Notes · Environmental ScienceUK · A-Levels

The living environment

This chapter sets out the conditions that make Earth uniquely able to support life and the way living organisms interact with the physical environment. It introduces the four interacting Earth spheres, the organisation of ecosystems, the ecological niche, and the abiotic factors that act as limiting factors and set the ranges of tolerance within which species can survive, closing with the global pattern of biomes and the idea of the biosphere as a self-regulating system.

5 sections~18 min reading time3 competenciesLevel Foundation 2 · Standard 2 · Advanced 1

T·0111 / 16

Exam profile

AO1 · Describe the conditions for life on Earth, the interacting spheres, and the structure of ecosystems, niches and toleranceAO2 · Apply the ideas of limiting factors and ranges of tolerance to unfamiliar species, habitats and environmental dataAO3 · Analyse tolerance and distribution data and evaluate models of a self-regulating biosphere such as Daisyworld

Operators:describeexplainapplyanalyseevaluatesuggest

basic level

AS-Level expects you to describe the conditions for life, the four spheres, the components of an ecosystem, the niche and the effect of abiotic limiting factors, and to recognise the major biomes.

higher level

The full A-Level adds the quantitative interpretation of tolerance curves and distribution data, the analysis of feedback in maintaining habitable conditions, and the synoptic use of these ideas across cycles, climate and conservation.

Depth

Text

Contents · 5 sections

The living environment

Why Earth is able to support life#

FoundationAQA 7447 3.1.1

The conditions that make Earth habitable

Fig. 1The conditions for life are interdependent: each supports the others in keeping the surface habitable.

Life as we know it depends on a narrow set of physical conditions, and the Earth is remarkable because it sits within all of them at once. The single most important requirement is liquid water, which acts as the solvent for the chemistry of life, and liquid water can only exist across a limited band of temperature and pressure. The Earth's distance from the Sun places it in the so-called habitable zone, where the average surface temperature keeps most water liquid rather than permanently frozen (as on Mars) or boiled away (as on Venus). This is often called the Goldilocks position: not too hot and not too cold.

Several other conditions reinforce this. The atmosphere provides the carbon dioxide and oxygen that photosynthesis and respiration depend on, exerts enough pressure to keep water liquid, and, through the natural greenhouse effect, warms the surface by roughly 33 degrees Celsius above what it would otherwise be, lifting the average from about

-18\,^{\circ}\text{C}

to around

+15\,^{\circ}\text{C}

. The stratospheric ozone layer absorbs most of the Sun's damaging ultraviolet radiation, and the planet's magnetic field deflects the charged particles of the solar wind that would otherwise strip the atmosphere away. Each of these is examined in detail in later chapters, but together they explain why the surface is habitable.

It is important to see these conditions as interlinked rather than as a checklist. A change in one, such as a change in atmospheric composition, alters temperature, which alters the state of water, which alters the biosphere. The idea that the living and physical parts of the Earth interact so as to keep conditions broadly stable is the theme of the whole subject, and is formalised at the end of this chapter as the concept of a self-regulating biosphere.

The habitability of Earth is not fixed for all time. Over geological history the Sun has brightened, the atmosphere has changed composition (most dramatically with the rise of oxygen produced by early photosynthesis), and the climate has swung between icehouse and greenhouse states. That life has persisted through these changes points to the buffering role of feedback, but it also shows that the conditions for life can be pushed towards their limits, which is exactly the concern raised by present-day human impacts.

Worked example

The warming effect of the natural greenhouse

Without its atmosphere the Earth's average surface temperature would be about -18 degrees Celsius; with it the average is about +15 degrees Celsius. Calculate the warming provided by the natural greenhouse effect and explain its importance for life.

Find the difference
Warming = final temperature - initial temperature = $15 - (-18)$ .
Evaluate
$15 - (-18) = 33\,^{\circ}\text{C}$ .
Interpret
This 33 degree warming keeps average surface temperatures above the freezing point of water over most of the planet, so liquid water and therefore life can persist.

Result: The natural greenhouse effect provides about 33 degrees Celsius of warming, which is essential for keeping most of the Earth's water liquid.

Typical mistakes

Listing conditions in isolation rather than explaining how they interact (for example that the atmosphere controls both temperature and the state of water).
Saying the greenhouse effect is inherently harmful; the natural greenhouse effect is essential and only its human enhancement is the problem.

Active revision

Explain why liquid water is regarded as the single most important requirement for life, and describe two conditions that allow it to exist on Earth.

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)

The four Earth spheres and their interactions#

FoundationAQA 7447 3.1.1

The four interacting spheres

Fig. 2The four spheres exchange energy and matter; environmental processes occur where they interact.

The Earth system is conventionally divided into four interacting spheres. The atmosphere is the envelope of gases surrounding the planet; the hydrosphere is all of the water (oceans, ice, rivers, lakes, groundwater and water vapour); the lithosphere is the solid outer Earth, the rocks and soils of the crust; and the biosphere is the sum of all living organisms and the zones they occupy. Everything in environmental science can be located within one or more of these spheres, and the interesting behaviour happens at the boundaries where they meet.

The spheres are open systems that constantly exchange energy and matter. Energy from the Sun drives almost all of the flows: it heats the atmosphere and drives its circulation, evaporates water to power the hydrological cycle, and is fixed by the biosphere in photosynthesis. Matter cycles between the spheres through the biogeochemical cycles, so that a carbon atom may move from the atmosphere into a plant (biosphere), into the soil (lithosphere) and into the ocean (hydrosphere) and back again. Understanding these transfers is the key to understanding both natural processes and human impacts.

Interactions between the spheres explain most environmental processes. Weathering is an interaction of the atmosphere and hydrosphere with the lithosphere that breaks down rock to form soil; the greenhouse effect is an interaction of the atmosphere with incoming and outgoing radiation; eutrophication is an interaction where nutrients from the lithosphere (via agriculture) enter the hydrosphere and disturb the biosphere. Because the spheres are coupled, a disturbance in one propagates to the others, which is why human activities that appear local, such as burning fossil fuels, have global consequences.

It helps to think of each sphere as a set of stores (where matter or energy resides) connected by flows or fluxes (the transfers between stores). A store in equilibrium has equal inputs and outputs, so its size is constant; an imbalance causes it to grow or shrink. This stores-and-flows way of thinking is used throughout the course, most explicitly in the biogeochemical cycles and the energy budget, and it is worth becoming fluent in it now.

Worked example

Locating a process in the spheres

Acid deposition forms when sulfur dioxide from burning coal reacts with water in the air and falls, acidifying a lake and killing fish. Identify the spheres involved at each stage.

Source
Sulfur dioxide is released into the atmosphere by combustion (a human activity acting on the lithosphere's fossil fuels).
Transformation
In the atmosphere it reacts with water vapour (hydrosphere) to form acid, which falls as precipitation.
Impact
The acid enters a lake (hydrosphere) and lowers its pH, harming the fish (biosphere).

Result: The pathway runs lithosphere to atmosphere to hydrosphere to biosphere, illustrating that the spheres are coupled.

Typical mistakes

Treating the spheres as isolated compartments rather than as open systems that exchange energy and matter.
Confusing the hydrosphere (all water, including ice and vapour) with only liquid surface water.

Active revision

Using the idea of stores and flows, describe how a single carbon atom could move between all four of the Earth's spheres.

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)

Ecosystems, niches and ecological organisation#

StandardAQA 7447 3.1.1

Levels of ecological organisation

Fig. 3The ecological hierarchy: the abiotic environment plus the community make up the ecosystem.

Ecology is organised into a hierarchy of levels. An individual organism belongs to a population, which is all the members of one species living in a defined area at a given time; the populations of all the different species living and interacting in that area form a community; and the community together with the physical, non-living environment it occupies forms an ecosystem. A habitat is the particular place where an organism lives, defined by its physical characteristics. Being precise about these terms is essential because examiners test them directly.

Every ecosystem has biotic and abiotic components. The biotic components are the living organisms and the interactions between them, such as predation, competition and mutualism. The abiotic components are the non-living physical and chemical factors, such as temperature, light, water availability, pH, salinity and dissolved oxygen. The structure and functioning of an ecosystem emerge from the interplay of these two sets of components, and a change to either can shift the whole system.

The ecological niche is the role and position a species has in its ecosystem: not just where it lives but how it lives, including what it eats, what eats it, when and how it reproduces, and the full range of physical conditions it can tolerate and resources it uses. The competitive exclusion principle states that two species cannot occupy exactly the same niche indefinitely in the same place, because the better competitor will eventually displace the other; this drives resource partitioning, where similar species divide up resources and so coexist. The niche is a powerful idea because it links an organism's biology to its distribution.

Ecosystems can be defined at any scale, from a rock pool or a rotting log to a whole lake or forest, and they are rarely closed. Most exchange energy and matter with neighbouring systems, for example through migrating animals, flowing water or falling leaves. Recognising that boundaries are chosen for convenience, and that ecosystems are open and dynamic, prevents the common error of treating them as fixed and self-contained.

Worked example

Applying competitive exclusion

In a laboratory two species of protist were grown separately and then together in the same tube on the same food. Grown separately both thrived; grown together, one species declined to extinction. Explain the result.

Identify the overlap
Grown on the same food in the same conditions, the two species occupy the same niche and so compete directly for the same limiting resource.
Apply the principle
By the competitive exclusion principle, the species that uses the resource more efficiently increases at the expense of the other.
Conclude
The weaker competitor's population falls to extinction, because two species cannot occupy an identical niche in the same place indefinitely.

Result: Complete niche overlap leads to competitive exclusion, so only one species persists.

Typical mistakes

Using habitat and niche interchangeably; a habitat is a place, a niche is a role including the conditions and resources used.
Describing an ecosystem as only the living organisms; it is the community together with its abiotic environment.

Active revision

Two species of bird feed in the same tree, one on the outer twigs and one on the trunk. Explain, using the niche concept, how they are able to coexist.

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)

Abiotic factors, limiting factors and tolerance#

StandardAQA 7447 3.1.1

A tolerance curve

Fig. 4Performance peaks at the optimum and falls to zero at the limits; the span between the limits is the range of tolerance.

Abiotic factors determine where organisms can live and how well they perform. For any factor, such as temperature, an organism performs best over a limited range around an optimum, performs less well as conditions move away from that optimum, and cannot survive beyond upper and lower limits. Plotting performance (growth, reproduction or survival) against the factor gives a tolerance curve that is typically hump-shaped, with the optimum at the peak and the range of tolerance stretching between the two limits. This is Shelford's law of tolerance.

A limiting factor is the abiotic factor that is in shortest supply relative to need, and so restricts the growth, abundance or distribution of a population at a given time. Liebig's law of the minimum captures the idea that a process is constrained by the single most limiting resource, however plentiful the others: a crop with abundant light and water but too little nitrogen is limited by nitrogen. As conditions change, the limiting factor may change; on a bright summer day plant growth may be limited by water or carbon dioxide rather than by light. Identifying the limiting factor is central to explaining distributions and to managing agriculture and conservation.

Species differ in the width of their tolerance. Eurytolerant (generalist) species have wide ranges of tolerance and can live in variable environments, whereas stenotolerant (specialist) species have narrow ranges and are restricted to stable conditions. Wide tolerance often means a wide geographical distribution, while narrow tolerance makes a species vulnerable to environmental change; this is directly relevant to predicting which species are most at risk from climate change and pollution. The presence or absence of species with known tolerances can also be used to indicate environmental conditions, an idea used later with indicator species in pollution monitoring.

Tolerance ranges also help to explain zonation, the arrangement of species in bands along an environmental gradient. On a rocky shore, for example, species are zoned up the shore according to how long they can tolerate exposure to air; on a mountainside, vegetation changes with altitude as temperature falls. Where a factor exceeds a species' tolerance, that species is absent, so the edges of a distribution map often coincide with the limits of tolerance for a key factor.

\text{range of tolerance} = \text{upper limit} - \text{lower limit}

Range of tolerance

The span of an abiotic factor over which a species can survive; a wide range indicates a generalist (eurytolerant) species.

Worked example

Identifying the limiting factor

In a greenhouse, tomato growth increased when extra carbon dioxide was added but did not increase further when extra light was then added. Later in the season, adding more light did increase growth. Explain these observations.

First stage
Adding carbon dioxide increased growth, so carbon dioxide was the limiting factor; adding light had no effect because light was already in surplus.
Apply the law of the minimum
Growth is controlled by the factor in shortest supply, so only raising that factor raises the rate.
Later stage
When conditions changed and light became the factor in shortest supply, adding light increased growth; the limiting factor had switched.

Result: Growth is set by the limiting factor, which changes as conditions change; only increasing the limiting factor raises the rate.

Typical mistakes

Confusing the optimum (best performance) with the limits of tolerance (where performance falls to zero).
Assuming light is always the limiting factor for plants; the limiting factor is whichever is in shortest supply and can change through the day or season.

Active revision

A plant species is found only on north-facing slopes in a warm region. Suggest, using the ideas of optimum and tolerance, which abiotic factor limits its distribution and explain your reasoning.

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)

Biomes and the self-regulating biosphere#

AdvancedAQA 7447 3.1.1

Negative feedback in Daisyworld

Fig. 5Daisyworld illustrates negative feedback: a temperature rise triggers a response that opposes it, stabilising the system.

A biome is a large-scale community of plants and animals characteristic of a particular climate, such as tropical rainforest, temperate deciduous forest, boreal forest (taiga), grassland, desert and tundra. The global distribution of biomes is controlled principally by climate, above all by temperature and precipitation, which are in turn set by latitude and the pattern of atmospheric circulation. Because climate governs biomes, a shift in climate shifts the boundaries of biomes, which is one of the mechanisms by which climate change threatens ecosystems.

Within a biome the vegetation reflects the climate: rainforests, with high year-round warmth and rainfall, support very high productivity and biodiversity; deserts, limited by water, support sparse, drought-adapted life; tundra, limited by cold and a short growing season, supports low, slow-growing vegetation. The organisms of each biome show convergent adaptations to their shared conditions, so unrelated species in similar biomes on different continents often look and function alike. This link between climate and biome is a recurring synoptic theme, connecting the atmosphere, the cycles and the living environment.

At the largest scale, the biosphere behaves in some respects as a single self-regulating system, a view associated with the Gaia hypothesis of James Lovelock and Lynn Margulis. The claim is not that the Earth is a purposeful organism but that life and the physical environment are so tightly coupled through feedback that conditions such as global temperature and atmospheric composition are held within limits suitable for life over long periods. Negative feedback, in which a change triggers a response that opposes it, is the mechanism of such stability; positive feedback, in which a change triggers a response that amplifies it, drives instability and is the concern behind climate tipping points.

The Daisyworld model is a deliberately simple thought experiment used to show how such regulation could arise without any foresight. On an imaginary planet, dark daisies absorb sunlight and warm their surroundings, while light daisies reflect it and cool theirs. As the sun brightens, cooling light daisies come to dominate and reflect more energy, while when the planet cools, warming dark daisies spread; the changing mix of daisies stabilises the planet's temperature through negative feedback. Daisyworld is not a description of the real Earth, and you should evaluate it as an illustrative model rather than as evidence, but it shows convincingly that self-regulation can emerge from ordinary competition and feedback.

Classifying the major biomes

Fig. 6Biomes grouped by their controlling climate; distribution follows temperature and precipitation.

Worked example

Classifying a feedback loop

As global temperature rises, permafrost thaws and releases methane, a greenhouse gas, which causes further warming. Identify the type of feedback and explain its significance.

Trace the loop
Warming leads to thawing permafrost, which releases methane, which enhances the greenhouse effect, which causes more warming.
Classify
The response (more warming) reinforces the original change (warming), so this is positive feedback.
Significance
Positive feedback amplifies change and can drive the system past a tipping point, making warming harder to reverse; it works against the stabilising negative feedbacks of the biosphere.

Result: This is positive feedback, which amplifies warming and is a mechanism behind climate tipping points.

Typical mistakes

Confusing positive and negative feedback; negative feedback opposes and stabilises, positive feedback amplifies and destabilises.
Treating the Gaia hypothesis as a claim that the Earth is a conscious organism, or treating Daisyworld as evidence rather than as an illustrative model.

Active revision

Explain, using the idea of feedback, how the melting of reflective sea ice could accelerate warming, and state whether this is positive or negative feedback.

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)

References & sources

Sources

AQA

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

Department for Education

GCE AS and A level subject content

The living environment

5 sections~18 min reading time3 competenciesLevel Foundation 2 · Standard 2 · Advanced 1

The living environment

Key points

The warming effect of the natural greenhouse

Key points

Locating a process in the spheres

Key points

Applying competitive exclusion

Key points

Identifying the limiting factor

Key points

Classifying a feedback loop

Sources

The living environment

Key points

The warming effect of the natural greenhouse

Key points

Locating a process in the spheres

Key points

Applying competitive exclusion

Key points

Identifying the limiting factor

Key points

Classifying a feedback loop

Sources