Energy flows, matter cycles
Two big rules govern ecosystems:
- Energy flows in one direction: from the Sun, through living things, and out as heat. It cannot be recycled.
- Matter cycles: atoms of carbon, nitrogen, water and other elements are reused again and again.
Energy flow through trophic levels
| Trophic level | Organisms | How they get energy |
|---|---|---|
| Producers (autotrophs) | Plants, algae, cyanobacteria | Photosynthesis (or chemosynthesis) |
| Primary consumers | Herbivores | Eat producers |
| Secondary consumers | Carnivores, omnivores | Eat herbivores |
| Tertiary consumers | Top predators | Eat other carnivores |
| Decomposers / detritivores | Fungi, bacteria, earthworms | Break down dead matter |
Primary productivity
- Gross primary productivity (GPP): total energy captured by producers.
- Net primary productivity (NPP): GPP minus energy producers use in their own respiration. NPP is what’s available to consumers.
Primary productivity is highest in tropical rainforests, coral reefs, estuaries and wetlands, and lowest in deserts and the open ocean (per unit area). However, the open ocean is so vast that it contributes about half of Earth’s total NPP.
The 10% rule
Ecological efficiency is typically about 10%, though it ranges from about 5% to 20%. Energy is lost at each step because:
- not all of an organism is eaten (bones, roots)
- not all that is eaten is digested
- most absorbed energy is used for respiration and lost as heat
This explains why:
- food chains rarely exceed 4–5 levels
- top predators are rare
- it takes far more land to produce meat than plant foods
Ecological pyramids
| Pyramid | Shows | Always upright? |
|---|---|---|
| Energy | Energy at each level | Yes |
| Biomass | Mass of living material | Usually (can be inverted in oceans, where fast-reproducing phytoplankton are eaten quickly) |
| Numbers | Number of organisms | No (one tree can support thousands of insects) |
Biomagnification
Some toxins, such as mercury and DDT, are not broken down and build up in fatty tissues. Their concentration increases at each trophic level. DDT thinned the eggshells of eagles and peregrine falcons before it was banned or heavily restricted in Canada and the U.S. in the 1970s
Biogeochemical cycles
A biogeochemical cycle moves an element between living things (bio), rocks and soil (geo) and the air and water (chemical).
The water cycle
Evaporation → condensation → precipitation → runoff and infiltration → back to oceans. Plants add water vapour by transpiration.
The carbon cycle
| Process | Effect on atmospheric CO₂ |
|---|---|
| Photosynthesis | Removes CO₂ |
| Cellular respiration | Adds CO₂ |
| Decomposition | Adds CO₂ |
| Combustion of fossil fuels and wood | Adds CO₂ |
| Ocean absorption | Removes CO₂ (but makes the ocean more acidic) |
| Formation of limestone and fossil fuels | Long-term storage |
Atmospheric CO₂ has risen from about 280 ppm before the Industrial Revolution to more than 420 ppm today, mainly from burning fossil fuels.
The nitrogen cycle
Nitrogen makes up about 78% of the air, but most organisms can’t use N₂ gas directly.
| Step | Process | Who does it |
|---|---|---|
| Nitrogen fixation | N₂ → ammonia (NH₃) | Bacteria in soil and in legume root nodules; lightning; the industrial Haber–Bosch process |
| Nitrification | Ammonia → nitrites → nitrates | Nitrifying bacteria |
| Assimilation | Plants absorb nitrates and build proteins | Plants |
| Ammonification | Dead matter and waste → ammonia | Decomposers |
| Denitrification | Nitrates → N₂ gas | Denitrifying bacteria |
The phosphorus cycle
Phosphorus has no significant gas phase. It cycles slowly from rocks to soil, to living things, to water and sediments, and back to rocks over millions of years. Phosphorus is needed for DNA, ATP and cell membranes.
Human impacts
- Burning fossil fuels adds carbon faster than natural sinks can remove it, driving climate change.
- Fertilizer runoff adds nitrogen and phosphorus to lakes and coastal waters, causing eutrophication: algal blooms die, decomposers use up oxygen, and "dead zones" form. Lake Erie and the Gulf of Mexico experience this regularly.
- Deforestation reduces carbon storage and disrupts the water cycle.
Common mistakes
- Saying energy is recycled. Matter is recycled; energy flows through and is lost as heat.
- Thinking plants take nitrogen from the air. They rely on bacteria to fix it.
- Thinking all pyramids are upright.
Try it yourself
- Producers in a meadow capture 50 000 kJ. Using the 10% rule, how much energy reaches tertiary consumers?
- Trace one carbon atom from a car’s exhaust to a rabbit’s muscle.
- Explain why farmers grow beans or clover to improve soil.
(Answers: 50 kJ; exhaust CO₂ → grass via photosynthesis → glucose → rabbit eats grass → muscle protein/glycogen; legumes host nitrogen-fixing bacteria)
Quick recap
- Energy flows one way and about 10% passes between trophic levels.
- Pyramids of energy are always upright; biomass and number pyramids may not be.
- Matter cycles through biogeochemical cycles: water, carbon, nitrogen and phosphorus.
- Nitrogen fixation by bacteria makes nitrogen usable to living things.
- Human activities alter these cycles, causing climate change and eutrophication.