IB Biology SLTopic 3 — Adapting to the EnvironmentPaper 1 & 2Practical skill~13 min read
Species Distribution (Skills)
You cannot count every plant on a beach, so you sample. And if you want to link a species to an abiotic factor, you cannot sample at random either — you have to sample along the thing that is changing. That is what a transect is for.
📚 What you need to know
Distribution means where a species is found within an ecosystem.
A limiting factor is any biotic or abiotic factor that restricts the growth of organisms.
Sampling means measuring a small area to represent a much larger one. Samples must be big enough to be representative.
A transect measures how abundance changes along an environmental gradient.
Line transects record what touches the tape; belt transects use quadrats. Each can be continuous or interrupted.
Measure the relevant abiotic factor at every sample point, and present the results as a kite diagram.
Why not just sample at random?
Random sampling is the right tool when you want an unbiased estimate of how much of something is in an area. But that is not the question here. Here you want to know whether a species is more common where a factor is high than where it is low.
Scatter quadrats randomly across a sand dune and you get one average figure for the whole dune. Lay a line from the sea inland and sample along it, and you can see the species change as you go — because the salt, the water and the shelter all change as you go. The gradient is the whole point.
The rule of thumb
Random sampling answers “how much?”
A transect answers “does it change along this gradient?”
Examiners often ask why a transect was used rather than random quadrats. The answer is always the same: the investigation is about an environmental gradient, and a transect deliberately samples across it.
The four types of transect
Type
What you record
Best for
Continuous line transect
Every species touching the tape, along its whole length
Short transects where you want complete detail
Interrupted line transect
Species touching the tape at set intervals, e.g. every metre
Long transects; much quicker than continuous
Continuous belt transect
Quadrats placed end to end along the whole line
Detailed abundance data over a short distance
Interrupted belt transect
Quadrats placed at set intervals along the line
The usual school choice: abundance data over a long gradient
The tape must run along the gradient, not across it. On a shore that means straight up the beach, not parallel to the water.
Measuring the abiotic factor
A transect that only records plants tells you a pattern with no explanation. You have to measure the abiotic factor at each sample point as well, or you cannot claim any link at all.
Only measure factors that are actually relevant. Water turbidity means nothing in a woodland, and there is no credit for recording it.
Abiotic factor
How you measure it
Air temperature
Thermometer or temperature probe
Light intensity
Electronic light meter
Humidity
Hygrometer, which reads the water vapour content of the air
Rainfall
Rain gauge: a funnel collecting water into a measuring cylinder
Soil or water pH
Indicator solution or a pH probe
Soil water content
Weigh a soil sample, dry it, weigh it again; the mass lost was water
Dissolved oxygen
Electronic oxygen sensor
Water turbidity
Turbidity meter, or lower a Secchi disc until it disappears and note the depth
Slope incline
Clinometer, a protractor that lets the angle be worked out
Landscape relief (height of land)
Contour lines on a map, or a GPS reading
Site aspect (which way it faces)
Compass
Use sensors where you can. A data logger removes human judgement from the reading and lets you take many measurements quickly, which makes the data more reliable than eyeballing a scale in the wind.
Kite diagrams
A kite diagram shows distribution and abundance on the same picture. Each species gets its own horizontal strip. The shape sits symmetrically around a central line, spreading out where the species is common and pinching to nothing where it is absent.
🧩 How to read a kite diagram
Each band is one species. Read the labels down the left first.
Left to right is distance along the transect, given on the x axis.
The width of the kite is abundance, usually percentage cover. Wider means more.
It is symmetrical on purpose. The shape extends the same distance above and below the centre line, so half the total width is the actual value.
Extra strips can show abiotic factors, such as elevation or pH, plotted against the same x axis.
Marram peaks on the dune crest where the sand is loosest and driest. Creeping willow appears only in the damp, sheltered hollow behind it.
🧠 The kite trap
The total height of a kite is twice the value. If a kite spans from 15% above to 15% below the line, the abundance is 30%, not 15%. Always check whether the scale is labelled on one side or across the whole width.
Worked examples
WORKED EXAMPLE
A 1 m by 1 m quadrat is divided into 100 equal squares. Marram grass fully covers 34 squares and covers about half of 12 more. Calculate the percentage cover. [2 marks]
Step 1: count the full squares34 squares fully covered = 34Step 2: count the part-covered squares as fractions12 squares at about half each = 12 × 0.5 = 6Step 3: add them, then convert to a percentage34 + 6 = 40 squares out of 100Percentage cover = 40%With 100 squares the count IS the percentage, which is exactly why quadrats are gridded that way.
WORKED EXAMPLE
Using the kite diagram above, state where marram grass is most abundant and suggest a reason for its distribution. [3 marks]
Step 1: read the widest point of the marram kite
The marram kite is widest at about 30 m from the high water line, which matches the highest point of the dune.
Step 2: quote the value
Percentage cover there is about 26%, falling to zero beyond roughly 60 m.
Step 3: give a biological reason
The dune crest has loose, dry, salty, nutrient-poor sand. Marram is adapted to those conditions and faces very little competition there, so it dominates. Further inland the soil is damper and more stable, so other species outcompete it.
Peaks at about 30 m at roughly 26% cover, on the exposed dune crest
WORKED EXAMPLE
Outline how you would investigate the effect of soil water content on the distribution of a plant species. [4 marks]
Step 1: set up along the gradient
Lay a tape measure as a transect running from wet ground to dry ground, so the transect crosses the soil water gradient.
Step 2: sample systematically
Place a quadrat at regular intervals, for example every 2 m, and record the percentage cover of the species in each one.
Step 3: measure the abiotic factor at the same points
At each quadrat take a soil sample, weigh it, dry it in an oven, and weigh it again. The mass lost gives the soil water content.
Step 4: repeat and present
Run several parallel transects and take a mean, then plot the results as a kite diagram with a soil water strip on the same x axis.
Transect along the gradient + regular quadrats + measure the factor + repeatThe repeat step is the one people leave out, and it is nearly always worth a mark.
💡 Exam tip
Say “along an environmental gradient” whenever you justify using a transect. It is the key phrase.
Always state the interval and the quadrat size when describing a method. Vague methods lose marks.
Quote numbers off the graph when interpreting a kite diagram. Description alone rarely gets full marks.
A correlation between a species and a factor does not prove that factor causes the distribution. Say “suggests” or “is consistent with”.
Larger samples and repeats make the data more representative and more reliable. Use the right word for the question.
Name real equipment. “A light meter” beats “something to measure light”.
⚠ Common mix-up
Reading only half a kite. The width of the whole shape is the value, split evenly either side of the line.
Laying the transect across the gradient instead of along it, so no change is detected.
Recording plants but not the abiotic factor. Without it you cannot link the two.
Calling a transect random sampling. It is deliberately systematic, and that is the point.
Confusing distribution with abundance. Distribution is where; abundance is how much. A kite diagram shows both.
Claiming one factor caused the pattern when several factors change along the same transect.
Up next: Abiotic Factors: Marine & Terrestrial — coral reefs, where a very narrow range of tolerance explains exactly why reefs sit where they do on a world map.
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