Energy Flows in an Ecosystem (Cambridge (CIE) IGCSE Environmental Management): Revision Note

Energy flow in an ecosystem

• All living organisms need energy to survive

• This energy originally comes from the Sun

• Plants capture sunlight during photosynthesis and turn it into chemical energy stored in glucose

• Animals get energy by eating plants or other animals

• Energy flow shows how energy moves

  • from the Sun → to producers → to consumers → to apex predators

• As energy moves through the ecosystem, some is always lost, which is why food chains never have endless levels

Food chains

A food chain is a simple model showing how energy passes from one organism to the next

• Example:

sun → grass → mouse → owl

• The grass is the producer

• The mouse is the primary consumer

• The owl is the secondary consumer

Food chains show the direction of energy transfer, not who eats the most

Food webs

• In reality, many animals eat more than one type of organism, and food chains combine to make food webs

• Food webs show:

  • All feeding relationships in an ecosystem

  • How energy moves through many species

  • How one change affects the whole ecosystem

  If one species disappears:

  • Prey populations may increase

  • Predator populations may fall

  • The whole food web becomes unbalanced

Trophic levels

• A trophic level is a feeding level in a food chain

  • If multiple organisms occupy the same position in a food chain, they are in the same trophic level

• Trophic levels:

  • Producers

  • Primary consumers

  • Secondary consumers

  • Tertiary consumers

  • Apex predators

• Each step up the chain gets less energy, so top predators are fewer in number

• Producers capture solar energy:

  • Producers (e.g., plants, algae, phytoplankton) convert solar energy into chemical energy through photosynthesis

  • Example: A grassland ecosystem where grass (producers) captures sunlight to grow

• Consumers transfer energy:

  • Primary consumers (herbivores) eat producers

  • Secondary and tertiary consumers eat other consumers, transferring energy upward in the food chain

  • Example: Earthworms decompose organic matter in forests, enriching the soil

The 10% rule

• The 10% rule explains how energy decreases as it moves through trophic levels in an ecosystem

  • Roughly 10% of energy is transferred from one trophic level to the next

  • For example, if a producer captures 1,000 kcal of energy, only 100 kcal is available to primary consumers

• So much energy is lost because of:

  • movement

  • respiration

  • digestion

  • heat loss

  • excretion

  • not all parts of an organism are eaten

• Because so much energy is lost:

  • Food chains are usually short

  • Apex predators need large spaces

  • There are more producers than consumers

• Energy pyramids show how total energy decreases at each trophic level

  • The base (producers) is widest

  • Each level above is narrower

  • The top has the least energy and the fewest organisms

• This helps explain why apex predators are rare

Ecological pyramids

• Ecological pyramids are diagrams that show how living things are organised in an ecosystem

• They help visualise patterns that can be hard to see in food chains

• Pyramids represent different types of information:

  • The number of organisms

  • The amount of energy available

• All pyramids show that producers form the widest base, because they capture sunlight and bring new energy into the system

• As you go up the pyramid, there is less energy and usually fewer organisms, because energy is lost at every trophic level

Why ecological pyramids are important

• They show how energy decreases through a food chain

• They help predict how ecosystems change if one level is affected

• They show why top predators are fewer in number

• They reveal the structure and stability of an ecosystem

Pyramids of numbers

• A pyramid of numbers shows how many organisms exist at each level of a food chain

• The width of the box indicates the number of organisms at that trophic level

• For example, consider the following food chain:

grass → vole → owl

• A pyramid of numbers for this food chain would look like the one shown below

  • Often, the number of organisms decreases along food chains, as there is a decrease in available energy since some energy is lost to the surrounding environment at each trophic level

  • Therefore, pyramids of numbers usually become narrower towards the apex (the top)

• Despite the name, a pyramid of numbers doesn’t always have to be pyramid-shaped

• For example, consider the following food chain:

oak tree → insects → woodpecker

• The pyramids of numbers for this food chain will display a different pattern to the first food chain

• When individuals at lower trophic levels are relatively large, like the oak tree, the pyramid becomes inverted or distorted

  • Only a single oak tree is needed to support large numbers of insects (which can then support large numbers of woodpeckers)

Pyramids of energy

• Shows the energy available at each trophic level

• Always measured over a period of time, for example, kJ per m² per year

  • The length of each box, or bar, represents the quantity of energy present

• These pyramids of energy are always pyramid-shaped

• Energy always decreases as you go up the trophic levels

• This is the most accurate way of showing ecosystem structure

• This supports the 10% rule of energy transfer

• Pyramids of energy never invert because energy is always lost through:

  • respiration

  • movement

  • heat

  • digestion

  • excretion

  • uneaten parts

• No feeding level can ever have more energy than the level below it

Respiration in an ecosystem

• Aerobic respiration is a chemical process that releases energy from glucose when oxygen is available

• It happens in the cells of plants, animals, fungi and many bacteria

• Respiration is not the same as breathing

  • Breathing brings oxygen into the body, but respiration is the chemical reaction that uses oxygen to release energy

• All organisms need energy for:

  • movement

  • growth

  • repair

  • keeping warm

  • staying alive

• Without respiration, cells cannot work and organisms cannot survive

• Respiration links to photosynthesis because:

  • Photosynthesis stores energy

  • Respiration releases energy

• Together, they form the core cycle that supports all life on Earth

The equation for respiration

glucose + oxygen → carbon dioxide + water ( + energy)

Where aerobic respiration happens

• Respiration happens inside cells

• You do not need to know the names of cell structures, but you should understand that:

  • Every living cell respires

  • Respiration releases energy that the cell can use

  • Plants respire all the time, even at night

  • Plants still need oxygen for respiration

• Respiration is happening 24 hours a day, even when photosynthesis stops in the dark

Why organisms need energy

• Organisms use the energy released by aerobic respiration for:

  • Movement such as running, swimming or flying

  • Active transport inside cells

  • Building new cells for growth

  • Repairing damaged tissues

  • Keeping a stable body temperature in mammals and birds

• This explains why animals need large amounts of food and oxygen — they must release lots of energy daily

Respiration in ecosystems

• Respiration affects ecosystem structure because:

  • It controls how much energy organisms can use from food

  • It reduces energy available to the next trophic level

  • It explains why only about 10% of energy is passed on

  • It helps drive the carbon cycle because it releases carbon dioxide

• Respiration is one of the main reasons food chains are short

How respiration links to the carbon cycle

• Respiration plays a vital role because:

  • Organisms take in carbon as glucose when they feed

  • They release carbon dioxide back into the atmosphere when they respire

  • This carbon dioxide is later used by plants in photosynthesis

• Respiration and photosynthesis form a continuous loop that keeps carbon cycling through the ecosystem

Aerobic respiration vs anaerobic respiration

• You only need to know aerobic respiration for this syllabus, but it helps to understand why oxygen is essential

• Aerobic respiration:

  • Uses oxygen

  • Releases large amounts of energy

  • Produces carbon dioxide and water

• Anaerobic respiration:

  • Happens without oxygen

  • Releases much less energy

  • Produces harmful waste products

• This explains why organisms prefer aerobic respiration whenever possible