What you'll learn
- How organisms are organised into populations, communities and ecosystems.
- How abiotic and biotic factors affect where organisms live.
- Why species in a community depend on each other, including parasitism and mutualism.
- How to use quadrats and belt transects to collect and process fieldwork data.
The basic ecological levels
Ecology is the study of how living organisms interact with each other and with their surroundings. Before you can explain fieldwork, you need the language for “how big” the biological system is.
Organism, population, community and ecosystem
An individual organism is one living thing. A population is all the organisms of one species in a habitat. A community is all the populations of different species living together. An ecosystem is the community plus the non-living surroundings it interacts with.
A habitat is the place where an organism lives, such as a pond, woodland, rocky shore or school field.
Increasing scale
The order is: individual organism → population → community → ecosystem. Each level includes the one before it.
Abiotic factors: non-living conditions
An abiotic factor is a non-living part of the environment that can affect organisms.
Important examples include:
- Temperature — affects enzyme-controlled reactions, growth and activity.
- Light intensity — affects the rate of photosynthesis in plants and algae.
- Water availability — affects survival, growth and distribution.
- Pollutants — harmful substances such as sewage, fertilisers, pesticides or toxic chemicals.
Abiotic factors help explain why organisms are not spread evenly everywhere. For example, mosses are often common in damp, shaded areas, while many flowering plants grow better where there is more light.
Explaining a plant distribution
A student finds many daisies in an open part of a field but very few under trees.
- Compare the abiotic conditions: the open field has higher light intensity, while the area under trees is more shaded.
- Link the condition to the organism: daisies are plants, so they need light for photosynthesis.
- Make the biological explanation: fewer daisies grow under the trees because lower light intensity reduces photosynthesis and growth.
Biotic factors: living interactions
A biotic factor is a living part of the environment that affects organisms.
The two key examples for this topic are competition and predation.
Competition and predation
Competition happens when organisms need the same limited resource, such as food, light, water, space or mates. Predation is when one organism, the predator, kills and eats another organism, the prey.
Plants may compete for light, water, mineral ions and space. Animals may compete for food, territory, nesting sites and mates.
Predation can change population sizes. If predator numbers increase, prey numbers may decrease. If prey numbers decrease too much, predator numbers may later fall because there is less food available.
Predicting effects of competition
Two plant species grow in the same soil. Species A has deeper roots than species B during a dry summer.
- Identify the limiting resource: in a dry summer, water is likely to be in short supply.
- Compare the adaptations: deeper roots allow species A to reach water lower in the soil.
- Predict the effect: species A may survive and grow better, while species B may decrease in number due to stronger competition for water.
Mixing up abiotic and biotic
Abiotic factors are non-living conditions such as light, temperature and water. Biotic factors involve living organisms, such as predators, prey, competitors, parasites and mutualistic partners.
Interdependence in a community
Interdependence means organisms in a community depend on each other for survival. This can include food, shelter, pollination, seed dispersal, cleaning, protection or nutrients.
For example:
- Bees depend on flowering plants for nectar.
- Flowering plants may depend on bees for pollination.
- Birds may depend on trees for nesting sites.
- Predators depend on prey for food.
- Decomposers depend on dead organisms and waste for energy.
Communities are connected
Changing one species can affect many others because food supply, competition, shelter and reproduction may all be linked.
A community is more stable when populations stay fairly balanced over time. A sudden change, such as pollution killing insects in a stream, can reduce food for fish and birds.
Parasitism and mutualism
Some relationships are especially close because the survival of one organism depends on another species.
Parasitism
Parasitism is a relationship where a parasite benefits and the host is harmed. The parasite usually lives on or inside the host.
Examples include fleas on mammals, tapeworms in the gut, and ticks feeding on blood. The parasite gains food or shelter, while the host loses resources and may be damaged or weakened.
Mutualism
Mutualism is a relationship where both species benefit.
Examples include bees and flowering plants: bees get nectar as food, while flowers are pollinated. Another example is bacteria in the human digestive system: bacteria gain a warm habitat and food, while humans gain help with digestion and vitamin production.
Classifying a species relationship
A fungus grows on a plant root. The fungus receives sugars from the plant. The plant absorbs more mineral ions because the fungus increases the root surface area.
- Identify what the fungus gains: it receives sugars, so it benefits.
- Identify what the plant gains: it absorbs more mineral ions, so it also benefits.
- Classify the relationship: because both species benefit, this is mutualism.
Why biologists sample habitats
It is usually impossible to count every organism in a habitat, especially if the area is large or the organisms are numerous. Instead, biologists take a sample: a smaller part of the habitat used to estimate what is happening in the whole area.
A good sample should be:
- Representative — it should reflect the habitat fairly.
- Large enough — more repeats usually make results more reliable.
- Unbiased — the scientist should not choose “interesting-looking” places.
Abundance and distribution
Abundance means how many organisms there are. Distribution means where organisms are found in a habitat.
Using quadrats
A quadrat is a square frame used to sample organisms in a known area. It is especially useful for plants or slow-moving animals.
For random quadrat sampling:
- Mark out the study area using two tape measures at right angles to make x and y axes.
- Generate random coordinates using a random number table, calculator or app.
- Place the quadrat at each coordinate.
- Count the organisms inside the quadrat, or estimate percentage cover if individuals are hard to count.
- Repeat many times and calculate a mean.
Count or percentage cover?
Count individuals when organisms are separate and easy to see, such as daisies. Use percentage cover for organisms that overlap or are difficult to separate, such as grass, moss or algae.
Random is not the same as careless
Random sampling means using random coordinates. Throwing a quadrat “wherever” is not properly random and can introduce bias.
Estimating the number of organisms in an area
Once you have quadrat data, you can estimate the total population in the whole habitat.
One useful method is:
estimated population=density×total area\text{estimated population} = \text{density} \times \text{total area}estimated population=density×total areawhere:
density=number counted in samplearea sampled\text{density} = \frac{\text{number counted in sample}}{\text{area sampled}}density=area samplednumber counted in sampleEstimating a population from quadrats
A student samples a field using eight quadrats. Each quadrat has an area of 0.25 square metres. They count 48 daisies in total. The whole field has an area of 40 square metres.
-
Calculate the total area sampled:
8×0.25=2.0 m28 \times 0.25 = 2.0\,\text{m}^28×0.25=2.0m2 -
Calculate the density of daisies:
482.0=24 daisies per m2\frac{48}{2.0} = 24\,\text{daisies per m}^22.048=24daisies per m2 -
Estimate the total number in the field:
24×40=960 daisies24 \times 40 = 960\,\text{daisies}24×40=960daisies
The answer is an estimate, not an exact count. It becomes more reliable if the quadrats are placed randomly and repeated enough times.
Using a belt transect
A transect is a line across a habitat used to study how organisms change with distance. A belt transect uses quadrats placed along the line, often at regular intervals.
Use a belt transect when there is an environmental gradient: a gradual change in an abiotic factor. Examples include:
- from bright open grassland into shaded woodland
- from dry ground to wet ground
- from the top of a rocky shore down towards the sea
- moving away from a polluted area
For a belt transect:
- Lay a tape measure across the gradient.
- Place a quadrat at regular distances along the tape, such as every 1 metre.
- Record abundance or percentage cover in each quadrat.
- Measure the abiotic factor if needed, such as light intensity or soil moisture.
- Look for a relationship between the abiotic factor and the organisms found.
Interpreting belt transect data
Along a transect from open grassland into woodland, light intensity decreases. A plant is found in 18, 15, 9, 3 and 0 percent cover as the quadrats move into deeper shade.
- Identify the pattern in the organism data: the percentage cover decreases along the transect.
- Link the pattern to the environmental gradient: light intensity also decreases towards the woodland.
- Give a cautious conclusion: the plant is more abundant in higher light conditions, suggesting low light may limit its growth in the woodland.
Correlation is not always proof
A transect can show that two things change together, such as light and plant abundance. To prove one factor causes the change, other factors should be controlled or considered.
Fieldwork quality: making results valid and reliable
Good fieldwork is not just about collecting numbers. You need to explain why the method is trustworthy.
A result is valid if it tests what it is meant to test. For example, using random coordinates helps make a population estimate valid because it avoids choosing only dense patches.
A result is reliable if repeating the method gives similar results. Repeating with many quadrats and calculating a mean improves reliability.
You should also:
- use the same quadrat size throughout
- sample enough locations
- record raw data clearly
- avoid damaging organisms or habitats
- keep safe, for example by washing hands after fieldwork and avoiding dangerous areas
In the exam
- For quadrat questions, mention random coordinates, many repeats and calculating a mean or density.
- For transect questions, say the transect crosses an environmental gradient and quadrats are placed at regular intervals.
- When explaining patterns, link the organism data to a named abiotic or biotic factor, but avoid claiming proof if the data only shows a relationship.
Check yourself
- What is the difference between a population, a community and an ecosystem?
- Why is random sampling better than choosing where to place a quadrat yourself?
- How would you use a belt transect to investigate how light affects plant abundance?
