Biology- Populations/ecosystems


  • Created by: T
  • Created on: 31-12-09 14:15


Populations= A group of individuals of the same species living in the same habitat

Ecosystems= All the living organisms that live and interact together along with physical and non physical factors.

Community= All of the organisms of all species in the ecosystem.

How to measure abiotic factors:

  • Temperature- Using a temperature meter or thermometer. (for field experiments use the meter as it is less fragile).Temperature effects metabolic systems.
  • pH- pH meter or litmus paper, enzymes are pH sensitive therefore disrupt metabolism if conditions too acidic or alkaline.
  • Light intensity- Light meter, often limiting factor in photosynthesis.
  • CO2 concentration- Essential in Photosynthesis and can be a limiting factor.
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Sampling techniques in field studies:

1) Quadrats- They allow you to sample different areas

  • Frame Quadrats are small areas of the ground that are small enough to be studied in a short time. They can be of 50 X 50 as it is manageable and poortable.
  • Place the quadrats randomly and use a representative sample of the area.
  • one way is by mapping out the area into squares and picking the squares randomly by either using a computer program to remove human bias.

2) Transects- They allow the researcher to measure change from one area to another.

  • Three types of transects: Continuous Belt transects, interrupted beelt transect and point transect.
  • C.B.transect- Is continuous measurement along the transect. The size of the transect depends on the area which is being investigated
  • I.B.transect- Is where the quadrats are placed along the transect at intervals. (can be regular or random)
  • P.transect- Is when species touching a particular point on the transect are recorded.
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Sampling techniques in field studies:

3) Mark-release-recapture- is used for estimating populations of living and moving animals.

  • Firstly capture the animal:

4 techniques for capturing animals:- Pitfall traps: Used to trap invertebrates that may be active in the soil or in the leaf litter. Use 10% methanal to kill predators that may want to eat the captured animals.

-Longworth traps: these traps are baited with cheese or other food, and let in the long grass where small mammals are found. As animals enter the trap is triggered and closes door behind the mammal.

-water traps: water is left on open land in different coloured containers. eg yellow attracts aphiods and white attract flies. Washing fluid can be added to reduce surface tension and insects sink into water

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  • Secondly mark the individuals caught: but make sure that the mark does not affect the animals ability to escape a predator or efffect their movement or make them more visible.
  • Release the marked animals
  • leave enough tim for the marked individuals to redistribute amongst the rest of the population.
  • Then capture another sample of the organism consisting of similar size.
  • count the number of individuals that have been caught before.

Use the formula: (M X C) / R= Estimated population size

M= Number of individuals caught the first time

C= the number of inviduals caught the second time

R= the number of individuals caught 2nd time that were marked

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Population size

Population size can be effected by biotic and abiotic factors. Each new population follows a general growth pattern.

These stages of population growth are:

A) Lag phase- A time of slow growth e.g. sexual reproduction which takes time.

B) Log phase(exponential phase)- a period of rapid growth e.g. unrestricted growth due to favourable conditions.

C) Growth slows due to limiting factors e.g. build up of toxic waste, or food beocoming scarce.

D) Population begins to stabilise at its carrying capacity (a size of a population which can be supported in a given area)

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Population and ecosystem

Factors affecting Population

No population can go on growing indefinately. These are some factors affecting population growth:

1) Competition Animals can compete for food, water, space and are locked in a battle for survival. There are two types of competition.

interspecific competition: Competition between individuals of different species e.g.for food

-intraspecific competition: competition between individuals of the same species

2) Predation key biotic factor. Predator eating prey. Usually more complex as predator have more than one prey.

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