4.2Unit 4: Energy, Environment, Microbiology and Immunity

Ecosystems & Energy Transfer — IAL Edexcel Linear Revision Notes & Topical Questions

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Lesson summary

4.2 Ecosystems and Energy Transfer — IAL EDEXCEL LINEAR notes: Define ecological terms, explain productivity and energy transfer, describe sampling methods and explain ecological succession.Habitat = the place where an organism lives, which can be large (a desert) or small (a tre…

Population
Biotic factors
Combined effect
Competitive exclusion
Distribution
Primary productivity

Part 1 of 10Ecological key terms

Ecological key terms

Habitat

Habitat the place where an organism lives, which can be large (a desert) or small (a tree, a microhabitat).

Specialists survive only in a very specific habitat, whereas generalists survive in many and are more likely to become invasive species.
Species released into a habitat can disrupt normal interactions.

Population

Population all the individuals of one species living in a habitat at the same time.

Abundance is the size of the population and distribution is where it lives.

Community

Community the populations of different species living and interacting in the same area.

Ecosystem

Ecosystem a community and its interactions with the non-living parts of its habitat.

Energy flows through an ecosystem, whereas nutrients are recycled.
Ecosystems vary in size and complexity, from a pond to a rainforest, and none is completely self-contained.

Part 2 of 10Biotic and abiotic factors

Biotic factors

Biotic factors predation, food availability, intraspecific competition, interspecific competition, cooperation, parasitism and disease.
They are usually density-dependent, because their effect increases as population density increases.
Predator and prey numbers cycle, with predator peaks lagging behind prey peaks.

Abiotic factors

Abiotic factors light intensity and wavelength, temperature, water turbidity, humidity, soil or water pH and salinity, soil composition and oxygen or carbon dioxide concentration.
They are usually density-independent, because the same proportion of the population is affected whatever its size, for example frost, fire or flood.

Combined effect

Combined effect abundance and distribution of a species are set by both biotic and abiotic factors.

Part 3 of 10Niche

Niche

Niche the role of a species within its habitat.

It includes what it eats, which species depend on it, when it is active and exactly where it lives and feeds.

Competitive exclusion

Competitive exclusion no two species can fill the same niche, so one out-competes the other and it dies out in that habitat.
Apparent overlaps have subtle differences, such as feeding at different times or on different foods.

Abundance

Abundance species in similar niches compete, so their populations and abundance are lower.

Distribution

Distribution species survive only in habitats to which they are adapted, so they move to a more suitable habitat if conditions change.

Part 4 of 10Ecological productivity

1Primary productivity

Primary productivity the rate at which producers convert light energy into chemical energy.

2Gross primary productivity (GPP)

Gross primary productivity (GPP) the rate at which chemical energy is converted into carbohydrates in photosynthesis.

3Net primary productivity (NPP)

GPP − R, the rate at which energy is stored in plant biomass.

R is respiratory loss, about 90% of the energy stored in glucose.
NPP is the energy available to primary consumers and decomposers.
Rearrangements are GPP NPP + R and R = GPP − NPP.

4Units

Units energy per unit area per time, such as kJ m⁻² yr⁻¹, or per unit volume (kJ m⁻³ yr⁻¹) in aquatic habitats.

Part 5 of 10Energy transfer through food chains

Transfer

Transfer chemical energy passes to primary consumers when plants are eaten, then up the chain, and to decomposers when organisms die.
Arrows in a food chain show transfer of energy by feeding between trophic levels.

Losses at the producer

Losses at the producer only about 1–3% of incident light becomes chemical energy.
Light passes through or is reflected from leaves.
Light hits non-photosynthetic parts such as bark and flowers.
Only certain wavelengths are absorbed.
Plants lose energy as heat in respiration.

Losses at consumers

Losses at consumers only about 10% of the energy ingested is stored in the consumer's tissues, and about 90% is lost.
Parts of the food are not eaten, such as roots, woody parts and bones.
Indigestible food, such as cellulose or fur, is egested in faeces.
Heat is lost in respiration.
Excretion of urea and other wastes removes energy.

Consequence

Consequence energy losses limit food chain length, because higher trophic levels need to eat much more to survive.

Net productivity of a consumer

Net productivity of a consumer energy ingested − (faeces + urine + respiratory loss).

Part 6 of 10Efficiency of energy and biomass transfer

Efficiency of energy and biomass transfer

Energy efficiency

(net productivity ÷ energy received) × 100.

Net productivity is energy received minus energy losses.
Efficiency is given as a percentage and is typically about 10%.

Biomass transfer efficiency

(biomass transferred ÷ biomass intake) × 100.

Dry biomass is used because water content varies.
Samples are dried at low heat and weighed repeatedly until the mass is constant, so an estimate is often used because the organism must be dead.
Both masses must be in the same units.

Link

Link biomass measures the chemical energy stored, so the two efficiencies give the same information.

Part 7 of 10Sampling methods in fieldwork

Sampling methods in fieldwork

Sampling

Sampling large or complex habitats cannot be counted fully, so samples estimate abundance and distribution.

Random sampling

Random sampling sample points are chosen at random to avoid bias and suit uniform areas.
Lay a grid over the area, generate random co-ordinates and place samples in the matching squares.

Systematic sampling

Systematic sampling samples are placed at fixed intervals,
- which avoids missing sections and shows the effect of an environmental feature along a line.

Frame quadrat

Frame quadrat a square frame used for sessile organisms.
- (for example 1 m² for herbs or limpets, 400 m² for trees)
Presence or absence, frequency (number of individuals), abundance on the ACFOR scale (abundant, common, frequent, occasional, rare, none) or percentage cover can be recorded.
Dividing the quadrat into small squares makes percentage cover easier to assess.

Point quadrat

Point quadrat a vertical frame with pins lowered through holes, useful in dense vegetation.
Every species touching a pin is recorded.
Percentage cover equals pins touched ÷ total pins × 100, so one pin of ten is 10%.

Population estimate

Population estimate mean number per quadrat × (total area ÷ quadrat area).

Part 8 of 10Transects, abiotic measurements and kite diagrams

Transect

Transect a tape laid along a gradient, such as away from a field margin or up a rocky shore, to show changes in abundance and distribution.
Continuous line transectevery species touching the tape is recorded.
Interrupted line transectspecies touching the line at regular intervals are recorded.
Continuous belt transectframe quadrats are placed end to end.
Interrupted belt transectframe or point quadrats are placed at regular intervals.

Abiotic factors

Abiotic factors only relevant ones are measured at each sample site, for example light with a light meter (lux), temperature in °C and soil pH with a probe.
Water turbidity is irrelevant in a woodland.

Kite diagram

Kite diagram each species has a horizontal line along the transect distance axis showing distribution, with the width of the kite showing abundance.
Extra sections can show abiotic factors such as height above sea level or soil pH.

Part 9 of 10Ecological succession

Succession

Succession the change of an ecosystem over time, during which the biotic and abiotic conditions change.

Primary succession

Primary succession colonisation of newly formed or newly exposed land,

such as cooled lava, bare rock after a landslide, retreating glaciers or sand dunes.

Wind-blown seeds and spores land and grow, and the first colonisers are pioneer species such as mosses and lichens.

Pioneers germinate easily and tolerate low nutrients and water, and they break up rock and form basic soil as they die and decompose.

Small plants and grasses grow in the shallow, nutrient-poor soil, and their roots hold the soil in place.

As they die the soil deepens and becomes richer, so larger plants, shrubs and small trees can grow.

Eventually the soil can support large trees, which become dominant in the climax community.

Climax community

Climax community the final stable community, such as rainforest in the tropics or deciduous woodland in temperate regions.
It is not always the most biodiverse stage.

Changing conditions

Changing conditions arriving species make the environment more suitable for later species and less suitable for earlier ones.
Lichens disappear once soil forms, and pioneers are out-competed for light by later species.

Secondary succession

Secondary succession it occurs on land that was previously occupied,
- such as after fire or deforestation,
- and begins at a later stage because soil is already present.

Human prevention

Human prevention mowing and grazing stop woody plants establishing, producing a stable plagioclimax such as heathland.

Part 10 of 10More on this topic

Energy transfer

Energy transfer only part of the energy at one trophic level reaches the next, because material is uneaten, undigested or used in respiration.

Sampling

Sampling quadrats estimate abundance or percentage cover in an area, and transects show change along an environmental gradient.
Random sampling reduces selection bias and repeated samples improve reliability.

Key terms

Term	Meaning
Ecosystem	A community and its interactions with the non-living parts of its habitat.
Niche	The role of a species within its habitat.
Gross primary productivity	Rate at which chemical energy is converted into carbohydrate by photosynthesis.
Net primary productivity	Gross primary productivity minus respiratory losses.
Trophic level	The feeding level of an organism in a food chain.
Pioneer species	The first species to colonise bare land.
Climax community	The final stable community at the end of succession.
Plagioclimax	A stable community maintained by human activity such as grazing or mowing.

Avoid these mistakes

Saying energy is recycled in ecosystems — energy flows and is lost as heat, while nutrients are recycled.

Giving GPP when NPP is asked — NPP is GPP minus respiratory loss.

Using fresh mass for biomass transfer — dry mass is used because water content varies.

Saying pioneer species remain in the climax community — they are usually out-competed.

Last updated: 12 October 2026