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2.8.3Additional lessons

Species Distribution Skills — IB Biology SL Revision Notes & Topical Questions

5 min read 6 parts Lesson 43 of 142

📚 IB Biology SL Hub / Topical Revision
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Curriculum Equation Key قوانين الرقم الهيدروجيني والاتزان الأيوني - pH & Acid-Base Equilibrium FormulasFast Exam Solution
pH = -log[H⁺] | pOH = -log[OH⁻] | pH + pOH = 14 | [H⁺][OH⁻] = 10⁻¹⁴
Word Equation & Conditions: الرقم الهيدروجيني = -لوغاريتم تركيز أيون الهيدروجين | pH + pOH = 14
Location & Specifications: الاتزان الأيوني، كيمياء 3 ثانوي، التحصيلي، و AP Chemistry.
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Part 1 of 6Sampling Methods

Random quadrats
  • Random quadrats Placing quadrats at random coordinates avoids bias and gives an estimate of abundance over an area.
Line transect
  • Line transect Recording species touching a tape laid along a line shows how organisms change across a habitat.
Belt transect
  • Belt transect Placing quadrats along the line at set intervals gives more detailed data about abundance along a gradient.
Sample size
  • Sample size Using more quadrats gives a more reliable mean and reduces the effect of chance.
Mobile animals
  • Mobile animals Animals can be sampled by capture-mark-release-recapture, since they cannot be counted in a fixed quadrat.

Part 2 of 6Measures of Abundance

Measures of Abundance
Density
  • Density The number of individuals per unit area, such as per square metre.
Percentage cover
  • Percentage cover The proportion of a quadrat covered by a species, useful for plants that are hard to count individually.
Frequency
  • Frequency The percentage of quadrats in which a species is found.
Abiotic readings
  • Abiotic readings Light, pH or soil moisture are measured at each quadrat position to compare with the species data.

Part 3 of 6Method for an Investigation

1
Decide the sampling design: random sampling for a whole area or a transect to follow a gradient.
2
Place the quadrats and count or estimate the abundance of the chosen species in each.
3
Measure an abiotic factor at the same positions with a suitable meter.
4
Calculate means and plot the data, for example abundance against distance along the transect.
5
Look for a pattern and consider whether a statistical test supports a relationship.

Part 4 of 6Evaluating the Data

Evaluating the Data
Reliability
  • Reliability Repeating the sampling and using a larger number of quadrats improves the reliability of the results.
Bias
  • Bias Choosing sampling positions with random numbers avoids selecting areas that look more interesting.
Conclusions
  • Conclusions State clearly what the data show and whether other factors might explain the distribution.

Part 5 of 6Worked Example

Question

  • A quadrat of area 0.25 m² is used 12 times.
  • A total of 36 plants are counted.
  • The whole field is 200 m².
  • Estimate the density and the population.

  1. Total area sampled = 12 × 0.25 = 3 m².
  2. Density = 36 ÷ 3 = 12 plants per m².
  3. Population estimate = 12 × 200 = 2400 plants.

Answer

  • Density 12 plants per m²;
    • about 2400 plants in the field.

Part 6 of 6Exam Checklist & Tips

2.8.3 – Key Points for the Exam

  • Quadrat A square frame of known area used to sample organisms.
  • Transect A line along which samples are taken to show change across a gradient.
  • Density The number of individuals per unit area.
  • Percentage cover The share of the sampled area occupied by a species.

Common Mistakes

  • Placing quadrats where plants look dense, which introduces bias.
  • Taking too few samples and treating the result as typical.
  • Forgetting that percentage cover can total more than 100 % when plants overlap.
  • Concluding that a gradient causes a pattern without controlling or testing other factors.

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