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4.1 Essential ideas

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Book: 4.1 Essential ideas
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Date: Sunday, 27 September 2020, 4:30 PM

4.1.1 Species, community and ecosystems

Ecology is the study of how organisms interact with one another and with their environments. Practically, this involves the study of ecosystems, which are sustainable systems consisting of biotic (living) and abiotic (non-living) components.

Important concepts related to the structure of ecosystems are described in the table below.

Concept Description
Species A group of organisms that can interbreed to produce fertile offspring. These may be reproductively isolated (i.e. geographically) or live together in populations.
Population Individuals of the same species living together in time and space
Community Populations (of different species) that interact with each other
Autotroph  A species that is able to produce its own food, usually through photosynthesis
Producer An autotroph
Heterotroph A species that feeds on plant or animal material (i.e. cannot make its own food)
Consumer A heterotroph that feeds on other animals or plants by ingesting them
Detritivore  A heterotroph that obtains nutrients by ingesting detritus (See Figures 4.1.1a-c)
Saprotroph  A heterotroph that obtains nutrients by externally digesting dead organisms (See Figure 4.1.1f)

Note: Both detritivores and saprotrophs are also known as decomposers, because they play an important role in recycling nutrients back into an ecosystem.

Many small animals, especially insects and earthworms, are detritivores. Shown here are a wood louse, a millipede and a dung beetle, all members of phylum Arthropoda.

woodlouseFigure 4.1.1a – Wood louse
millipedeFigure 4.1.1b – Millipede
dung beetleFigures 4.1.1c – Dung beetle

Communities consist of all the biotic components of an ecosystem. Parts of the community interact with each other through:

  • competition – both intraspecific (within a single species) and interspecific (between species) – for food and resources
  • trophic relationships, for example predation, herbivory, parasitism
  • symbiosis, which means ‘living together’; types of symbiotic relationship are described in the following table.




Commensalism One organism benefits while the other is unharmed Spiders build webs on tree trunks
Mutualism Both organisms benefit from the relationship Bees pollinate flowers while collecting nectar
Parasitism One organism benefits while the other is harmed (See image 4.1.1g-h) Strangler figs grow around other trees, preventing their growth

Interaction of biotic and abiotic components

A community forms an ecosystem by its interactions with the abiotic environment. Abiotic factors can be quantitatively observed.


Examine the picture of a freshwater ecosystem shown in Figure 4.1.1d and do the following:

  • Identify examples of autotrophs, heterotrophs, populations, communities.
  • Suggest names for species of saprotrophs and detritivores (common names are acceptable) that might be found in this ecosystem. Indicate on the image where they might be found.
  • Outline the role of the following abiotic factors in the functioning of this ecosystem: pH, humidity, temperature, carbon dioxide concentration, oxygen concentration, salinity, precipitation, soil composition, air composition.
  • In groups of two or three, discuss methods that you might use to measure how one of the abiotic factors above influences the structure of this ecosystem.

freshwater ecosystemFigure 4.1.1d – A freshwater ecosystem

Sustainability of ecosystems

A mature ecosystem has the potential to be sustainable over long periods of time. In ecological terms, sustainability refers to an ecosystem’s ability to maintain a level of biodiversity and productivity that does not consume resources or cause destruction of the ecosystem’s structure. Consider the following facts:

  • Autotrophs obtain inorganic nutrients from the abiotic environment, and synthesise food for heterotrophs.
  • The supply of inorganic nutrients is maintained by nutrient cycling. Nutrients are cycled when bacteria and other decomposers return inorganic nutrients to the system.

the amount of nutrients cycling through an ecosystemFigure 4.1.1e – The amount of nutrients cycling through an ecosystem is constant (unless there is disturbance)

In Figure 4.1.1e we see that nutrients cycle through the system, and energy is lost as heat. No new mass is being added – it is simply being transformed between one trophic level and another. Sustainable ecosystems do not become more or less productive.

saprotroph funghiFigure 4.1.1f – Saprotroph
Most fungi are are saprotrophs – they obtain nutrients by digging long hyphae into the woody bark of trees and digesting cellulose. Some species of bacteria are also saprotrophic.

Language tools

  • The word ‘species’ is both singular and plural.
  • Students often confuse the terms detritivore and saprotroph. The important distinction is the location of digestion: internal digestion for detritivore, external digestion for saprotroph.
  • Large scavengers such as vultures and hyenas are not considered detritivores, since they consume most of the organic matter, rather than return it to the soil.

Course link

Learn more about biotic interactions, the ecological role of different species, and sustainability in Option Topic C: Ecology and Conservation.

Language tools

Most ecological terms can be deconstructed into their Latin or Greek roots. For example, autotroph comes from the Greek suffix auto-, meaning ‘self’ and the word trophos meaning ‘food’. If you can remember a few root words, it will help you to understand some of the new terms you encounter. 

strangler treeFigure 4.1.1g – Parasitism by strangler trees
Many members of the genus Ficus grow around, and eventually kill, host trees.

parasitic plantFigure 4.1.1h – Hyobanche sanguinea is a parasitic plant that requires a host plant to get its nutrition

International mindedness

Keeping the productivity of ecosystems in mind:

  • How can we increase food production (i.e. to alleviate world hunger) and promote sustainability at the same time?
  • What are some methods to promote sustainability in other important human activities such as forestry, sewage and water treatment, or the manufacturing of consumer goods?

In the lab


In biology textbooks from the early 20th century and up until about 1980, the ‘balance of nature’ was often mentioned in reference to ecosystems. More recently, ‘dynamic equilibrium’ is the term used to describe the resilience of ecosystems to outside disturbances. Is this a case of old wine in a new bottle?

Nature of Science

Scientists are always looking for patterns and trends. New scientific knowledge comes out of discrepancies in the trends. For example, some plants are heterotrophic!

4.1.2 Energy flow

The supply of inorganic nutrients is relatively constant in a stable ecosystem, since nutrients are cycled repeatedly through the non-living and living components. Energy, on the other hand, must constantly be supplied.

  • Most ecosystems rely on sunlight as an energy source. There is seasonal variation in quantity, but light energy is continuously available.
  • Photosynthetic autotrophs, like plants and some algae, convert light energy from the sun into chemical energy in the form of carbon compounds. The rate at which they do this is related to how productive an ecosystem is.
  • Chemical energy is transferred through the food chain in various forms by means of feeding. The energy stored in chemical bonds is converted into different forms, in order to do work like movement and digestion.
  • In general, less than 10% of total available energy is transferred to the next trophic level. Most of the stored chemical energy is lost as heat, a by-product of respiration.
  • The number of trophic levels in a food chain and the total biomass at the higher trophic levels is limited by energy losses.

food chainFigure 4.1.2a – A simple food chain showing the flow of energy through an ecosystem
Normally, the sun is not included in a food chain, but is shown here for illustrative purposes. This food chain has three trophic levels: a producer, a primary consumer and a secondary consumer.

Energetics of food chains and pyramids of energy

Most food chains have a maximum of four trophic levels. This makes sense when the energetics of a food chain is considered. As heat energy cannot be converted back into chemical energy, it dissipates into the surroundings and is lost.

To illustrate this point, let’s imagine a food chain with four trophic levels:

Food chain: Moss → Caterpillar → Sparrow → Hawk
Tropic level Producer Primary consumer Secondary consumer Tertiary consumer
Energy available
(% of first
trophic level)
100 10 1 0.1

Assuming a 10% transfer rate, which is very efficient, four trophic levels leaves only 0.1% of the energy stored in plants available to secondary consumers.

In addition to energy lost as heat, biomass is also lost in the form of urea, faeces, carbon dioxide and water. This explains why biomass of the large carnivores is small compared with that of the producers.

The energetics of food chains can be represented as a pyramid of energy.

Pyramid of energyFigure 4.1.2b – A pyramid of energy is a scaled diagram that shows how much energy is in each trophic level of a food chain.

Important points about energy pyramids:

  • The pyramid should be stepped and not triangular. The lowest step represents the producers, and each successive step upwards represents a consumer: primary, secondary, tertiary (and quaternary, when appropriate).
  • A pyramid of energy is a scaled diagram. This means that the area of the box in each step is proportional to the amount of energy represented by that step.
  • The units used in a pyramid of energy are kilojoules (kJ) or, more appropriately, a unit of energy per unit area per unit of time, kilojoules per square metre per year (kJ m-2 year-1).
  • Pyramids of energy will never appear inverted, as some of the energy stored in one source is always lost when transferred to the next source.

This is an application of the second law of thermodynamics.


Use 1mm graph paper to draw a pyramid of energy to represent the following food chain. (Hint: Make sure your pyramid is scaled.)

Trophic level Energy
kJ m-2 year 1)
Producers 8805
Primary consumers 1453
Secondary consumers 79
Tertiary consumers 12

Study tip

Repeat this mantra: ‘Nutrients cycle, but energy flows. Nutrients cycle, but energy flows …’

Consider this

Archaea are a group of ancient bacteria that are able to live in conditions that resemble the very ancient earth, such as volcanic vents. Some of these bacteria feed on heavy metals and gases produced in the Earth’s core. These ecosystems would survive the death of the Sun since they don’t rely on its energy!

CyanobacteriaFigure 4.1.2c – Cyanobacteria
The poorly named ‘blue-green algae’ is actually an entire phylum of photosynthetic bacteria. They are found in almost every terrestrial and aquatic ecosystem.

International mindedness

The productivity of different ecosystems is an important factor to consider when making judgements on which environments need the most protection. For example, the productivity of tropical rainforests (37,000 kJ m-2 year-1) is about three times more than temperate grasslands (12,500 kJ m-2 year-1), and more than ten times more than Arctic tundras (2,500 kJ m-2 year-1) (all values approximate).

rainforestFigure 4.1.2d – Tropical rainforest biomes are the most productive in the world

tundraFigure 4.1.2e – Arctic tundra is one of the least productive biomes in the world, especially in winter

temperate grasslandFigure 4.1.2f – Temperate grassland productivity is very high in the summer and very low in the winter

Nature of Science

Theories explain phenomena. Energy flow is a theoretical concept that explains why food chains are limited in length.

4.1.3 Carbon cycling

Energy flows and nutrients cycle. Carbon cycles through ecosystems in two general forms: as inorganic free carbon dioxide (CO2) and as fixed organic carbon compounds, especially carbohydrates like glucose, C6H12O6, or gases like methane, CH4. Further detail on the carbon cycle is given below:

Figure 4.1.3a – Carbon cycle
A carbon store is so named because these are places where carbon accumulates and is stored, either long term (i.e. in the lithosphere), or short term (i.e. in biomass).

Major processes of the carbon cycle

  1. Carbon dioxide makes up about 0.04% of the atmosphere.
  2. It is also present in aquatic ecosystems, dissolved in gaseous form, or as hydrogen carbonate ions (HCO3-).
  3. CO2 diffuses from the atmosphere (or from water) into the bodies of autotrophs, such as plants or cyanobacteria. These organisms convert inorganic carbon dioxide into organic carbon compounds, such as carbohydrates.
  4. Under anaerobic conditions, organic carbon compounds are digested by bacteria to produce methane gas. Methane accumulates in the ground or is diffused into the atmosphere.
  5. When exposed to oxygen, methane is oxidised to form carbon dioxide and water, which is returned to the atmosphere or water bodies by diffusion.
  6. In aerobic conditions, organic carbon compounds are digested by other heterotrophs, especially decomposers, to produce carbon dioxide, a product of respiration.
  7. A portion of the digested organic compounds is incorporated into the bodies of organisms in various forms. For example, reef-building corals and molluscs have shells composed of calcium carbonate (CaCO3).
  8. Organic matter that is not fully decomposed is converted to peat if it is exposed to anaerobic or acidic conditions in waterlogged soils.
  9. Partially decomposed organic matter is converted over long periods of time to form gas, oil and coal that accumulate in porous rocks. Hard-bodied organisms such as molluscs and corals become fossilised in limestone.
  10. Carbon dioxide is also added to the atmosphere by human activity, especially burning fossilised matter, coal, oil, gas, and biomass (i.e. fossil fuels).

Activity 1

1a: Annotate your diagram

  • Download the Carbon cycle activity (PDF) and print this out.
  • Annotate your diagram using the numbers 1-10 to indicate where each of the corresponding processes occurs.
  • Check your answers by scrolling over yellow dots on Figure 4.1.3a above.

1b: Skill - Practise drawing a simplified diagram of the carbon cycle

  • Place carbon compounds in boxes and use arrows to represent processes.
  • Label arrows with these processes: photosynthesis, carbon fixation, feeding, death, combustion, oxidation, cellular respiration, anaerobic respiration, decomposition, fossilisation.
  • Check your answers >

Carbon flux

There are two types of process involved with the carbon cycle:

  1. Those that remove carbon dioxide from the atmosphere and oceans, including photosynthesis and fossilisation.
  2. Those that add carbon dioxide to the atmosphere and oceans, especially cellular respiration and combustion of fossil fuels.

The amount of carbon that is transferred by different processes is called carbon flux.

carbon fluxesFigure 4.1.3b – Annual carbon fluxes in natural systems (green arrows) and anthropogenic systems (red arrows)
One gigatonne is equal to one billion metric tonnes.

Activity 2

Using information from Figure 4.1.3b:

  • List the carbon stores in order of decreasing size
  • Estimate the amount of carbon that accumulates as a result of human activities
  • Comment on the balance of carbon flux in the natural systems compared with the human systems.

Course link

Learn about cycling of two other important nutrients, and the vulnerability of agricultural soils in 14.1.6 Nitrogen and phosphorus cycles.

Language note

  • Bacteria that produce methane gas from carbon dioxide are called methanogenic archaeans. Some methanogenic bacteria can metabolise other carbon compounds as well.
  • When carbon is converted from a gaseous form into more complex organic compounds, this is called carbon fixation. Once it’s fixed, carbon remains fixed until it is released as gas, either carbon dioxide or methane. 

methanogenic bacteriaFigure 4.1.3c – Methanogenic archaeans have recently been found at Mount Everest base camp1
Scientists think that melting ice caps on Everest are to blame for uncovering this source of greenhouse gas. In this photo, you can see methane gas bubbles accumulating under the ice.

Study tip

There are many more processes that put carbon dioxide into the atmosphere, like rising ocean temperatures, deforestation and increased weathering of limestone due to acid rain. You do not need to include them all in your drawing of the carbon cycle. The processes listed above are sufficient.

weathering limestoneFigure 4.1.3d – Weathering limestone
This limestone has a distinctive weathering pattern. As limestone weathers naturally, it releases carbon dioxide into the atmosphere.

International mindedness

Biofuels are made with what have been traditionally food crops, such as maize and cane sugar. What are the ethical implications of diverting food crops for fuel? Are biofuels an ecological alternative to fossil fuels?

Nature of Science

In order to understand the carbon cycle, making accurate, quantitative measurements of atmospheric greenhouse gases is very important for reliability of the evidence.


  1. "Psychrophilic Methanogenic Bacteria Found at Everest Base Camp: What Are the Implications for Global Warming?" Solar Cities. N.p., 2 June 2011.

4.1.4 Climate change

Gases in the atmosphere are warmed by the energy reflecting from the surfaces of land and oceans. This natural greenhouse effect is what keeps the temperatures on Earth suitable for life.

Gases that are able to absorb the reflected longwave radiation (i.e. heat energy) are called greenhouse gases.

Primary greenhouse gases
Absorb most heat
Secondary greenhouse gases
Absorb some heat
Other atmospheric gases (not greenhouse gases)
Do not absorb heat
Carbon dioxide (CO2) Methane (CH4) Oxygen/ozone (O2/03)
Water vapour (H2O) Nitrogen oxides (NOx) Nitrogen (N2)

The enhanced greenhouse effect

Greenhouse gases that retain the most heat (carbon dioxide and water) have the greatest impact on global temperatures and climate patterns. When the concentration of atmospheric carbon dioxide increases, more longwave radiation is absorbed and more heat is retained.

greenhouse effectFigure 4.1.4a – The greenhouse effect

Not all the infrared energy is absorbed by the Earth. Some is reflected into the atmosphere, where it is trapped as greenhouse gases. When concentrations of atmospheric carbon dioxide increase, more heat is reflected back from the atmosphere to the surface.

Atmospheric concentrations of carbon dioxide are rising primarily as a result of combustion of fossil fuels. Most scientists agree that increasing carbon dioxide levels are the main reason for the increase of average global temperatures over the last 200 years.

carbon dioxide and temperatureFigure 4.1.4b – Carbon dioxide and temperature
There is a positive correlation between increasing atmospheric carbon dioxide concentrations and average global temperatures.

Consequences of ocean acidification

Carbon dioxide released by burning fossil fuels diffuses into the oceans. Some of the carbon dioxide gas dissociates in water to form weak carbonic acid. The result is that, along with atmospheric temperatures, the acidity of the oceans is also increasing.

CO2 (aq) + H2O ← → H2CO3 ← →  H+ +  HCO3-

Carbon dioxide + water ← → Carbonic acid ← → Hydrogen ions + Hydrogen carbonate ions

Increasing ocean temperatures also contribute to ocean acidification. Recall from Figure 4.1.3b that deep oceans are a significant store of carbon dioxide. As temperatures rise, carbon dioxide becomes less soluble in water, so more gas is released from the cold, deep ocean into the warm, shallow ocean.

solubility of CO2 in waterFigure 4.1.4c – Solubility of CO2 in water at different temperatures

Increasing ocean acidification has disastrous consequences for shallow-water corals and other shelled animals because carbonic acid dissolves calcium carbonate.

dissolving shellfishFigure 4.1.4d – Dissolving shellfish1
Corals, crustaceans and molluscs are all at risk because of ocean acidification

Coral reef ecosystems are especially vulnerable to small changes in the environment because they have a very small zone of tolerance for temperature and pH. Ocean acidification has been implicated as a cause of coral reef bleaching, when normally very colourful coral reef formations appear white or very pale. The change in colour is also associated with changes in mass and productivity.

coral bleachingFigure 4.1.4e – Coral reef bleaching

Corals normally live in symbiotic relationships with colourful algae. When the temperature or acidity is disturbed, corals expel the algae and appear white. Corals can survive a bleaching event. Usually, they episodes are short-lived, but are very stressful on the organisms involved.

Study tip

There is a layer of stratospheric ozone that protects the Earth from about 97% to 99% of ultraviolet radiation from the Sun. The ozone layer is shrinking due to human activities. This phenomenon, though important for other biological reasons, is not related to climate change.

Language tool

Longwave radiation is also called infrared radiation.

Analysing data

  • Outline the trend shown in Figure 4.1.4a.
  • Discuss whether this correlation is evidence that increasing carbon dioxide concentration is a cause of rising global temperatures  

positive feedback loopFigure 4.1.4f – A positive feedback loop

Consider this

When surface temperatures increase, more water evaporates. Water vapour is also a greenhouse gas, so temperatures increase again. This is an example of positive feedback – one result causes the amplification of the same result. The relationship between ocean acidification and rising ocean temperatures is another example of positive feedback.

International mindedness

Greenhouse gases are released on a small scale but are a global problem. Consider how international agreements, like the Kyoto Protocol or the UNFCCC (United Nations Framework Convention on Climate Change), make an impact. 

crabFigure 4.1.4g – Crab
clamsFigure 4.1.4h – Clams
In addition to coral, other hard-shelled animals such as crabs and clams are at risk from increasing ocean acidification.

Did you know?

Scientists predict there will be more frequent coral bleaching events in the future.

Further reading

Evaluating claims: scientists have also implicated changes in sun exposure, certain chemicals and human disturbance as causes of coral bleaching. To learn more about bleaching, see:
Sapp, J. (2003) What is Natural? Coral Reef Crisis. New York: Oxford University Press.


  1. Figure 4.1.4d from "Ocean Acidification: Global Warming's Doppelgänger." Seattle Magazine