growth were increasing numbers of individuals continue to reproduce. The population doubles, steeper gradient.

3) a period when the population growth declines until the size remains stable. slight flucuations is down to food supply and predation.

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Abiotic Factors that effects the size of a populat

  • These are non-biological factors that make up the environemt these include:
  • Tempreture:each species has a different optimum tempretur at which it is best able to survive, to high enzyme denature, decrease population
  • Light: the source of energy for an ecosystem. effects rate of photosynthesis, increased growth so better chance of increased population.
  • pH: enzymes have different optimums if this met then population will increase.
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intraspesific competition

  • Intraspesific competition: occurs with individuals of the same species compete for: food, water and breeding.
  • The greater the availablity, the larger the population.
  • The lower the availablity, the smaller the population.
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Human populations size and growth rate

  • 2 major recent events that has caused to an explosion in the human population:
  • The development of agriculture
  • The development of manufacturing and tade that created the industrial revelution
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Population structure

  • Demographic Transition- change from when life expetancy was short and birth rates are high to those where life expectancy is long and birth rates are low.
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populations and ecosystems


  • An ecosystem is made up of all the interacting abiotic and biotic features of a spesific area.
  • 2 main processes are: flow of energy through the system and cycling of elements within a system
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factor effecting the growth and size of human popu

  • Main factors that affect human populations:
  • Birth Rate
  • Death Rate
  • Immergration rate
  • Emmergration rate 

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Factors affecting death rates

  • Age profile- the greater the number ofelderly people the higher the death rate will be
  • Life expecatancy- at birth- more econmoically developed countries live loner than less developed ones.
  • Food supply- An adequate and balanced diet reduces death rate
  • safe drinking water- reduce death rate of water borne diseases such as cholera.
  • Better medical care
  • natural disaster
  • war
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Mark-releas-recapture 2

  • Few if any births or deaths
  • method of marking is non toxic and doesnt make individuals more liable to predation.
  • mark doesnt come off.


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Factor affecting birth rate

  • Economic Conditions- less econmoiclly developed countries tend to have higher birth rates
  • Culture/Religion- some religions oppose birth control.
  • Social pressure and coonditions- in some countries larger families improve social standings.
  • Birth control
  • Political factors
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