The Organism in the Environment Notes

Edexcel IGCSE Biology: Revision notes

Key facts

  • A population is one species in a habitat; a community is all the populations; an ecosystem is the community plus the non-living environment.
  • Quadrats are placed randomly to avoid bias when estimating population size.
  • Biodiversity is the variety of species in an area.
  • Abiotic factors are non-living (light, temperature, water, pH, oxygen and carbon dioxide).
  • Biotic factors are living influences: predation, competition, disease and food availability.

Population to ecosystem

These four terms describe organisms at different scales, from one species up to the living and non-living parts together.

A population is all the organisms of one species in a habitat at one time. A community is all the populations of different species living and interacting there. The habitat is the place where an organism lives. An ecosystem is the community together with the non-living (abiotic) parts of the environment.

  1. 1

    Population

    all the organisms of one species in a habitat

  2. 2

    Community

    the populations of all the different species living there

  3. 3

    Ecosystem

    the community plus the non-living (abiotic) environment

Each level includes the one before it

Population

  • One species

Community

  • All species in a habitat

Ecosystem

  • Community + non-living environment

A pond contains frogs, fish, algae, water and sediment. Which term describes all of this together?

Sampling with quadrats

Count organisms in randomly placed quadrats, find the mean, then scale up to the whole habitat.

A quadrat is a square frame of known area used to sample a habitat. Place quadrats randomly, for example with random coordinates, to avoid bias. More quadrats give a more reliable estimate.

  1. 1

    Place quadrats randomly

    e.g. using random coordinates, to avoid bias

  2. 2

    Count individuals

    of the chosen species in each quadrat

  3. 3

    Calculate the mean

    number of individuals per quadrat

  4. 4

    Scale up

    mean x (total area of habitat / area of one quadrat)

Estimating population size

Worked example

A field has an area of 400 m2^2. Ten 1 m2^2 quadrats contain a mean of 3 daisies each. Estimate the population.

Why are quadrats placed randomly?

Biodiversity

Biodiversity is the variety of species in an area; measure it by recording every species in random quadrats.

Biodiversity is the variety of different species in an area. Many species in reasonably even numbers means high biodiversity; one dominant species means low.

To measure it, record every species present (not just one) across many random quadrats. A transect places quadrats along a line to show how distribution changes, for example away from a hedge or up a shoreline.

010203040Species 1Species 2Species 3Species 4Species 5SpeciesIndividuals counted
  • High biodiversity site
  • Low biodiversity site
Illustrative values: 50 individuals counted in each site. Even numbers across five species mean high biodiversity; one dominant species means low

High biodiversity

  • Many species
  • Fairly even numbers

Low biodiversity

  • Few species
  • One species dominates

How does a transect differ from random quadrat sampling?

Abiotic factors

Abiotic factors are non-living physical and chemical conditions that limit where organisms can live and how many there are.

Abiotic factors are the non-living conditions in an environment. Each affects populations differently, as shown below.

510152025303540246810Temperature / °CNumber of organismsoptimumPopulation size
Illustrative values: a population is largest near the optimum value of an abiotic factor (here temperature) and falls away towards the extremes

Factor

  • Light intensity
  • Temperature
  • Water availability
  • pH of soil or water
  • Oxygen and carbon dioxide

Effect

  • Rate of photosynthesis; limits plant growth
  • Rates of enzyme reactions such as photosynthesis and respiration
  • Limits where organisms needing water can live
  • Some species tolerate only a narrow range
  • Affects respiration and photosynthesis, especially in water

Name an abiotic factor that limits plant growth.

Biotic factors

Biotic factors are living influences, such as predators, competitors, disease and food supply.

Biotic factors are the living influences on a population.

  • Predation: more predators reduces prey numbers.
  • Competition for food, light, space or mates, within a species or between species, limits growth.
  • Disease: pathogens reduce numbers, especially at high density.
  • Food availability sets how many consumers a habitat supports.
  1. 1

    Drought

    an abiotic factor

  2. 2

    Less plant growth

    less food for herbivores

  3. 3

    Herbivore numbers fall

    a biotic knock-on effect

Abiotic and biotic factors interact

Which is a biotic factor?

Try an exam question

A student estimates the number of buttercups in a field using quadrats. Describe how the student should do this.

[4 marks]

That's the notes covered.

Carry on to the next subtopic.