Productivity and ecosystems — Edexcel International A Level Biology
Productivity and energy transfer, ecological terms, biotic and abiotic factors, niche, the ecology core practical, and succession.
Revision notes
6 short notes, in the order of the specification. Each one in short:
Gross primary productivity (GPP) is the rate at which plants fix chemical energy by photosynthesis. Net primary productivity (NPP) is what remains after the plants’ own respiration (R) is subtracted: NPP = GPP − R. NPP is the energy available to primary consumers and decomposers, and is stored as new biomass.
The efficiency of a transfer between trophic levels is the energy (or biomass) passed to the next level, divided by the energy (or biomass) available from the previous level, multiplied by 100. Efficiency is low because energy is lost in respiration, as heat, and in uneaten, egested and excreted material.
A population is all the organisms of one species in a habitat at one time. A community is all the populations of different species living together in a habitat. A habitat is where organisms live, and an ecosystem is the community plus its abiotic environment. Biotic and abiotic factors control the numbers and distribution of organisms.
A niche is the role of a species in its habitat: what it eats, where and when it is active, and how it interacts with other organisms and the abiotic environment. Two species cannot share the same niche in one habitat for long, because one outcompetes the other. Niche helps explain where organisms are found and how many there are.
Core practical 11 is a study of a habitat using quadrats and transects to measure the distribution and abundance of organisms, together with measurements of abiotic factors. Random sampling is used to estimate abundance across an area, and systematic sampling along a transect shows how distribution changes along an environmental gradient.
Succession is the gradual change in the species of a community over time, from colonisation by pioneer species to a stable climax community. Pioneers change the abiotic conditions, making them less hostile for other species. Biodiversity and biomass rise as larger, more competitive species replace earlier ones, until a stable climax community forms.
Exam questions
9 exam-style questions (34 marks), each with its mark scheme.
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18 cards: flip them, mark what you knew, and practise the rest.
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The whole of energy flow, ecosystems and the environment on one page, so you can see where this subtopic fits.
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Quick check questions
Write the equation linking NPP, GPP and R.
Show answer
NPP = GPP − R.
Give two ways in which energy is lost between trophic levels.
Show answer
Respiration (heat), egestion, excretion, uneaten parts.
What is a niche?
Show answer
The role of a species in its habitat, including its interactions with biotic and abiotic factors.
Why are quadrats placed at random?
Show answer
To avoid bias in sampling.
What is a climax community?
Show answer
The stable community at the end of succession, in balance with the abiotic conditions.
Frequently asked questions
What is the difference between GPP and NPP?
Gross primary productivity is the total chemical energy fixed by plants in photosynthesis. Net primary productivity is the energy left after the plants’ own respiration is subtracted (NPP = GPP − R). NPP is the energy stored as new biomass and available to the next trophic level.
Why is energy transfer between trophic levels inefficient?
Only a small fraction passes on because energy is lost in respiration as heat, in egested faeces, in excretory products such as urea and in parts that are not eaten, such as bone. These losses leave less energy available at each successive trophic level.
What is the difference between a habitat and a niche?
A habitat is the place where an organism lives. A niche is the role of the species in that habitat, including what it feeds on, when it is active, the conditions it tolerates and how it interacts with other organisms. Species in the same habitat have different niches.
What are biotic and abiotic factors?
Biotic factors are caused by living organisms, such as competition, predation, parasitism and disease. Abiotic factors are non-living physical and chemical factors, such as temperature, light intensity, pH and water availability. Both kinds of factor control the numbers and distribution of organisms in a habitat, for example by setting where a species can survive.
What happens during ecological succession?
Pioneer species colonise bare ground and change the abiotic conditions, for example by forming soil. This allows new species to replace them in stages. Diversity and biomass increase until a stable climax community forms, which is in balance with the climate.
Written and checked against the Edexcel International A Level Biology (YBI11) specification · Updated October 2026