4.1.3Community Level Systems
Levels of Organisation in an Ecosystem — OCR Gateway GCSE Combined Science A J250 - Biology Revision Notes & Topical Questions
5 min read 9 parts Lesson 4 of 6
Section 1 of 9From individual to ecosystem
Ecology is the study of how living things relate to each other and to their surroundings. To describe this clearly, biologists use a set of terms that move from small to large scales.
A pond, a hedgerow, a woodland, a rock pool and even a garden lawn can all be described as ecosystems. They vary in size but all follow the same rules.
Section 2 of 9Biotic and abiotic factors
Everything in an ecosystem that affects the organisms can be sorted into two types.
| Biotic factors | Abiotic factors | |
|---|---|---|
| Meaning | Caused by living things | Caused by non-living parts of the environment |
| Examples | Food availability, predators, pathogens, competition | Light intensity, temperature, moisture, soil pH, wind speed, oxygen level in water, carbon dioxide level |
Changes in either type can change the size of a population. For instance, a rise in the number of predators (biotic) can reduce prey numbers, while a very dry summer (abiotic) can reduce the number of plants that grow.
Section 3 of 9Habitat, niche and adaptation
A habitat is the place where an organism lives, such as a rocky shore. Within a habitat, many different species survive because each one is adapted to its own way of life. An adaptation is a feature that increases an organism's chance of survival and reproduction. Adaptations can be:
Organisms that live in extreme conditions are called extremophiles; examples include bacteria living in hot springs or in very salty water. They have enzymes and membranes that still work in conditions that would stop most organisms.
Section 4 of 9Studying an ecosystem
To describe a community, ecologists need to know what lives there and how many. Two common field techniques are used at GCSE.
- The organisms inside are counted, or the percentage cover is estimated.
- They are best for plants and slow-moving or fixed animals.
- To avoid bias, quadrats are placed at random positions found using random coordinates from a number generator on a measured grid.
The mean number per quadrat is multiplied up to estimate a whole population: estimated population = mean number per quadrat x (total area / area of one quadrat). A larger number of quadrats makes the estimate more reliable.
Section 5 of 9Interdependence and stability
Organisms in a community depend on each other for food, shelter, pollination and seed dispersal.
Section 6 of 9Required practical: investigating the effect of a factor on distribution
A common practical asks how the distribution of a plant, such as daisies or buttercups, changes along a gradient, for example from a shaded hedge into open grass.
Section 7 of 9Exam-style questions
A scientist counts 14 daisies in a 0.25 m2 quadrat on average. The field is 500 m2. Estimate the number of daisies in the field. [2]2 marks
1. Number of quadrats that fit in the field = 500 / 0.25 = 2000. [1]
2. Estimated population = 14 x 2000 = 28 000 daisies. [1]
Explain the difference between a population and a community, using a pond as an example. [3]3 marks
Exam-style questions1. A population is all the organisms of one species in an area, for example all the frogs in the pond. [1]2. A community is all the different populations of species living together, for example frogs, fish, pond weed and insects. [1]3. The community plus the abiotic factors (water, light, temperature) forms the ecosystem. [1]Suggest why quadrats should be placed at random positions when sampling a field. [2]2 marks
1. To avoid bias, such as choosing areas that look to have more plants. [1]
2. So the sample is representative of the whole field and the mean can be used to estimate the population. [1]
Section 8 of 9Key terms
| Term | Meaning |
|---|---|
| Ecosystem | A community of organisms together with the non-living environment and their interactions. |
| Community | All the populations of different species living together in one area. |
| Population | All the organisms of one species in one area. |
| Habitat | The place where an organism lives. |
| Biotic factor | A living influence on an organism, such as a predator or competitor. |
| Abiotic factor | A non-living influence, such as temperature or light. |
| Quadrat | A square frame used to sample organisms in an area. |
| Transect | A line along which samples are taken to show changes across a habitat. |
| Interdependence | Reliance of species in a community on each other. |
Section 9 of 9Exam technique
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