IB Biology HLB4.1 — Adaptation to EnvironmentPaper 1 & 2Practical skill~11 min read
Species Distribution
Saying “this plant likes damp soil” is a hunch. Turning it into evidence means going out with a tape measure, sampling in a sensible pattern, measuring the damp bit properly, and drawing the result so the pattern shows. That is the whole of this page, and the IB expects you to be able to design the study yourself.
📘 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 large one, because counting everything is impossible.
A transect measures how abundance changes along an environmental gradient.
Four types: continuous and interrupted, line and belt.
You must measure the relevant abiotic factor at each sample point, using the right equipment.
Results are drawn as a kite diagram, which shows distribution and abundance at once.
Distribution and limiting factors
Two definitions worth memorisingDistribution = where a species is found within an ecosystem Limiting factor = any biotic or abiotic factor that restricts the growth of organisms
The two link straight together. A limiting factor sets the range of tolerance, and the range of tolerance sets the distribution. If soil water becomes severely limiting, a plant falls outside its range of tolerance and simply cannot survive in that area any more.
Why we sample
You cannot count every organism in a habitat. There are far too many, and you would still be counting next year. So you measure a small area and use it to represent the whole.
Sampling saves time, which is the practical reason.
The sample must be representative of the whole area, which is the scientific reason.
That means the sample size must be large enough. A handful of readings tells you very little.
Transects
A transect is a line laid across a site. You lay a tape measure along the gradient you are interested in and take samples along it.
Transects are the right tool whenever abundance might change gradually across a habitat — at increasing distance from the water’s edge on a rocky shore, or from the beach inland across a dune system.
Type of transect
What you record
Continuous line
Every species that touches the tape measure, all the way along.
Interrupted line
Species touching the line at regular intervals only, e.g. every metre.
Continuous belt
Quadrats placed end-to-end along the whole line.
Interrupted belt
Quadrats placed at regular intervals, e.g. every metre, along the line.
Interrupted versions are used far more often in the field, simply because sampling every centimetre of a long transect takes too long.
Choose the transect direction on purpose. If you want to see the effect of soil water, lay the tape along a high-to-low water gradient, not across it. A transect laid the wrong way shows nothing.
Measuring the abiotic factor
A transect on its own only tells you where the species is. To link it to a cause you must also measure the abiotic factor at each sample point.
Only record the factors that are relevant. A study may only be interested in one, and some factors make no sense in a given habitat — water turbidity means nothing in a woodland.
Abiotic factor
How to measure it
Air temperature
Thermometer or temperature probe
Rainfall
Rain gauge — a funnel collecting water into a measuring cylinder
Humidity
Hygrometer — an electronic sensor for water vapour in the air
Dissolved oxygen
Electronic oxygen sensor
Water turbidity
Turbidity meter, or lower a Secchi disc until it disappears and record the depth
Light intensity
Electronic light meter
Landscape relief (height of land)
Contour lines on a map, or a GPS
Site aspect (direction the site faces)
Compass
Slope incline
Clinometer — a protractor that gives the angle by trigonometry
Soil or water pH
Indicator solution or a pH probe
Soil water content
Mass of a soil sample before drying, minus its mass after drying
Kite diagrams
A kite diagram is the standard way to show transect results, because it puts distribution and abundance on the same picture.
Each section of the diagram is one species, with its own central horizontal line.
Distance along the transect runs along the x-axis, parallel to that line.
Abundance is shown by the width of the kite around the central line.
It is called a kite because the shape spreads an equal distance on each side of the line.
Extra sections can be added on top to show how an abiotic factor changes along the same transect, such as height above sea level or soil pH.
Read a kite diagram by looking at where each shape is widest. Sea rocket is a pioneer of the bare sand nearest the sea; heather only appears once the dunes are older and more stable.
Designing your own investigation
🧩 A method that would score well
Pick one abiotic factor and one species, and state what you expect to find.
Lay the tape along the gradient of that factor, from high to low.
Sample at regular intervals, e.g. a quadrat every 5 m, so the whole gradient is covered.
Record abundance of the species in each quadrat, e.g. as percentage cover.
Measure the abiotic factor at every single sample point, using a sensor where you can.
Repeat along several parallel transects and take means, so one odd patch does not skew the result.
Plot a kite diagram and look for whether abundance tracks the abiotic factor.
Where you can do it. A natural habitat, or a semi-natural habitat — an area shaped by human activity that still contains wild species. A school field counts.
Worked examples
WORKED EXAMPLE
A soil sample has a mass of 42.0 g. After drying it has a mass of 31.5 g. Calculate the soil water content as a percentage. [2]
Step 1: find the mass of water lost42.0 g − 31.5 g = 10.5 gStep 2: express it as a percentage of the original mass(10.5 ÷ 42.0) × 100 = 25Soil water content = 25%divide by the mass before drying, not the dry mass.
WORKED EXAMPLE
Outline how you would investigate the effect of light intensity on the distribution of a plant species in a woodland. [5]
Step 1: set up along the gradient
Lay a tape from the open woodland edge into deep shade, so light intensity changes along it.
Step 2: sample systematicallyPlace a quadrat every 5 m and record the percentage cover of the species.Step 3: measure the abiotic factorUse a light meter at each quadrat, held at the same height each time.Step 4: make it reliableRepeat along three parallel transects and calculate a mean for each distance.Step 5: plot abundance against light intensity and describe the pattern“same height each time” is the kind of control detail that earns the extra mark.
WORKED EXAMPLE
Using the kite diagram above, state where marram grass is most abundant and suggest one reason for this. [2]
Read off the widest point of the marram kiteMarram grass is widest at about 20 m from the shore.Now suggest a cause, linked to an abiotic factorMarram tolerates dry, salty, low-nutrient sand better than the species around itread values off the axis; do not guess “in the middle”.
💡 Exam tip
You may be asked to design and carry out a study on one abiotic factor. Practise writing the method in steps.
Say which transect type you would use and why. Belt transects give abundance; line transects only give presence.
Always measure the abiotic factor at every sample point, not just at the two ends.
Use sensors where you can — the syllabus specifically mentions them.
When reading a kite diagram, look for where each kite is widest and read the distance off the x-axis.
Mention repeats and means in any method question. It is nearly always a mark.
⚠ Common mix-up
Hygrometer, not hydrometer. A hygrometer measures humidity; a hydrometer measures the density of a liquid. Textbooks get this wrong surprisingly often.
Using a transect where random quadrats are needed. Transects are for gradients; random sampling is for estimating abundance across a uniform area.
Confusing distribution with abundance. Distribution is where; abundance is how much. A kite diagram shows both.
Thinking a wider kite means a wider area. Width means more of that species, not more ground covered.
Recording every abiotic factor going. Only the relevant ones. Turbidity in a wood scores nothing.
Forgetting that limiting factors can be biotic too. The definition says biotic or abiotic.
Claiming a transect proves cause. It shows a correlation with the gradient; say “suggests” rather than “proves”.
Up next: Abiotic Factors: Marine & Terrestrial — the same ideas at global scale, starting with why coral reefs sit only in a narrow band of the world’s oceans.
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