Biology IA · C · Interaction and interdependence · topic 19 of 25
Duckweed: how fast does a population really grow? · IB Biology IA idea: guide, sample graph and step-by-step plan
cheap, safe, fast. a great first IA :)A few tiny fronds on water turn into a green carpet in two weeks. Then the carpet stops growing. Your job is to find out how hungry it is, and when it runs out of room.
- ✓ you like living things that grow while you watch
- ✓ you can photograph the same tubs every day
- ✓ you want population ecology without catching animals
The big idea
← a RATE, not a final count!Early on, each frond makes new fronds at a steady rate, so numbers climb exponentially; later, crowding and shortage of nutrients slow it. Because the early phase is exponential, the relative growth rate is the sensible thing to measure, not the final count. Change the nutrient strength and you change that rate, probably with diminishing returns at the top. You will predict the shape, count carefully, and decide what the data can and cannot support. Clear variables, safe, cheap and repeatable: a good first IA.
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 →
Photograph from above with a ruler in the frame and measure the green area with free image software.
→ Your dependent variable becomes area growth rate; you must justify the thresholding and explain why area copes better than counting once fronds overlap.Keep the others at full strength and vary only one nutrient, or use a fertiliser at several dilutions.
→ Your independent variable becomes a single nutrient or a named fertiliser; the prediction (a limiting factor) is sharper and the controls need more care.Run one culture for longer and fit a logistic curve to find the carrying capacity.
→ You analyse a whole growth curve, not a single rate; you need a model fitted with an uncertainty, and a sensible link to density-dependent factors.Hold nutrients constant and vary the distance from the lamp or the number of layers of mesh over the tubs.
→ The independent variable becomes light intensity, which you must measure (a phone light-meter app is a start); the evaluation asks how reliable that measurement is.Grow two duckweed species or two strains together and apart, if your school has them.
→ You can discuss competition; the dependent variable becomes growth relative to growing alone, and you need more tubs and more careful counting.Link it to a pond bloom, wastewater clean-up or a livestock-feed story.
→ Gives the introduction a real purpose; the method and the graph stay the same.Your Biology IA research question: run the check
Write your own question first. Then make sure it ticks every box:
- Names the nutrient (or the nutrient solution) and gives its range as percentages of the recommended strength, with how many values.
- Names the dependent variable as a rate (per day) and says exactly how it is calculated from counts or area.
- Says the time period and how often you will record.
- Names the controls: light, temperature, container, volume, starting number and how each is kept constant or checked.
- Is small enough to run with at least three replicate tubs per concentration in about 10 days of growth.
The set-up
One way to arrange it. Draw your own, with your real volumes and distances, for the Method section.
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.
Your question; why duckweed (fast, small, floating) is a good model for population growth; what you predict the nutrient will do and why.
Exponential growth and what limits it; what nutrients do for a plant; how a relative growth rate is defined and why it is better than a final count.
Independent, dependent and controlled variables, with how each is set, measured or monitored.
Source and species of the duckweed, how you made up each concentration, container, volume, light, starting number, how and when you counted or photographed, and the number of replicate tubs.
Specific to duckweed: it can be invasive, so say how you will kill it and bin it, and what you will not pour away. Plus electrical safety near water.
Counts or areas for every tub and every recording day, with units and the counting uncertainty stated.
From counts to relative growth rate, with one worked example; how you handle the uncertainty and the replicate spread.
Growth curves (log scale helps), then rate against nutrient strength with SD error bars; the model you choose for the trend; ANOVA across strengths or Spearman's rank for the trend, with the tub (not the frond) as the replicate.
Answer the question with values and spread; compare with published duckweed growth rates (cite them); say what range your claim covers.
Weaknesses ranked by their effect on your rate; evaporation, counting in crowded tubs, algae and light position are likely candidates, but only if your data show them.
Every source for theory, growth rates and method.
What your Biology IA graph could look like
show the spread of your replicate tubs ✓Made-up data, to show you the form of a strong graph, not the answer. Yours will look different, and may even be flat. example only
- The dependent variable is a rate with its unit (per day), not a final count.
- Error bars are the spread of the replicate tubs, and the caption says n.
- Two candidate models drawn, so the reader can see which one the data support. Judge it, don't just draw it.
- A zero-nutrient point is useful: growth there tells you what the plant brings with it.
- Another graph to consider: ln(frond count) against day for each concentration. In the exponential phase it is close to a straight line, and the gradient is the relative growth rate.
Which uncertainty matters most?
Rank by the effect on your result, the growth rate, not on the raw counts. Numbers here are for a made-up tub that went from 10 to about 67 fronds. Put your own in, and the biggest bar tells you what to fix.
Your game plan
the trial run decides if your counting worksTick steps off as you go. We'll remember on this device.
0 of 10 done
- Read and predictFind out what limits duckweed growth and how a relative growth rate is calculated. Before you start, write down the shape you expect (steady rise? levelling off?) and why.
- Trial run (about 5 days)Grow a few tubs at two strengths. Can you count reliably? When does it get crowded? Do the fronds stick to the walls? How much water goes in a day? Fix the problems now.
- Decide range, replicates and recordingChoose at least five strengths including zero, at least three tubs each, and a recording schedule. Write down why: this becomes your method justification.
- Set up identical tubsSame volume, container, start number, light position and temperature. Mark the water line. Randomise where tubs sit so a bright corner doesn't line up with one concentration.
- Record every dayPhotograph from the same height with a ruler in frame, and record counts or areas straight into your table. Note anything odd: yellowing, algae, a tub that dried out.
- ProcessTurn counts into relative growth rates with an uncertainty on each tub. Decide what you will do about tubs that got crowded or contaminated, and why.
- Graph and fitPlot rate against strength with error bars. Decide what model the points suggest and what a test between strengths needs (conditions, sample size) before you pick one.
- Compare and concludeWhat does your data say about the range you tested? Where does it agree with published growth rates? How big are your differences compared with your spread?
- EvaluateRank your weaknesses by their effect on the rate. Tie each improvement to a specific problem you saw in the data.
- 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
- Biology HL past-paper analysis →
- Biology SL past-paper analysis →
- Uncertainties and error bars (physics examples, same ideas) →
Traps to avoid in this Biology IA
- Counting overlapping fronds in a crowded tub: the count stops meaning what you think. Switch to area, or stop recording before it happens.
- Water lost to evaporation, so the nutrient is more concentrated at the end than at the start.
- Using the final count as the dependent variable when the tubs began with different numbers.
- One tub per concentration: no spread, so no way to judge whether two concentrations really differ.
- Algae or fungus growing in the nutrient: it competes, and it changes the colour you rely on for counting.
- Treating the early exponential phase and the later levelling off as one process.
- Pouring the culture down the sink or into a pond at the end.
More Biology IA ideas like this
- Tea bags: how much soil moisture do decomposers need?●●○
- Does maltose slow amylase down?●●○
- Enzyme beads: does size matter?●●○
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.