Biodiversity, Distribution and Limiting Factors

What this lesson covers in the specification

Incandio is aligned to this specification. It is not published by, endorsed by or affiliated with Pearson, and it reproduces none of Pearson's wording — the statement numbers are given so you can check every lesson against your own copy.

• 4.3 — The term biodiversity (bold B statement — Paper 2 only)
• 4.4 — Practical: investigate distribution and measure biodiversity using quadrats (bold B statement — Paper 2 only) (required practical)
• 4.5 — How abiotic and biotic factors affect population size and distribution

Specification section 4(a) The organism in the environment — also taught in Populations, Ecosystems and Sampling with Quadrats.

1 · Understand it

No exam language yet. The only question this section answers is: do I actually understand what is happening?

Walk from an open field into the shade under a hedge and the plants change completely within a few metres. Nothing has moved them. Each species is where the conditions suit it and absent where they do not, and the boundary between the two is often startlingly sharp. This page is about what draws those boundaries.

The two kinds of factor that decide where an organism can live

1. ABIOTIC factors are the NON-LIVING conditions: light intensity, temperature, water availability, oxygen and carbon dioxide concentration, soil pH, and the mineral ions available.
2. BIOTIC factors are the effects of OTHER LIVING ORGANISMS: competition for resources, predation, disease, and the availability of food.
3. Either kind can limit a population — that is, prevent it from growing further or from occupying an area at all.
4. A useful test for which you are looking at: could it happen on a lifeless planet? Light, temperature and pH could. Competition, predation and disease could not.

The important idea is that a population is usually limited by whichever factor is in shortest supply, not by all of them at once. Add fertiliser to a field of crops growing in poor soil and the yield rises — but keep adding it and eventually the yield stops responding, because something else has become the limiting factor: light, or water, or space. That is exactly the shape of the limiting-factor graphs from photosynthesis, and it is the same idea one level up.

Think of it like a queue with several counters

A queue moves at the speed of its slowest counter. Speed up any of the others and nothing changes — the line still crawls, because the bottleneck is somewhere else. Fix the slow counter and the queue speeds up until a different counter becomes the slowest one, and that one now controls everything. A population works the same way: it is held back by one thing at a time, and relieving that constraint simply hands control to the next one. It is why 'this species needs light' is never a complete answer to why it is absent.

Now the other question this topic asks, which is not about quantity at all. BIODIVERSITY is the variety of different species present in an area. Two fields can hold identical numbers of plants while one contains twenty species and the other only one, and the difference matters enormously — a community with high biodiversity is more likely to withstand a disease, a drought or a new predator, because it is unlikely that every species is affected in the same way. Low biodiversity is the ecological version of the identical-banana problem from Topic 3.

Measuring distribution rather than abundance — what changes about the method

1. When the question is HOW MANY, quadrats are placed randomly, because a representative sample is what is needed.
2. When the question is HOW DOES IT CHANGE across a gradient — from open ground into shade, or up a beach from the water — random placement scatters the data across the gradient and destroys the pattern.
3. So a TRANSECT is used instead: a line laid across the gradient, with quadrats placed at regular intervals along it.
4. At each quadrat, record which species are present and how much of each — and record the ABIOTIC factor as well, with a light meter, a thermometer or a pH probe.
5. Plotting abundance against distance along the transect, alongside the abiotic measurement, is what allows a conclusion about cause rather than just a description of the pattern.

For biodiversity itself, the count is of SPECIES rather than individuals: place quadrats randomly, list every different species found in each, and the total number of species across all quadrats is the measure. It is worth being honest that this is a crude measure — it treats a species represented by one straggling plant as equal to one covering half the field — but it is the measure the specification asks for, and it makes the comparison between two areas straightforward.

One caution about what a transect can and cannot show. If bluebells become scarcer as you move out of woodland into open field, and the light intensity rises along the same line, you have found a CORRELATION. That is genuine evidence and worth stating — but temperature, soil moisture and competition from grasses all change along that same line too, and any of them could be the actual cause. Establishing which requires an experiment in which one factor is changed and the others held constant, which is not what a transect does.

2 · Grade 9 Notes

A different job from the section above. You have already understood it; this is the precise set of things to LEARN — definitions to reproduce word for word, processes in order, equations with units, and the answers that score full marks.

Learn this definition · Biodiversity

The variety of different species present in an area.

Statement 4.5 — abiotic against biotic factors

Required practical 4.4 — investigating distribution and biodiversity

1. FOR BIODIVERSITY: place at least ten quadrats at random, using coordinates from random numbers, in each area to be compared.
2. Record every different species present in each quadrat, and the abundance of each as a count or as percentage cover.
3. The total number of different species recorded across all the quadrats is the measure of biodiversity for that area. Repeat in the second area and compare.
4. FOR DISTRIBUTION: lay a tape measure as a transect along the gradient being investigated — for example from open ground into the shade of a hedge.
5. Place a quadrat at regular intervals along the line, such as every two metres.
6. At each position record the abundance of the chosen species AND the abiotic factor, using a light meter, thermometer or pH probe.
7. Plot abundance against distance along the transect, with the abiotic measurement on the same axis, and describe how they change together.

Variables

Independent (changed) — distance along the transect — or, for biodiversity, the area being sampled
Dependent (measured) — abundance of the species, or the number of different species present

Sources of error

Which sampling method to use — the choice is decided by the question

A. RANDOM quadrats: use when estimating abundance or comparing biodiversity between areas, because an unbiased representative sample is what is needed.

B. A TRANSECT with quadrats at regular intervals: use when investigating how a species changes ACROSS A GRADIENT, because random placement would scatter the data and hide the pattern.

Interpreting a transect — what a good conclusion says and what it avoids

• DESCRIBE the pattern first: as distance from the hedge increases, the abundance of the species falls and light intensity rises
• QUOTE figures from the data to support it — a described trend with numbers scores above one without
• SUGGEST the abiotic factor responsible, and say HOW it affects the organism
• STATE that this is a correlation: other factors change along the same line and any of them could be the cause
• SAY what would be needed to establish cause — an experiment varying one factor with the others controlled

Not this: A population stops growing because it has run out of everything it needs.

This: It is limited by whichever single factor is in shortest supply. Relieving that one allows growth until a different factor becomes limiting, exactly as with photosynthesis.

Model answer [4 marks]

Explain the difference between an abiotic and a biotic factor, giving two examples of each. [4]

An abiotic factor is a non-living physical or chemical condition of the environment, such as light intensity or soil pH. A biotic factor is an effect caused by other living organisms, such as competition for resources or predation. Both can limit the size of a population or prevent a species from occupying an area, but abiotic factors would exist even if no other organisms were present.

Model answer [4 marks]

A student finds that a species of moss is abundant close to a wall and scarce further away, and that light intensity increases with distance from the wall. Evaluate the conclusion that light intensity limits the distribution of the moss. [4]

The data show a correlation between the abundance of the moss and light intensity, which is genuine evidence and consistent with the conclusion. However, other factors also change with distance from the wall — the soil is likely to be drier and warmer further away, and there may be more competition from other plants. Any of these could be the actual cause of the pattern. To establish that light is responsible, an experiment would be needed in which light intensity is varied while temperature, water availability and competition are kept constant.

Model answer [3 marks]

Explain why a community with high biodiversity is more likely to survive a new disease. [3]

High biodiversity means many different species are present. A disease usually affects only one species or a few closely related ones, so most of the species present are unlikely to be susceptible to it. The community therefore continues to function even if one species is destroyed, whereas in a community with very few species the loss of one may cause the whole community to collapse.

Measuring biodiversity — what to record and what it does NOT tell you

What is counted

The number of different SPECIES found across all the quadrats — not the number of individuals.

How the quadrats are placed

At random, using coordinates from random numbers, and the same number and size in each area compared.

Why high biodiversity matters

A disease, drought or new predator is unlikely to affect every species alike, so the community is more likely to survive a change.

The limitation of the measure

It treats a species represented by one straggling plant as equal to one covering half the area.

Mark-losing trap. Abiotic = non-living conditions. Biotic = effects of other organisms. Predators and disease are BIOTIC.

Mark-losing trap. Biodiversity is the variety of SPECIES, not the number of individuals. A field of one million grass plants has very low biodiversity.

Mark-losing trap. A transect goes ACROSS the gradient, not along it. Along it, every quadrat records the same conditions.

Mark-losing trap. A transect gives a CORRELATION. Say so, and name what else changes along the same line.

Mark-losing trap. Name the ONE limiting factor. A population is held back by whichever is in shortest supply, not by all of them together.

3 · Prove it — the five questions

The five questions climb Grade 6 → Grade 7 → Grade 8 → Grade 9 → Grade 9 challenge, and are marked inside Incandio on your own device, by rule, with an authored diagnosis of the mistake you actually made. The mark schemes stay in the app so that the practice is worth doing; the questions themselves are here.

1. Grade 6 · Identify [1 mark] — Which of these is a biotic factor affecting the size of a population?
2. Grade 7 · Explain [2 marks] — A student wants to find out how the abundance of a moss changes as you move away from the shade of a wall. Explain why they should use a transect rather than placing quadrats at random.
3. Grade 8 · Explain [5 marks] — A pond is stocked with fish, and the population grows rapidly at first and then levels off, even though food is still being added daily. Explain why the population stops growing. Select every statement that belongs in a full-mark explanation.
4. Grade 9 · Calculate [3 marks] — In an unmown meadow, ten quadrats contain a total of 34 different plant species between them. In a mown lawn of the same area, ten quadrats of the same size contain 8 different species. Calculate the percentage decrease in the number of species recorded in the mown area compared with the meadow.
5. 9+ · Analyse [5 marks] — A transect is run from the base of a north-facing wall out into open ground. Moss cover falls steadily with distance from the wall, while light intensity, temperature and soil dryness all increase. A student concludes that high light intensity prevents the moss from growing. Select every statement that belongs in a full-mark evaluation.

The people behind this science

Two ways into the same idea — the one who showed what removing one species does, and the one who explained why competition decides who lives where. Inside Incandio each of them answers knowing exactly which lesson you have just finished.

Rachel Carson — the one who showed what removing one species does

This page claims that biodiversity protects a community and that biotic factors bind species together. Carson's Silent Spring is the demonstration. She traced how pesticides sprayed to kill insects accumulated through food chains and destroyed bird populations that had never been the target — showing that the species in a community are connected so tightly that you cannot remove one without consequences you did not intend. She was attacked ferociously by the chemical industry and testified to the Senate while dying of cancer.

• “How can killing insects end up killing birds?”
• “Why can you not remove one species from a community without affecting the others?”
• “What made you certain the decline was caused by pesticides and not something else?”
• “How did you answer the people who said you were exaggerating?”
• “What should we be measuring before we change an environment?”

Charles Darwin — the one who explained why competition decides who lives where

Statement 4.5's biotic factors are Darwin's territory. His argument required that more offspring are produced than can survive, and that competition therefore decides who does — which is why a species may be missing from an area whose abiotic conditions suit it perfectly, simply because a better competitor is already there. He tested this directly, clearing small plots of turf and counting how many seedlings emerged and how many were destroyed by competition and grazing before they could establish.

• “Why might a plant be absent from ground that would suit it perfectly?”
• “What did you find when you cleared a plot of turf and watched what grew?”
• “How much of what happens in a field is competition rather than climate?”
• “Is being outcompeted different from being unable to survive the conditions?”

Carry on through the course

• Previous lesson: Populations, Ecosystems and Sampling with Quadrats
• Next lesson: Food Chains, Webs and Pyramids
• Populations, Ecosystems and Sampling with Quadrats — Four nested words for where organisms live — and how to count a population you could never count in full.
• Food Chains, Webs and Pyramids — Who eats whom, drawn three different ways — and why only one of the three can come out upside down.
• Energy Transfer Along a Food Chain — About nine tenths of the energy disappears at every step. Here is exactly where it goes — and what that means for how the world is fed.
• The Carbon and Nitrogen Cycles — The same atoms, used again and again — and the four bacteria that do the work nothing else can.
• Air Pollution and the Greenhouse Gases — Two entirely different problems that both begin with burning — one that poisons what it touches, and one that changes the energy balance of the planet.
• All of Biology · Biology key terms · required practicals · Incandio Science · The 4BI1 specification