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5.1.1Topic 5 · On the Wild Side

Ecosystems- Key Terms — Edexcel A Level Biology A (SNAB) 9BI0 Revision Notes & Topical Questions

5 min read 8 parts Lesson 4 of 34 1 quiz

📚 Edexcel A Level Biology A (SNAB) 9BI0 Hub / Topical Revision
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5.1.1 Ecosystems- Key Terms — Edexcel A Level Biology A (SNAB) 9BI0 notes: Define the key terms used in ecology and describe how the components of an ecosystem interact.An ecosystem is a community of living organisms interacting with each other and with the non-living (abiotic) c…

  • individual
  • population
  • Decomposers
  • food chain
  • trophic level
  • calorimeter

Section 1 of 8The ecosystem and its levels of organisation

An ecosystem is a community of living organisms interacting with each other and with the non-living (abiotic) components of their environment, within a particular area.

The ecosystem and its levels of organisation
Ecosystems can be as small as a rock pool or a rotting log, or as large as a rainforest or an ocean.
They are dynamic: energy flows through them in one direction, whereas chemical elements such as carbon and nitrogen are recycled.

Ecologists describe a hierarchy of organisation.

Individual
An individual is a single organism.
Population
A population is all the individuals of one species living in a particular place at the same time and able to interbreed.
Community
A community is all of the populations of different species living and interacting in the same area.
The ecosystem includes the community plus the abiotic environment.
Habitat
The habitat is the place where an organism lives, such as a woodland floor or a stream bed,
  • whereas the niche is the role of the species in the ecosystem.
    • (considered in more detail in 5.1.3)

Section 2 of 8Producers, consumers and decomposers

Organisms are classified by how they obtain energy. Producers (autotrophs) such as plants, algae and some bacteria convert light energy into chemical energy during photosynthesis and form the first trophic level. Consumers (heterotrophs) obtain energy by eating other organisms: primary consumers feed on producers, secondary consumers on primary consumers, and so on. Decomposers, mainly bacteria and fungi, break down dead material and waste by secreting enzymes and absorbing the products, releasing mineral ions that producers can take up again. Decomposers therefore recycle nutrients, and are essential to the continued function of an ecosystem.

Some organisms occupy more than one trophic level, so a food chain, showing a single route of energy transfer, is a simplification. A food web shows the interconnected feeding relationships in a community and more realistically represents how energy and biomass move around.

Section 3 of 8Trophic levels, biomass and energy

A trophic level is the position of an organism in a food chain.

Arrows in a food chain show the direction of energy transfer, from the organism being eaten to the eater.
Biomass
At each stage energy is lost, so there are rarely more than four or five trophic levels. Biomass is the mass of living material, usually measured as dry mass per unit area (for example g m-2),
  • and is a measure of the energy stored in organisms.
Dry mass is used because water content varies and water stores no useful energy.
Calorimeter
Energy in biomass can be estimated using a calorimeter, burning a dry sample and measuring the temperature rise of water.

A pyramid of biomass or of energy shows the amount in each trophic level. Pyramids of energy are always pyramid-shaped, since energy is lost at each step, but biomass pyramids can be inverted (for example in the sea, where tiny phytoplankton have a high turnover rate but a low standing crop at any instant).

Section 4 of 8Biotic and abiotic components, and some further terms

TermMeaning
Biotic factorLiving influence such as predation, competition, disease
Abiotic factorNon-living influence such as temperature, light, pH, rainfall
Carrying capacityMaximum population size that an environment can support sustainably
BiodiversityVariety of living organisms, including species and genetic diversity
SuccessionDirectional change in the species composition of a community over time

The distinction between the terms matters in exam answers.

Biotic and abiotic components, and some further terms
For example, one cannot define a population simply as a group of organisms: the answer should state same species, same place, same time.
Similarly, a community includes all species but not the abiotic factors, whereas the ecosystem includes both.
These terms provide the vocabulary for later topics: succession (5.1.5), productivity (5.1.6) and energy efficiency (5.1.7).

Section 5 of 8Energy flow versus nutrient cycling

A key idea is the contrast between energy and matter.

1
Energy enters ecosystems as light, is transferred along food chains as chemical energy and leaves as heat from respiration; it cannot be recycled,
  • so an ecosystem needs a constant supply of sunlight.
2
Matter, by contrast, is recycled: carbon moves between the atmosphere, producers, consumers, decomposers and back,
  • and nitrogen passes through the nitrogen cycle by the action of bacteria.
3
Decomposers and saprotrophs are therefore as important as producers.
4
Pyramids of numbers can also be drawn,
  • but these can be misleading because they ignore the size of the organisms: one oak tree supports thousands of insects,
  • giving an inverted pyramid.
5
Biomass and energy pyramids give a more reliable picture, which is why ecologists prefer them.

Section 6 of 8Exam-style questions

  1. Distinguish between a population and a community. [2]2 marks

  2. Explain why dry mass, rather than fresh mass, is used to estimate the biomass of a sample. [2]2 marks

  3. Explain the role of decomposers in an ecosystem. [3]3 marks

Section 7 of 8Key terms

TermMeaning
EcosystemCommunity of organisms interacting with each other and their abiotic environment.
PopulationAll the individuals of a species in a given area at one time.
CommunityAll populations of different species living in the same area.
HabitatThe place where an organism lives.
Trophic levelPosition of an organism in a food chain.
BiomassMass of living material, usually as dry mass per unit area.
ProducerOrganism that makes organic molecules using light energy by photosynthesis.
DecomposerMicroorganism that breaks down dead material and recycles nutrients.

Section 8 of 8Exam technique

1
Defining an ecosystem as only the living organisms: it includes the abiotic environment too.
2
Confusing habitat (where it lives) with niche (what it does and how it fits in).
3
Putting arrows the wrong way in a food chain: they show energy flow, from prey to predator.
4
Using fresh mass for biomass estimates without justification.
5
Saying a population is all organisms in an area, without stating same species.

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