Pearson Edexcel International A Level Biology: Productivity, Populations, Niche and Succession (YBI11)

Productivity, populations, niche and succession are taught here from the beginning, following statements 5.9 to 5.15 of Topic 5 in the Pearson Edexcel International Advanced Subsidiary/Advanced Level Biology specification (YBI11), Issue 2, with Core Practical 11 included. Within the specification, IAS is Topics 1 to 4 and IA2 is Topics 5 to 8, so this unit sits in IA2. No tiers apply. Companion pages: revision notes and practice questions. The energy that producers capture is explained in the photosynthesis guide. For the wider course, use the course hub and the printable checklist; the free diagnostics help you find a starting point.

Scope of the unit

Statement	What you must be able to do
5.9 (i)	Explain how GPP, NPP and plant respiration (R) are related
5.9 (ii)	Calculate net primary productivity
5.10	Calculate the efficiency of biomass and energy transfers between trophic levels
5.11	Define population, community, habitat and ecosystem
5.12	Explain how biotic and abiotic factors control numbers and distribution
5.13	Use the niche concept to account for distribution and abundance
5.14	Core Practical 11: study a habitat with quadrats, transects and abiotic measurements
5.15	Describe succession from colonisation to a climax community

5.9 Gross and net primary productivity

Gross primary productivity (GPP) is the total rate at which producers convert light energy into chemical energy in organic molecules. Plants then use part of it in respiration (R). What remains is net primary productivity (NPP), the rate at which energy is stored in new plant biomass.

NPP = GPP − R

Rearranged, GPP = NPP + R and R = GPP − NPP. NPP is the energy that is available to primary consumers and decomposers, so it sets the limit on everything above the producers. Productivity is a rate, so it carries a time unit: kJ m⁻² year⁻¹ for energy, or g m⁻² year⁻¹ for dry mass.

Worked example. A grassland plot has GPP of 18 400 kJ m⁻² year⁻¹. Plant respiration uses 7 360 kJ m⁻² year⁻¹.

1. NPP = 18 400 − 7 360 = 11 040 kJ m⁻² year⁻¹.
2. R as a percentage of GPP = 7 360 ÷ 18 400 × 100 = 40.0%.
3. NPP is therefore 60.0% of GPP.

State the unit every time. A value with no unit and no time period loses its meaning.

5.10 Efficiency of transfers between trophic levels

Only part of the energy or biomass at one trophic level reaches the next. The efficiency of a transfer is:

efficiency (%) = (energy or biomass passed to the next level ÷ energy or biomass available at the level below) × 100

Use the same units in the top and bottom lines. Compare like with like: dry mass with dry mass, kJ with kJ.

Losses explain why the figure is always below 100%. Not all of the organism is eaten, because roots, bones or shells may be left. Not all of what is eaten is digested, and the undigested part is egested in faeces. Of the energy absorbed, much is used in respiration and released as heat. Some is lost in excretory products. Only the remainder becomes new tissue, which is what the next level can feed on. Because the losses at each step are large, food chains are short.

Worked example. Using the grassland above, primary consumers gain 1 325 kJ m⁻² year⁻¹ and secondary consumers gain 106 kJ m⁻² year⁻¹.

1. Producers to primary consumers = 1 325 ÷ 11 040 × 100 = 12.0%.
2. Primary to secondary consumers = 106 ÷ 1 325 × 100 = 8.0%.
3. Producer NPP to secondary consumers overall = 106 ÷ 11 040 × 100 = 0.960%.

Worked example with biomass. A caterpillar eats 4.80 g dry mass of leaf, produces 1.20 g dry mass of faeces and gains 0.72 g dry mass of body tissue.

1. Mass absorbed = 4.80 − 1.20 = 3.60 g, which is 75.0% of the intake.
2. Efficiency of conversion of intake to growth = 0.72 ÷ 4.80 × 100 = 15.0%.
3. Efficiency of conversion of absorbed material to growth = 0.72 ÷ 3.60 × 100 = 20.0%.
4. The other 2.88 g of absorbed material was used in respiration or lost in excretion.

Always say which quantity is the denominator. The two answers in steps 2 and 3 are both valid, but they answer different questions.

5.11 Population, community, habitat and ecosystem

Term	Meaning
Population	All the organisms of one species living in a place at the same time
Community	Every population of every species that shares a place and interacts there
Habitat	The place where an organism lives, with its physical and biological surroundings
Ecosystem	The community together with the abiotic factors it interacts with

An example: all the common frogs in a pond are a population. Frogs, pondweed, snails, dragonfly larvae and bacteria in the pond form the community. The pond water, the mud and the margin plants are the habitat. The pond with its light, temperature, pH and dissolved oxygen as well as its living things is the ecosystem.

5.12 Biotic and abiotic factors

Abiotic factors are the non-living ones, such as temperature, light intensity, water supply, pH, salinity, oxygen level and wind exposure. Biotic factors are living: competition, predation, grazing, parasitism, disease and the availability of food or pollinators.

Both control the numbers and distribution of a species. A plant may be absent from a site because the soil is too acidic (abiotic) or because a faster-growing competitor shades it (biotic). Competition within one species is intraspecific. Competition between different species is interspecific. When you explain a distribution, name the factor, say how it acts on the organism and say what happens to survival or reproduction.

5.13 Niche

A niche is the role of a species in its habitat. It includes the abiotic conditions the species tolerates, the resources it uses and its interactions with other organisms, such as what it feeds on and what feeds on it.

The niche accounts for where a species is found and how many there are. A species is present where its requirements for temperature, moisture, food and shelter are all met. It is abundant where those resources are plentiful and few competitors use them.

If two species have very similar niches, they compete for the same limiting resource. One species usually does better, and the other declines locally. Species can coexist when they use the habitat differently, for example by feeding at different heights, at different times of day or on different foods.

A well-known case is two barnacle species on a rocky shore. One lives high on the shore, where it tolerates drying out. The other lives lower down, where it outcompetes the first for space. Biotic and abiotic factors together fix the distribution.

For adaptations (behavioural, anatomical, physiological) see the classification and Hardy-Weinberg guide.

5.14 Core Practical 11: studying the ecology of a habitat

The aim is to measure the distribution and abundance of organisms and to relate them to abiotic factors.

Quadrats are used to sample sessile or slow-moving organisms. For a random sample, mark out the area with two tape measures and generate pairs of coordinates with random numbers, then place a quadrat at each point. This avoids bias in where you place it. Choose the quadrat size to suit the organisms: a small quadrat for dense, small plants.

Abundance can be recorded in three ways:

• Density: the number of individuals per unit area.
• Frequency: the percentage of quadrats in which the species occurs.
• Percentage cover: the proportion of the quadrat covered, used for plants that are hard to count as individuals, such as grass.

Transects are used when conditions change across the habitat. A line transect uses a tape laid along the gradient, with the organisms touching the line recorded at set intervals. A belt transect uses quadrats placed along the line, at regular intervals, for more detail. Because the sampling is systematic, a transect shows how distribution changes with distance, for example from the shore inland.

Abiotic factors are measured at each sampling point: light intensity with a light meter, soil moisture from the mass lost on drying a soil sample, soil pH with a probe or indicator, and temperature with a thermometer. Take the same measurement in the same way each time.

Worked example: random quadrats. An 800 m² field is sampled with twelve 0.5 m × 0.5 m quadrats. Counts of dandelions are 3, 0, 5, 2, 4, 1, 0, 6, 2, 3, 4, 2.

1. Quadrat area = 0.5 × 0.5 = 0.25 m².
2. Total = 32, so the mean = 32 ÷ 12 = 2.67 per quadrat.
3. Density = 2.67 ÷ 0.25 = 10.7 per m².
4. Estimated population = 10.67 × 800 = about 8 500 dandelions.
5. Frequency = 10 of 12 quadrats contain the species, which is 83.3%.
6. The sample standard deviation is 1.87, which shows how variable the counts are. A large spread means the estimate is uncertain.

More quadrats make the estimate more reliable. A null hypothesis could be: there is no relationship between light intensity and the percentage cover of grass.

Worked example: a transect. A belt transect runs from a hedge into woodland.

Distance from hedge (m)	0	5	10	15	20
Light intensity (lux)	11 800	7 400	3 100	1 200	650
Grass cover (%)	64	45	22	7	2

Describe: as light decreases from 11 800 to 650 lux, grass cover falls from 64% to 2%. The relationship is a positive correlation. Explain: less light reduces photosynthesis and NPP of the grass, so it grows less. Evaluate: the data show a correlation. They do not prove that light is the cause, because soil moisture, leaf litter and competition from trees also change with distance. A correlation coefficient can be used to test the strength of such a relationship, and five pairs of points is a small sample.

Risk and ethics: handle plants and animals gently, return stones and logs to their place, keep to paths, wash your hands after contact with soil and water, and avoid damaging the habitat you are studying.

5.15 Succession

Succession is the change in the community of a habitat over time, from the first colonisers to a stable climax community.

Primary succession starts on bare ground with no soil, such as new volcanic rock or a sand bank. Secondary succession starts where soil is already present, such as after a fire or on abandoned farmland, and is usually faster.

Stage	Typical organisms	Change that follows
Colonisation	Pioneer species such as lichens and mosses, adapted to little water and few nutrients	Rock weathers and dead organisms add organic matter, forming thin soil
Early	Small plants such as grasses	Deeper soil, more humus, better water retention
Intermediate	Herbs, shrubs	More shade, more nutrients, more habitats for animals
Climax	Trees such as oak in a temperate climate	Stable community, high biodiversity and biomass

Each stage changes the abiotic environment. The change makes it less suitable for the species that caused it and more suitable for the next species. Pioneers die out as taller, faster-growing competitors shade them. Species diversity increases, food webs become more complex and the amount of biomass rises.

The climax community is relatively stable, with little change in species. It is determined mainly by the climate of the area. In a temperate region, it is often woodland.

Common errors

• Using NPP = GPP + R. It is GPP − R.
• Dividing by the wrong trophic level when calculating efficiency.
• Writing “population” when you mean “community”, or “habitat” when you mean “niche”.
• Describing the niche as only a place. It is a role.
• Using the mean from a non-random, biased sample as an estimate.
• Saying a correlation proves a cause.
• Describing pioneer species as the climax community.

Next steps

Move on to the condensed revision notes, then attempt the practice set under timed conditions.

Official syllabus

Pearson Edexcel International Advanced Subsidiary/Advanced Level in Biology (YBI11), Specification, Issue 2, February 2021, Pearson Education Limited. Statements 5.9 to 5.15, Topic 5.