Topic 5: On the Wild Side | Pearson Edexcel Biology A (Salters-Nuffield) / 9BN0
Ecosystems: living things and their environment
5.1 / populations, communities, habitats and ecosystems
Ecology studies relationships between organisms and their environment. Use the correct level of
organisation when explaining change: the number of one species can change even when the total number of
organisms stays similar.
Term Meaning Example
Population All organisms of one species in a particular area at a All oak trees in a
particular time. woodland.
Community All populations of different species living and interacting Oaks, fungi, birds and
in an area. insects.
Habitat The place where an organism lives. A pond or a woodland
floor.
Ecosystem A community together with its physical environment and A pond including its
their interactions. water, light, minerals and
organisms.
An ecosystem includes processes
Energy transfer: Light energy is captured by producers and passed through feeding relationships.
Energy is eventually dissipated as heat.
Nutrient cycling: Elements move between organisms and their surroundings through feeding,
excretion and decomposition.
Interactions: Predation, competition and mutualism can influence population sizes.
A pond contains microhabitats: open water, mud, shaded banks and submerged leaves. Each has a different
combination of conditions, so different organisms may be found there.
A community contains living organisms. An ecosystem includes the community and abiotic
surroundings. Matter cycles; energy flows through the system.
Topic 5: On the Wild Side
,Topic 5: On the Wild Side | Pearson Edexcel Biology A (Salters-Nuffield) / 9BN0
Distribution, abundance and the niche
5.2-5.3 / explain where organisms live and how many occur
Distribution describes where a species occurs. Abundance describes how many individuals occur, or how
much of a sampled area they occupy. Both depend on biotic and abiotic factors.
Factor Examples Possible effect
Abiotic: non-living Temperature, light, water, soil pH, Soil pH may prevent a plant from
mineral ions, salinity and oxygen. obtaining sufficient mineral ions.
Biotic: living Competition, grazing, predation, Grazing may remove seedlings even
disease, food and pollinators. where the climate is suitable.
The niche explains more than the habitat
A niche includes the resources a species uses, the conditions it tolerates and its interactions with other
species. Two plants can occupy the same woodland but exploit different light levels or soil depths.
Different resource use can reduce competition.
Within-species competition: Members of one species often need the same resources. Food, light or
breeding sites can become limiting as density increases.
Between-species competition: Different species compete when their requirements overlap.
Competition can exclude a species from part of its potential range.
Predator and prey: Prey availability affects predators; predation can alter prey abundance. Several
factors usually act together.
Build a causal explanation
Low light reduces photosynthesis in a shade-intolerant plant. Less sugar is available for respiration and
growth, so fewer individuals establish. Explain the mechanism rather than simply stating that light affects
distribution.
A correlation between moisture and abundance does not prove that moisture alone causes the pattern.
Other variables may change at the same time.
Topic 5: On the Wild Side
, Topic 5: On the Wild Side | Pearson Edexcel Biology A (Salters-Nuffield) / 9BN0
Core Practical 10: random quadrats
5.2-5.3 / estimate abundance without choosing attractive sites
1. Define the study area and species. Lay out two measuring tapes at right angles to provide coordinates.
2. Generate random coordinate pairs. Place the same-sized quadrat at each selected position using a
consistent rule.
3. Record number, percentage cover or frequency. Use one boundary rule for organisms touching the
quadrat edge.
4. Take enough independent samples across the habitat. Include representative variation rather than
repeatedly sampling one patch.
5. Calculate a mean or percentage, state units and describe uncertainty. Avoid unnecessary disturbance
to organisms.
Choose the measurement to match the organism
Measure What you record Useful when...
Density Number per unit area. Individuals are easy to distinguish.
Percentage cover Area covered by the species / quadrat area x Individual boundaries are unclear.
100.
Frequency Quadrats containing the species / total You want to describe how widespread
quadrats x 100. it is.
Worked estimate
Five 0.25 m2 quadrats contain 3, 5, 4, 2 and 6 daisies. Mean count = = 4 per quadrat. Density = 4 /
0.25 = 16 daisies m-2. Estimated total in 50 m2 = 16 x 50 = 800 daisies, if the sample represents that area.
Allow for quadrat area before multiplying by habitat area. More representative sampling improves an
estimate; it does not make it an exact census.
Topic 5: On the Wild Side
Ecosystems: living things and their environment
5.1 / populations, communities, habitats and ecosystems
Ecology studies relationships between organisms and their environment. Use the correct level of
organisation when explaining change: the number of one species can change even when the total number of
organisms stays similar.
Term Meaning Example
Population All organisms of one species in a particular area at a All oak trees in a
particular time. woodland.
Community All populations of different species living and interacting Oaks, fungi, birds and
in an area. insects.
Habitat The place where an organism lives. A pond or a woodland
floor.
Ecosystem A community together with its physical environment and A pond including its
their interactions. water, light, minerals and
organisms.
An ecosystem includes processes
Energy transfer: Light energy is captured by producers and passed through feeding relationships.
Energy is eventually dissipated as heat.
Nutrient cycling: Elements move between organisms and their surroundings through feeding,
excretion and decomposition.
Interactions: Predation, competition and mutualism can influence population sizes.
A pond contains microhabitats: open water, mud, shaded banks and submerged leaves. Each has a different
combination of conditions, so different organisms may be found there.
A community contains living organisms. An ecosystem includes the community and abiotic
surroundings. Matter cycles; energy flows through the system.
Topic 5: On the Wild Side
,Topic 5: On the Wild Side | Pearson Edexcel Biology A (Salters-Nuffield) / 9BN0
Distribution, abundance and the niche
5.2-5.3 / explain where organisms live and how many occur
Distribution describes where a species occurs. Abundance describes how many individuals occur, or how
much of a sampled area they occupy. Both depend on biotic and abiotic factors.
Factor Examples Possible effect
Abiotic: non-living Temperature, light, water, soil pH, Soil pH may prevent a plant from
mineral ions, salinity and oxygen. obtaining sufficient mineral ions.
Biotic: living Competition, grazing, predation, Grazing may remove seedlings even
disease, food and pollinators. where the climate is suitable.
The niche explains more than the habitat
A niche includes the resources a species uses, the conditions it tolerates and its interactions with other
species. Two plants can occupy the same woodland but exploit different light levels or soil depths.
Different resource use can reduce competition.
Within-species competition: Members of one species often need the same resources. Food, light or
breeding sites can become limiting as density increases.
Between-species competition: Different species compete when their requirements overlap.
Competition can exclude a species from part of its potential range.
Predator and prey: Prey availability affects predators; predation can alter prey abundance. Several
factors usually act together.
Build a causal explanation
Low light reduces photosynthesis in a shade-intolerant plant. Less sugar is available for respiration and
growth, so fewer individuals establish. Explain the mechanism rather than simply stating that light affects
distribution.
A correlation between moisture and abundance does not prove that moisture alone causes the pattern.
Other variables may change at the same time.
Topic 5: On the Wild Side
, Topic 5: On the Wild Side | Pearson Edexcel Biology A (Salters-Nuffield) / 9BN0
Core Practical 10: random quadrats
5.2-5.3 / estimate abundance without choosing attractive sites
1. Define the study area and species. Lay out two measuring tapes at right angles to provide coordinates.
2. Generate random coordinate pairs. Place the same-sized quadrat at each selected position using a
consistent rule.
3. Record number, percentage cover or frequency. Use one boundary rule for organisms touching the
quadrat edge.
4. Take enough independent samples across the habitat. Include representative variation rather than
repeatedly sampling one patch.
5. Calculate a mean or percentage, state units and describe uncertainty. Avoid unnecessary disturbance
to organisms.
Choose the measurement to match the organism
Measure What you record Useful when...
Density Number per unit area. Individuals are easy to distinguish.
Percentage cover Area covered by the species / quadrat area x Individual boundaries are unclear.
100.
Frequency Quadrats containing the species / total You want to describe how widespread
quadrats x 100. it is.
Worked estimate
Five 0.25 m2 quadrats contain 3, 5, 4, 2 and 6 daisies. Mean count = = 4 per quadrat. Density = 4 /
0.25 = 16 daisies m-2. Estimated total in 50 m2 = 16 x 50 = 800 daisies, if the sample represents that area.
Allow for quadrat area before multiplying by habitat area. More representative sampling improves an
estimate; it does not make it an exact census.
Topic 5: On the Wild Side
