4a The Organism in the Environment¶
Part of 4 Ecology and the Environment.
Ecology works at several scales, from a single organism to a whole ecosystem, and the term for each scale has an exact meaning. Populations are measured by sampling with quadrats, and their size and distribution depend on non-living (abiotic) and living (biotic) factors.
What You Need to Learn¶
Further detail: Pearson Edexcel International GCSE Biology specification.
The ecological levels from individual to ecosystem, how to sample populations and biodiversity with quadrats and transects, and how abiotic and biotic factors affect the size, distribution and survival of populations.
Ecological Levels¶
Each level contains the one before it. A pond shows all five:
| Level | Meaning | Pond example |
|---|---|---|
| Individual | A single organism of one species | One frog |
| Population | All the organisms of one species living together in a habitat | All the frogs in the pond |
| Community | All the populations of different species living together in a habitat | The frogs, fish, snails and pondweed together |
| Habitat | The specific place where an organism lives | The pond |
| Ecosystem | The community together with the non-living parts of the environment and their interactions | The pond community plus its water, light and temperature |
Populations in a community are often interdependent: frogs eat snails, and snails eat pondweed. Organisms are adapted to the conditions of their habitat.
Exam technique
The usual slip is to mix up population and community. A population is one species only, and a community is all the species. If a question gives a list of different organisms in one place, the answer is community.
Quadrats, Distribution and Biodiversity¶
A quadrat is a square frame of known area. Placing a quadrat on a small, representative part of a large habitat lets you estimate how many organisms live there and how they are spread out, without counting the whole area.
Quadrats on a transect. A transect is a line or strip across the habitat. Quadrats are placed at regular intervals along it, and the number or percentage cover of each species is recorded. This shows how distribution changes along an environmental gradient, such as light level from a woodland edge into open ground.
Random placement. Quadrats can instead be placed at random coordinates, chosen with a random number generator. Random placement avoids sampling bias, such as unconsciously placing quadrats on the patches with the most plants, so the estimate is more reliable. Counting gives a more precise value than percentage cover, which is quick but subjective.
Improving reliability. Take more samples, use a longer transect or a larger area, repeat the sampling, and keep the quadrat size the same each time.
Biodiversity is the variety of species in an ecosystem. To measure it, count the number of different species in each quadrat, not just the individuals of one species. More samples and random placement make the estimate more reliable.
Worked example: estimating a population
A student places ten quadrats at random in a field of 400 m². Each quadrat covers 0.25 m². The numbers of daisies counted are 4, 7, 3, 5, 6, 2, 5, 4, 6 and 8.
Mean per quadrat = 50 ÷ 10 = 5 daisies
Number of quadrat areas in the field = 400 ÷ 0.25 = 1600
Estimated population = 5 × 1600 = 8000 daisies
The result is an estimate. More quadrats would reduce the effect of an unusually high or low count.
Exam technique
When asked how to make a sampling method more reliable, give a specific change and a reason, such as "use more quadrats, so that unusual counts have less effect on the mean". "Do it again" alone is weaker. For random placement, say that it avoids bias.
Abiotic and Biotic Factors¶
Abiotic and biotic factors affect the size, distribution and survival of populations.
| Abiotic (non-living) factor | How it affects organisms |
|---|---|
| Light intensity | Needed for photosynthesis, so it affects plant growth and therefore the food and shelter available to other species |
| Temperature | Affects enzyme activity and metabolic rate, and decides which species can survive |
| Moisture levels | Plants and animals both need water to survive |
| Soil pH and mineral content | Soil pH affects the rate of decomposition, and mineral levels decide which plants can grow, which affects the whole food web |
| Biotic (living) factor | How it affects organisms |
|---|---|
| Food availability | More food allows more successful breeding, so the population increases |
| New predators | A new predator can rapidly reduce the prey population |
| New pathogens | A disease to which a population has no resistance can cause rapid decline |
| Competition | When two species compete for a resource, the better-adapted species tends to win, and the other population falls |
Worked example: reading a transect
A transect runs from the edge of a wood into open ground. Fern cover falls from 60% under the trees to 5% in the open, while light intensity rises along the same line.
The pattern suggests that ferns grow best in shade. It does not prove that light is the cause, because moisture and soil could also change along the transect. To test light as the cause, change only light intensity and keep the other factors the same.
Common Confusions¶
- Population vs community: A population is one species in a habitat, and a community is all the populations together.
- Habitat vs ecosystem: A habitat is the place an organism lives, and an ecosystem includes the community and the non-living environment as well.
- Biodiversity is not abundance: Biodiversity counts the variety of species. A quadrat with 100 daisies and nothing else is less biodiverse than one with 10 individuals of five species.
- Correlation is not cause: A pattern along a transect suggests which factor matters but does not prove it.
Check Yourself¶
- Define the terms population and community, using one habitat as an example.
- Describe how to use quadrats and a transect to find out how the number of a plant species changes from a hedge into an open field.
- Explain why quadrats should be placed at random when estimating a population.
- A field of 200 m² is sampled with eight quadrats of 0.5 m². The total count of buttercups is 24. Estimate the population of buttercups.
- Name one abiotic and one biotic factor that could reduce the population of a plant species, and explain how each acts.
- A transect shows that moss is common where soil moisture is high and rare where it is low. Explain why this does not prove that moisture is the cause.
Answers
- A population is all the organisms of one species in a habitat, for example all the frogs in a pond. A community is all the populations of different species living together there, for example the frogs, fish, snails and pondweed.
- Lay a transect line from the hedge across the field. Place a quadrat at regular intervals along it, and at each position count the plants of the species or record the percentage cover. Compare the results along the line.
- Random placement avoids bias, for example choosing places that look good. The sample is then more representative, so the estimate of the population is more reliable.
- Total area sampled = 8 × 0.5 = 4 m². Mean per m² = 24 ÷ 4 = 6. Estimated population = 6 × 200 = 1200 buttercups.
- Abiotic: low light reduces photosynthesis, so growth and reproduction fall. Biotic: a new herbivore eats the plants, or competition from another species takes light and water. Any suitable pair with a reason.
- Other factors, such as light, soil pH or temperature, may also change along the transect. The pattern suggests moisture matters, but only a controlled test that changes moisture alone could show that it is the cause.
Key Terms¶
- Population: all the organisms of one species in a particular area.
- Community: all the populations of different species living together in an area.
- Habitat: the place where an organism lives.
- Ecosystem: the community of organisms and the non-living environment interacting together.
- Biodiversity: the variety of species living in an ecosystem.
- Quadrat: a square frame of known area used for ecological sampling.
- Transect: a defined line across a habitat along which samples are taken at regular intervals.
- Abiotic factor: a non-living environmental factor that affects the distribution and abundance of organisms.
- Biotic factor: a living factor in the environment that affects organisms.
- Distribution: the spread or pattern of where organisms occur in a habitat.