The Organism in the Environment Notes

Edexcel GCSE Biology: Revision notes

Key facts

  • Levels of organisation: organism, population, community, ecosystem.
  • Abiotic factors are non-living; biotic factors are living.
  • Parasitism: the parasite benefits and the host is harmed. Mutualism: both benefit.
  • Quadrats and belt transects sample populations.
  • Estimated total = mean per quadrat × (total area ÷ quadrat area).
  1. 1

    Organism

    one individual

  2. 2

    Population

    one species in a habitat

  3. 3

    Community

    different species

  4. 4

    Ecosystem

    community plus abiotic environment

Levels of organisation

Levels of organisation

Ecology works through four levels, from one organism to a whole ecosystem.

A population is all of one species in a habitat. A community is the populations of different species. An ecosystem is the community plus the abiotic environment.

  1. 1

    Organism

    one individual

  2. 2

    Population

    all of one species in a habitat

  3. 3

    Community

    populations of different species

  4. 4

    Ecosystem

    community plus abiotic environment

Levels of organisation

All the rabbits in a field are a...

Abiotic and biotic factors

Non-living (abiotic) and living (biotic) factors both affect organisms.

Abiotic factors are non-living, such as temperature, light, water and pollutants. Biotic factors are living, such as competition and predation.

  1. 1

    Abiotic factors

    non-living: temperature, light, water, pollutants

  2. 2

    Biotic factors

    living: competition, predation

  3. 3

    Together they decide

    which organisms survive and how many there are

Both kinds of factor shape a community.

Abiotic

  • Temperature
  • Light
  • Water
  • Pollutants

Biotic

  • Competition
  • Predation
  • Parasitism
  • Mutualism

Which is a biotic factor?

Interdependence

Parasitism harms one organism; mutualism benefits both.

Interdependence means organisms rely on each other in a community. In parasitism the parasite benefits and the host is harmed. In mutualism both benefit.

  1. 1

    Plant gives the bacteria

    sugars and a home in its root nodules

  2. 2

    Bacteria give the plant

    nitrates made from nitrogen in the air

  3. 3

    Both benefit

    this is mutualism; in parasitism only the parasite benefits

Mutualism example: Rhizobium bacteria in root nodules of legumes.

Parasitism

Organism 1:
Benefits
Organism 2:
Harmed

Mutualism

Organism 1:
Benefits
Organism 2:
Benefits

In mutualism...

Core practical: quadrats and transects

Place quadrats randomly to sample an area, or along a belt transect to sample a gradient.

Quadrats: place randomly, then count organisms or estimate percentage cover. Belt transects place quadrats at intervals along a line to sample a gradient.

  1. 1

    Mark out

    the area to be sampled

  2. 2

    Random coordinates

    use a random number generator

  3. 3

    Place quadrat

    at each position

  4. 4

    Count

    organisms or estimate cover

  5. 5

    Repeat

    and calculate the mean

Random quadrat sampling

Why are quadrats placed randomly?

Estimating population size

Scale the mean count per quadrat up to the whole area.

Take the mean count per quadrat, then multiply by the number of quadrats that fit in the area.

  1. 1

    Count organisms in each quadrat

    use random positions

  2. 2

    Calculate the mean per quadrat

    total count ÷ number of quadrats

  3. 3

    Find how many quadrats fit the area

    total area ÷ quadrat area

  4. 4

    Multiply

    mean per quadrat × number of quadrats

Estimating a population with quadrats.
  • Estimated totalmean per quadrat×total areaquadrat area\text{mean per quadrat} \times \dfrac{\text{total area}}{\text{quadrat area}}

Worked example

The mean is 4 per 1 m² quadrat and the field area is 200 m². Estimate the population.

Mean 5 per m² quadrat, field 500 m². What is the estimate?

Try an exam question

Describe how you would estimate the population of daisies in a field using quadrats.

[4 marks]

That's the notes covered.

Carry on to the next subtopic.