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Notes/Environmental Science/Life processes in the biosphere
Notes · Environmental ScienceUK · A-Levels

Life processes in the biosphere

This chapter examines how energy and matter move through living systems. It covers photosynthesis and respiration and the productivity they set, the flow and inevitable dissipation of energy along food chains and webs, the ecological pyramids that display this, the dynamics of populations from exponential to logistic growth, and ecological succession and nutrient cycling. Quantitative skills run throughout: calculating net primary productivity, energy-transfer efficiency and population growth rates.

5 sections·~17 min reading time·3 competencies·Level Standard 4 · Advanced 1

T·0222 / 16
Exam profile
AO1 · Describe energy flow, primary productivity, population dynamics and ecological successionAO2 · Calculate net primary productivity, energy-transfer efficiency and population growth rates from dataAO3 · Interpret ecological pyramids and growth curves and evaluate the factors that limit productivity and population size
Operators:describeexplaincalculateanalyseevaluateinterpret

basic level

AS-Level expects you to describe energy flow and losses along food chains, draw and interpret pyramids, and describe exponential and logistic growth and succession.

higher level

The full A-Level requires confident calculation of NPP, energy-transfer efficiency and growth rates, and the analysis and evaluation of what limits productivity and population size.

Depth

Reading depth: In depth

Text

Text size: Standard

Contents · 5 sections▾
  1. Life processes in the biosphere
    • 01Photosynthesis, respiration and primary productivity◐
    • 02Energy flow through trophic levels and food webs◐
    • 03Ecological pyramids and energy-transfer efficiency◐
    • 04Population growth, carrying capacity and survival strategies●
    • 05Ecological succession and nutrient cycling◐
§ 01

Photosynthesis, respiration and primary productivity#

●●○StandardLPAQA 7447 3.1.1

Productivity as a balance of photosynthesis and respiration

Primary productivityGraph, solar energy → gross primary productivity (GPP), gross primary productivity (GPP) → respiration by producers (R), gross primary productivity (GPP) → net primary productivity (NPP = GPP - R), net primary productivity (NPP = GPP - R) → energy available to consumerssolar energygross primaryproductivity(GPP)respiration byproducers (R)net primaryproductivity(NPP = GPP −R)energy availableto consumersphotosynthesislost as heat
Fig. 1Net primary productivity is what remains of gross productivity after the producers' own respiration.

Key points

Almost all the energy in the biosphere enters through photosynthesis, in which producers (green plants, algae and some bacteria) capture light energy and use it to convert carbon dioxide and water into glucose and oxygen. This fixes light energy as chemical energy in organic molecules and is the base of virtually every food chain. Respiration is the reverse balance sheet: all organisms, producers included, break down organic molecules to release the stored energy for their own life processes, returning carbon dioxide and water. The two processes together move both energy and carbon through ecosystems and link this chapter to the carbon cycle.
The rate at which producers fix energy is measured as productivity. Gross primary productivity (GPP) is the total rate at which producers capture and fix energy by photosynthesis. However, producers use some of this energy for their own respiration (R), so the energy actually available to be stored as new plant biomass, and therefore passed on to consumers, is the net primary productivity (NPP), given by NPP=GPP−RNPP = GPP - RNPP=GPP−R. NPP is the key measure because it represents the energy available to the rest of the ecosystem, and it is usually expressed as an amount of energy per unit area per unit time, for example kJ m−2 yr−1\text{kJ m}^{-2}\,\text{yr}^{-1}kJ m−2yr−1.
Productivity varies enormously between ecosystems because it depends on the abiotic factors that limit photosynthesis: light, temperature, water and nutrient availability. Tropical rainforests and estuaries have very high NPP because warmth, light, water and nutrients are all plentiful; deserts and the open ocean and tundra have low NPP because water, nutrients or warmth are limiting. This variation explains the global pattern of biomes and the distribution of the world's most productive fisheries and farmland, so it recurs in later chapters on agriculture, forests and fisheries.
The same logic applies one level up: secondary productivity is the rate at which consumers build new biomass from the food they assimilate, after their own respiratory and excretory losses. Because much of the energy in food is lost as heat in respiration or is never assimilated, secondary productivity is always far lower than the primary productivity beneath it, which is the fundamental reason food chains are short and top predators are rare.
NPP=GPP−RNPP = GPP - RNPP=GPP−R

Net primary productivity

Net primary productivity equals gross primary productivity minus the energy lost by the producers in respiration; it is the energy available to consumers, in units such as kJ m−2 yr−1\text{kJ m}^{-2}\,\text{yr}^{-1}kJ m−2yr−1.

Worked example

Calculating net primary productivity

In a woodland the producers fix 26 000 kJ per square metre per year (GPP) and lose 11 000 kJ per square metre per year in respiration. Calculate the net primary productivity.

  1. 01Recall the equation

    NPP=GPP−RNPP = GPP - RNPP=GPP−R.

  2. 02Substitute

    NPP=26 000−11 000NPP = 26\,000 - 11\,000NPP=26000−11000.

    NPP=26 000−11 000=15 000 kJ m−2 yr−1NPP = 26\,000 - 11\,000 = 15\,000\ \text{kJ m}^{-2}\,\text{yr}^{-1}NPP=26000−11000=15000 kJ m−2yr−1
  3. 03State the answer with units

    The net primary productivity is 15 000 kJ per square metre per year, the energy available to consumers.

Result: NPP = 15 000 kJ per square metre per year.

Exam focus

  • Define GPP, R and NPP and use NPP=GPP−RNPP = GPP - RNPP=GPP−R to calculate any one from the other two, with correct units.
  • Explain why NPP differs between ecosystems in terms of the abiotic factors that limit photosynthesis.

Typical mistakes

  • Confusing GPP and NPP; NPP is what is left after the producers' own respiration.
  • Omitting the units of area and time (per square metre per year) from a productivity value.

Active revision

A grassland has a gross primary productivity of 9000 kJ per square metre per year and the plants respire 3200 kJ per square metre per year. Calculate the net primary productivity and state what it represents.

Active recall

Recall the key points — then reveal.

Sources: AQA AS and A-level Environmental Science (7447) specification (AQA) · GCE AS and A level subject content (Department for Education)

§ 02

Energy flow through trophic levels and food webs#

●●○StandardLPAQA 7447 3.1.1

Energy flow and losses along a food chain

Energy flow through trophic levelsGraph, producers → primary consumers, primary consumers → secondary consumers, secondary consumers → tertiary consumers, producers → heat, faeces, death, primary consumers → heat, faeces, death, secondary consumers → heat, faeces, deathproducersprimaryconsumerssecondaryconsumerstertiaryconsumersheat, faeces,deathheat, faeces,deathheat, faeces,deathabout 10%about 10%about 10%
Fig. 2Energy is lost at every transfer, so only about a tenth passes to the next trophic level.

Key points

A trophic level is a feeding position in a food chain: producers form the first level, primary consumers (herbivores) the second, secondary consumers (carnivores) the third, and so on, with decomposers acting on every level. A food chain is a single pathway of feeding relationships, but in reality organisms have several food sources and are eaten by several others, so feeding relationships form a branching food web. Food webs are more realistic than food chains and show how the removal of one species can ripple through many others.
Energy flows in one direction through this structure and is dissipated at every step. Only a fraction of the energy at one trophic level is passed on to the next, because energy is lost as heat in respiration, in undigested material egested as faeces, and in nitrogenous excretion; some organisms and parts are also never eaten. As a rule of thumb only about 10%10\%10% of the energy at one trophic level is fixed as biomass at the next, though the real figure varies. This inefficiency is why food chains rarely have more than four or five links and why there is far less biomass at the top than at the bottom.
Because energy is lost and not recycled, ecosystems depend on a continuous input of solar energy; unlike matter, energy cannot cycle. This is a crucial distinction: nutrients such as carbon and nitrogen are recycled through the biogeochemical cycles and used again, but the energy that flowed with them is ultimately radiated to space as heat and must be constantly replaced from the Sun. Keeping the one-way flow of energy separate from the cycling of matter is one of the most examined ideas in the subject.
The inefficiency of energy transfer has direct human relevance. Feeding grain to animals and then eating the animals wastes most of the energy in the grain, so a given area of land can feed more people on a plant-based diet than on a meat-based one; this argument reappears in agriculture and food security. It also explains why top predators, which sit at the end of long chains and depend on a large energy base, are especially vulnerable to habitat loss and to the biomagnification of persistent pollutants.
Worked example

Following energy along a chain

Producers in a pond fix 40 000 kJ per square metre per year. If about 10% of the energy is transferred at each step, estimate the energy reaching the secondary consumers.

  1. 01First transfer

    Producers to primary consumers: 40 000×0.10=4000 kJ m−2 yr−140\,000 \times 0.10 = 4000\ \text{kJ m}^{-2}\,\text{yr}^{-1}40000×0.10=4000 kJ m−2yr−1.

  2. 02Second transfer

    Primary to secondary consumers: 4000×0.10=400 kJ m−2 yr−14000 \times 0.10 = 400\ \text{kJ m}^{-2}\,\text{yr}^{-1}4000×0.10=400 kJ m−2yr−1.

  3. 03Interpret

    Only about 1% of the original fixed energy reaches the secondary consumers, which is why they are far less abundant.

Result: About 400 kJ per square metre per year reaches the secondary consumers, roughly 1% of the energy fixed by the producers.

Exam focus

  • Explain why energy transfer between trophic levels is inefficient, naming the routes of energy loss.
  • Explain the difference between the one-way flow of energy and the cycling of matter, and why food chains are short.

Typical mistakes

  • Saying energy is recycled; energy flows one way and is lost as heat, whereas nutrients are recycled.
  • Treating the 10% figure as exact rather than as a typical value that varies between ecosystems and levels.

Active revision

Explain why a hectare of land can support more people growing wheat than raising cattle, using the idea of energy-transfer efficiency.

Active recall

Recall the key points — then reveal.

Sources: AQA AS and A-level Environmental Science (7447) specification (AQA) · GCE AS and A level subject content (Department for Education)

§ 03

Ecological pyramids and energy-transfer efficiency#

●●○StandardLPAQA 7447 3.1.1

A pyramid of energy

Pyramid of energypyramid, 4 tiers, Data: producers, primary consumers, secondary consumers, top carnivoresproducers40 000primary consumers4000secondary consumers400top carnivores40Energy per square metre per year (illustrative values)
Fig. 3A pyramid of energy always tapers upwards because energy is lost at each transfer.

Key points

Ecological pyramids display the quantities at each trophic level as horizontal bars stacked with producers at the base. A pyramid of numbers shows the number of organisms at each level; a pyramid of biomass shows the dry mass of living material; and a pyramid of energy shows the energy (or productivity) at each level. Pyramids of energy are always broadest at the base and taper upwards, because energy is lost at every transfer, which makes them the most reliable of the three.
Pyramids of numbers and biomass can occasionally be inverted or irregular. A single large tree supports many insects, so a pyramid of numbers based on it is top-heavy; in the open ocean the tiny, fast-reproducing phytoplankton have a small standing biomass at any instant yet support a larger biomass of zooplankton, so a pyramid of biomass can be inverted. These anomalies arise because numbers ignore the size of organisms and biomass is only a snapshot that ignores turnover, whereas a pyramid of energy measured over a year cannot be inverted.
Energy-transfer efficiency between two trophic levels is calculated as the energy at the higher level divided by the energy at the lower level, expressed as a percentage: efficiency=energy at level nenergy at level n−1×100\text{efficiency} = \dfrac{\text{energy at level } n}{\text{energy at level } n-1} \times 100efficiency=energy at level n−1energy at level n​×100. Working out these efficiencies from data is a standard quantitative task, and the answers explain the shape of the pyramid: a low efficiency produces a steeply tapering pyramid. Efficiencies tend to be higher for carnivores than herbivores, because animal food is more digestible than plant food with its indigestible cellulose.
Pyramids are a compact way to compare ecosystems and to detect disturbance. A change in the shape of a pyramid over time, or a marked difference between a polluted and a clean site, can reveal that energy flow has been disrupted, for example by the loss of a trophic level. Being able to construct a pyramid from a table of data, and to interpret an unusual shape, is exactly the kind of applied, data-handling skill the examination rewards.
energy-transfer efficiency=energy at trophic level nenergy at trophic level n−1×100\text{energy-transfer efficiency} = \frac{\text{energy at trophic level } n}{\text{energy at trophic level } n-1} \times 100energy-transfer efficiency=energy at trophic level n−1energy at trophic level n​×100

Energy-transfer efficiency

The percentage of energy passed from one trophic level to the next; a typical value is around 10% but it varies.

Worked example

Calculating transfer efficiency

In a meadow the producers hold 18 000 kJ per square metre per year and the primary consumers 1620 kJ per square metre per year. Calculate the energy-transfer efficiency between these two levels.

  1. 01Recall the equation

    efficiency=higher levellower level×100\text{efficiency} = \dfrac{\text{higher level}}{\text{lower level}} \times 100efficiency=lower levelhigher level​×100.

  2. 02Substitute

    162018 000×100\dfrac{1620}{18\,000} \times 100180001620​×100.

    162018 000×100=9%\frac{1620}{18\,000} \times 100 = 9\%180001620​×100=9%
  3. 03Comment

    An efficiency of 9% is close to the typical figure of about 10%, so this transfer is unremarkable.

Result: The energy-transfer efficiency is 9%, close to the typical value of about 10%.

Exam focus

  • Calculate energy-transfer efficiency between trophic levels from tabulated data.
  • Explain why a pyramid of numbers or biomass can be inverted but a pyramid of energy cannot.

Typical mistakes

  • Dividing the lower level by the higher level, giving an efficiency greater than 100%.
  • Assuming all pyramids are upright; pyramids of numbers and biomass can be inverted in some ecosystems.

Active revision

A table gives the energy at producers as 20 000 and at primary consumers as 2400 kJ per square metre per year. Calculate the energy-transfer efficiency and comment on whether it is typical.

Active recall

Recall the key points — then reveal.

Sources: AQA AS and A-level Environmental Science (7447) specification (AQA) · GCE AS and A level subject content (Department for Education)

§ 04

Population growth, carrying capacity and survival strategies#

●●●AdvancedLPAQA 7447 3.1.1

Exponential and logistic population growth

Patterns of population growthGraph of exponential (J), y-intercept at y = 20, increasing, on the interval x from 0 to 20, Graph of logistic (S), y-intercept at y = 10, increasing, on the interval x from 0 to 20510152020040060080010001200fastest growth(K/2)carryingcapacity (K)exponential (J)logistic (S)population sizetime
Fig. 4Exponential (J) growth is unlimited; logistic (S) growth levels off at the carrying capacity as resistance builds.

Key points

A population changes size through births and immigration, which add individuals, and deaths and emigration, which remove them. When resources are unlimited a population grows exponentially, producing a J-shaped curve in which the number added each generation keeps increasing; the growth rate can be expressed per unit time or per head as a percentage. Exponential growth cannot continue indefinitely because resources are finite, so it is seen only briefly, for example when a species colonises a new habitat or recovers from a crash.
As a population grows, environmental resistance builds up: food and space become scarce, waste accumulates, competition intensifies and predators and disease increase. These density-dependent factors slow growth as the population rises, producing a logistic or S-shaped curve that levels off at the carrying capacity, the maximum population the environment can support sustainably. The population then fluctuates around the carrying capacity through negative feedback, rising when it is below and falling when it is above. Density-independent factors such as drought, fire or a hard winter can also cut a population regardless of its size.
Species differ in their reproductive strategy along a continuum from r-selected to K-selected. r-selected species (many insects, weeds and pioneer species) reproduce early and prolifically, produce many small offspring with little parental care, and have short lives; they exploit disturbed or temporary habitats and their populations boom and bust. K-selected species (large mammals, trees) reproduce later and produce few large offspring with much parental care, live longer, and maintain populations near the carrying capacity. K-selected species recover slowly from population crashes, which makes them more vulnerable to over-exploitation and extinction, a point that matters in conservation and fisheries.
In communities, populations interact. Predator and prey populations often cycle, with the predator lagging behind its prey: a rise in prey allows predators to increase, which then drives the prey down, which in turn starves the predators, and so on. Interpreting such coupled fluctuations, and distinguishing genuine cause from mere correlation, is a common data-analysis task and connects population dynamics to the wider management of exploited and conserved species.
percentage growth rate=change in populationoriginal population×100\text{percentage growth rate} = \frac{\text{change in population}}{\text{original population}} \times 100percentage growth rate=original populationchange in population​×100

Population growth rate

The change in a population as a percentage of its starting size over a stated time; a positive value means growth, a negative value means decline.

Worked example

Calculating a percentage growth rate

A bacterial culture grows from 2000 to 5000 cells in one hour. Calculate the percentage growth rate over that hour.

  1. 01Find the change

    Change = 5000−2000=30005000 - 2000 = 30005000−2000=3000 cells.

  2. 02Divide by the original and convert

    30002000×100\dfrac{3000}{2000} \times 10020003000​×100.

    30002000×100=150%\frac{3000}{2000} \times 100 = 150\%20003000​×100=150%
  3. 03Interpret

    The population grew by 150% in one hour, characteristic of rapid exponential growth while resources are plentiful.

Result: The percentage growth rate is 150% per hour.

Exam focus

  • Interpret J-shaped and S-shaped growth curves and identify the carrying capacity and the phase of fastest growth.
  • Distinguish r-selected from K-selected species and relate the strategy to vulnerability to exploitation and extinction.

Typical mistakes

  • Confusing exponential (unlimited, J-shaped) with logistic (levels off at carrying capacity, S-shaped) growth.
  • Muddling density-dependent factors (competition, disease, predation, which intensify as density rises) with density-independent ones (weather, fire).

Active revision

A population of 500 deer rises to 650 over one year. Calculate the percentage growth rate, and explain what will happen to the growth rate as the population approaches the carrying capacity.

Active recall

Recall the key points — then reveal.

Sources: AQA AS and A-level Environmental Science (7447) specification (AQA) · GCE AS and A level subject content (Department for Education)

§ 05

Ecological succession and nutrient cycling#

●●○StandardLPAQA 7447 3.1.1

Stages of primary succession

Primary successionGraph, bare rock (no soil) → pioneers: lichens, mosses, pioneers: lichens, mosses → grasses and herbs, grasses and herbs → shrubs, shrubs → climax woodlandbare rock (nosoil)pioneers:lichens, mossesgrasses andherbsshrubsclimax woodlandcolonisationsoil builds
Fig. 5Each seral stage changes conditions, allowing the next to establish, until a climax community is reached.

Key points

Ecological succession is the directional change in a community over time as one group of species modifies the environment and is replaced by another. Primary succession begins on bare ground with no soil, such as a new lava flow or bare rock, and is started by pioneer species (lichens, mosses) that tolerate harsh conditions and begin to build soil. Secondary succession begins where a disturbance (fire, clearance, abandonment) has destroyed the community but left the soil, so it starts from a more advanced point and proceeds faster.
Succession proceeds through a series of transitional communities, the seral stages, each of which changes the conditions, for example by adding organic matter, retaining water, providing shade and altering nutrient availability, so making the site suitable for the next group of species while often making it less suitable for the current one. Diversity and biomass generally increase through succession. It ends, at least in theory, at a stable climax community determined by the climate, which persists until the next major disturbance. In practice human activity often halts succession at an earlier stage, producing a plagioclimax such as heather moorland maintained by grazing and burning.
Underpinning all of this is the recycling of nutrients by decomposers. Detritivores and decomposers (bacteria and fungi) break down dead organic matter and waste, releasing the mineral nutrients locked in it back into the soil, where they can be taken up again by plants. Without this decomposition, nutrients would remain trapped in dead material and productivity would collapse; decomposers therefore close the loop that lets the same atoms of nitrogen, phosphorus and other elements be used over and over, linking this chapter directly to the biogeochemical cycles.
Understanding succession is important for conservation and land management. Knowing that a habitat is a stage in succession explains why it must sometimes be actively managed to prevent it from changing, why abandoned farmland reverts to scrub and then woodland, and how a damaged ecosystem might recover after restoration. It also underlies the deliberate use of early succession in techniques such as leaving field margins or restoring quarry sites, where pioneer communities are encouraged to rebuild soil and diversity.
Worked example

Explaining a halted succession

A meadow is left unmanaged. Predict and explain the sequence of communities that will develop over the following decades.

  1. 01Starting point

    Soil is already present, so this is secondary succession starting from grassland.

  2. 02Sequence

    Tall herbs and coarse grasses first dominate, then shrubs such as bramble and hawthorn colonise, shading out the grasses, and finally trees establish to form woodland.

  3. 03Explain the mechanism

    Each stage modifies conditions, particularly light and soil organic matter, making the site suitable for taller, more competitive species until a climax woodland is reached.

Result: Without management the meadow undergoes secondary succession through scrub to climax woodland, as each community alters conditions for the next.

Exam focus

  • Describe the stages of primary and secondary succession and explain how each stage changes conditions for the next.
  • Explain the role of decomposers in recycling nutrients and the idea of a climax community and plagioclimax.

Typical mistakes

  • Confusing primary succession (starts on bare ground with no soil) with secondary succession (starts where soil remains).
  • Forgetting that many valued habitats are plagioclimaxes maintained by human management, not natural climax communities.

Active revision

Explain why heather moorland must be regularly burned and grazed to prevent it from being replaced by woodland, using the idea of succession.

Active recall

Recall the key points — then reveal.

Sources: AQA AS and A-level Environmental Science (7447) specification (AQA) · GCE AS and A level subject content (Department for Education)

Contents

Section -- / 05

    • 01Photosynthesis, respiration and primary productivity◐
    • 02Energy flow through trophic levels and food webs◐
    • 03Ecological pyramids and energy-transfer efficiency◐
    • 04Population growth, carrying capacity and survival strategies●
    • 05Ecological succession and nutrient cycling◐

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Life processes in the biosphere

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References & sources

Sources

AQA

  • AQA AS and A-level Environmental Science (7447) specification

Department for Education

  • GCE AS and A level subject content

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