Practical: Investigating Population Size (Edexcel IGCSE Biology)

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Practical: Investigating Population Size

  • Ecology is the branch of biology that studies species distribution, abundance, interactions between species, and their interactions with the abiotic environment

  • Ecologists investigate ecosystems, often using tools like quadrats to study population size

Quadrats

  • Quadrats are square frames made of wood or wire

  • They can be a variety of sizes, e.g. 0.25 m2 or 1 m2

  • They are placed on the ground and the organisms within them are recorded

  • Plants species are commonly studied using quadrats to estimate their abundance

Quadrat in use, downloadable IGCSE & GCSE Biology revision notes

Quadrats are used to investigate population size or species distribution

  • Quadrats can be used to measure abundance by recording:

    • the number of individuals of a single species, e.g. buttercups

    • species richness: the number of different species present

    • percentage cover: the approximate percentage of the quadrat area in which an individual species is found

      • this method is used when it is difficult to count individuals of the plant species being recorded, e.g. grass or moss

Estimating percentage cover of one or more species, downloadable IGCSE & GCSE Biology revision notes

Percentage cover can be assessed using a quadrat

Investigating population size in two different areas using quadrats

Apparatus

  • 2 tape measures

  • Quadrat

  • Random number generator

  • Species key

Method

RP Ecosystems_ Estimating Population Size Method (1)_1, downloadable IGCSE & GCSE Biology revision notes
Comparing population sizes across 2 different study areas 2, downloadable IGCSE & GCSE Biology revision notes

The population size of a plant species in a survey area can be estimated using quadrats

Results

  • Once the results have been collected and the averages calculated, we can compare the abundance of the species in each survey area

  • Species abundance is likely to be influenced by biotic factors such as:

    • competition

    • predator-prey relationships

    • interactions with other organisms within the food chain or food web

  • The abundance will also be influenced by abiotic factors such as:

    • light intensity

    • mineral availability

    • water availability

    • pH

    • temperature

    • salinity

Limitations

  • It can be easy to miss individual organisms when counting in a quadrat, especially if they are covered by a different species

    • Solution: Use a pencil or stick to carefully move leaves out of the way to check if there is anything else underneath

  • Identifying species may be tricky

    • Solution: Use a species key to identify the species

Applying CORMS to practical work

  • When working with practical investigations, remember to consider your CORMS evaluation

CORMS evaluation, downloadable AS & A Level Biology revision notes

CORMS Evaluation

  • In this investigation, your evaluation should look something like this:

    • C - We are changing the study area where we are collecting the data

    • O - We will count the same species of organism in each quadrat

    • R - We will repeat the investigation several times to ensure reliability

    • M1 - We will count the number of the designated study species found across all quadrats

    • M2 -  this isn't really relevant in this scenario

    • S - We will control the size of the quadrat, the random way that quadrats are placed on the ground, the day that the results were collected

Examiner Tips and Tricks

Take care with your spelling of the word 'quadrat' it is commonly written as 'quadrant' by students in examinations.

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Lára Marie McIvor

Author: Lára Marie McIvor

Expertise: Biology Lead

Lára graduated from Oxford University in Biological Sciences and has now been a science tutor working in the UK for several years. Lára has a particular interest in the area of infectious disease and epidemiology, and enjoys creating original educational materials that develop confidence and facilitate learning.