Biology IA · A · Unity and diversity · topic 5 of 25

Species–area on a school field · IB Biology IA idea: guide, sample graph and step-by-step plan

a lovely equation to test outdoors

Bigger areas hold more species. You'll find out by how much, and fit a number to it.

Difficulty●●○ Medium
KitBasic: pegs, string, tape measure, plant ID guide, spreadsheet
TimeAbout 10 hours
FitsSL and HL
  • ✓ you like being outdoors
  • ✓ you like a power-law you can fit
  • ✓ you want ecology with a real equation

The big idea

← this is your RQ hook!

Ecologists describe how species number grows with area using S = cAᶻ. Taking logs makes this a straight line with gradient z, so you can fit it and compare your z with published values. You count plant species in nested squares from tiny to large, fix your rules for what counts, and repeat in several places so your answer isn't just one patch. Your IA is a fieldwork design with a clean equation to test, a log-log graph and a real discussion of why z might differ.

Make it yours: pick one twist

Copying a classic won't show your choices. Pick one of these (or invent your own) and your IA becomes yours.

Swipe for more twists →

Compare two habitats

Repeat the nests in two habitats, such as mown lawn and long grass.

→ Compares z and c between habitats. Your analysis now compares two fitted lines.
Compare different area ranges

Fit the first half and the second half of the area range separately.

→ Tests whether one power law holds across all sizes. Your graph may show a bend.
Rare species and z

Compare species counted with species seen once or twice (singletons).

→ Asks how much rare species affect z. Your data table gets a second column.
Scale up the area

Add larger squares of 8 or 16 m² if your site and time allow.

→ Extends the range, so the fit is better but sampling takes longer and plant identification becomes harder.
Add soil or light

Record one environmental measure in each nest.

→ Asks whether the habitat explains the differences you see in z.

Your Biology IA research question: run the check

Write your own question first. Then make sure it ticks every box:

  • Names the independent variable (area sampled, with the range in m²) and the dependent variable (number of plant species).
  • Names the site and the habitat, and says how nests are placed.
  • Says what counts as 'in' a quadrat (a rule for plants on an edge) and how species are identified.
  • Says how many nests you repeat and why.
  • Says which model you will fit (S = cAᶻ) and what you compare z with.

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The set-up

One way to set it up. Draw your own plan, with your real square sizes and positions, for the Method section.

0.01 m²0.04 m²0.25 m²1 m²4 m²second nestadd up the species as thesquare gets biggerfixed corner peg;string marks each squarerepeat in other places:squares in one nest aren't independent

How your Biology IA could be laid out

The usual order for a topic like this, what each part needs to show, and roughly how much of your 3,000 words it takes. The words are all yours.

Research question and context~250
Your question; the site and habitat; what you expect and why.
Background biology~450
The species-area relationship, its equation and what z means; published values for similar habitats (cite); what your log-log graph should look like.
Variablestable
Independent, dependent and controlled variables, each with how you set, measure or hold it, and why it matters.
Method~400
Site and nest positions; how squares are marked; the counting rules (edge rule, identification); number of nests; enough detail to repeat.
Safety, ethics, environment~60
Specific: permission to survey, ticks and uneven ground, not damaging plants or taking protected species.
Raw datatable
Cumulative species count for every square in every nest, with units and notes on uncertain identifications.
Processing~250
Logs of area and species, means across nests and their spread; one worked example per calculation type.
Graphs and analysis~350
Log-log plot with error bars, best-fit line, gradient z with its uncertainty, and the constant c.
Conclusion~450
Answer with z and its uncertainty; compare with published values for a similar habitat; keep your claim within the sizes you sampled.
Evaluation~650, the biggest part!
Weaknesses ranked by effect (nest independence, identification at small sizes, edge rule, one habitat); linked improvements.
Referencesnot counted
Every source for theory, published z values and plant identification.

What your Biology IA graph could look like

gradient = z. give it an uncertainty

Made-up data, to show you the form of a strong graph, not the answer. Yours will look different. example only

-2.0-1.5-1.0-0.50.00.51.00.20.40.60.81.01.21.4log₁₀(area A / m²)log₁₀(number of species S) (error bar: SD of nests)line of best fit: log S = log c + z log Amean of several nests, with SDthe gradient is z:find its uncertaintyILLUSTRATIVE · MADE-UP DATA
  • Both axes are logs, so the line has gradient z and intercept log c.
  • Error bars come from the spread between your repeated nests, not from a single nest.
  • The tiniest squares often have the largest fractional errors, because a few species make a big difference.
  • Say which points you used for the fit, and why. A curve may mean one power law does not cover all sizes.
  • Another graph to consider: plot S against A on normal axes first, to show why logs are used.

Your game plan

decide the edge rule in the trial run

Tick steps off as you go. We'll remember on this device.

0 of 10 done

  1. Read and predictFind the species-area equation, what z means and typical values for your type of habitat. Write down your prediction before you go outside.
  2. Get permission and choose the siteAsk the school or landowner. Choose a habitat that is uniform, and note where the nests will go.
  3. Trial run (1 lesson)Lay out one nest and count each square. How long does it take? Which plants are hard to identify? Is your edge rule clear? Fix the problems now.
  4. Decide the area sizes and number of nestsChoose at least 5 sizes across a wide range and how many nests to repeat. Write down why for each.
  5. Collect the dataCount cumulatively as the square grows. Record species counts straight into the table and photograph unknowns.
  6. Take logs and averageConvert area and species counts to logs, and average across nests. Decide how you handle a count of zero.
  7. Fit and find zFit a straight line to the log-log data and find the gradient with an uncertainty. Check whether a straight line is a good description.
  8. Compare with published valuesFind z values for similar habitats. Is yours within your uncertainty? What explains a difference?
  9. EvaluateWhich weakness in your design mattered most for z? Make every improvement specific.
  10. Check before your teacher's draftRun the checklist below, then get your RQ or draft checked. Your teacher reads one draft only, so make it count.

Biology you'll need

Traps to avoid in this Biology IA

  • One nest only: nested squares are not independent, so the spread tells you nothing.
  • Counting new species per square instead of the cumulative total for that area.
  • No rule for plants on a quadrat's edge.
  • Fitting a line on normal axes and calling z the gradient.
  • Taking the log of a zero count without saying how it was handled.
  • Claiming your z applies to every habitat, or to areas bigger than you sampled.

More Biology IA ideas like this

Make sure it's yours

This guide shows how a topic like this is usually approached. It is not an IA. Your question, choices, data, analysis and words must be your own: change something, tell your teacher where the idea came from, and follow your school's rules on outside help.

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