Structure of an ecosystem
Biotic (living) components:
- Producers: green plants
- Consumers: animals
- Decomposers: fungi and bacteria
Abiotic (non-living) components: sunlight, water, soil, temperature and air.
Stratification: plants grow in vertical layers. In a forest: tall trees (top) → shrubs → herbs and grasses (bottom).
Ecosystems can be terrestrial (forest, grassland, desert) or aquatic (pond, lake, sea). They can also be man-made, like a crop field or an aquarium.
Productivity: how much food plants make
| Term | Meaning |
|---|---|
| Primary production | Organic matter (biomass) made by plants through photosynthesis, per unit area over a time period |
| GPP (Gross Primary Productivity) | Total food made by photosynthesis |
| NPP (Net Primary Productivity) | Food left over after the plants use some for respiration: NPP = GPP − R |
| Secondary productivity | New organic matter made by consumers |
Yearly NPP (organic matter, dry weight):
| Region | NPP (billion tons per year) |
|---|---|
| Land | ~115 |
| Oceans | ~55 |
| Whole Earth | ~170 |
Decomposition: nature's recycling
- Fragmentation: detritivores (earthworms) break detritus into small pieces.
- Leaching: water-soluble nutrients sink into the soil.
- Catabolism: bacterial and fungal enzymes break it down chemically.
- Humification: dark, spongy humus forms (it decays very slowly).
- Mineralisation: humus slowly releases inorganic nutrients.
| Decomposition is FAST when... | Decomposition is SLOW when... |
|---|---|
| Detritus is rich in nitrogen and sugars (water-soluble substances) | Detritus is rich in lignin and chitin |
| Warm and moist | Cold |
| Oxygen is available (aerobic) | No oxygen (anaerobic) |
Energy flow
- The Sun is the only source of energy for almost all ecosystems (except deep-sea hydrothermal vent ecosystems).
- Less than 50% of sunlight is PAR (photosynthetically active radiation, the light plants can use).
- Plants capture only 2–10% of PAR. But this small amount runs the whole living world!
- Energy flow is one-way (unidirectional): Sun → producers → herbivores → carnivores. It never flows backwards.
The 10% law (Lindeman)
Only about 10% of energy passes to the next level. The rest is lost as heat (respiration) at each step.
| Trophic level | Energy (example) |
|---|---|
| Producers | 10,000 J |
| Herbivores | 1,000 J |
| Primary carnivores | 100 J |
| Secondary carnivores | 10 J |
Food chains and food webs
| Grazing food chain (GFC) | Detritus food chain (DFC) |
|---|---|
| Starts with living plants | Starts with dead organic matter |
| Grass → goat → man | Dead leaves → earthworms → birds |
| In land ecosystems, more energy flows through the DFC than the GFC |
- A food web is many food chains linked together, which is more realistic.
- Standing crop = the mass of living material at each level at a given time.
Trophic levels:
- 1st level: producers
- 2nd level: primary consumers (herbivores)
- 3rd level: secondary consumers (carnivores)
- 4th level: tertiary consumers (top carnivores)
Ecological pyramids
| Pyramid of | Usually | Exception |
|---|---|---|
| Energy | Always upright | Never inverted |
| Number | Upright | Inverted in a tree ecosystem (1 tree → many insects → fewer birds) |
| Biomass | Upright | Inverted in the sea (small biomass of phytoplankton supports a large biomass of fish) |
Limitations of ecological pyramids:
- They don't show a species that belongs to 2 or more trophic levels.
- They assume a simple food chain, not a food web.
- They don't include saprophytes (decomposers), even though decomposers are very important.
Ecological succession
- Sere = the whole sequence of communities. Each stage is a seral stage (seral community).
| Primary succession | Secondary succession |
|---|---|
| Starts where no life existed before: bare rock, a new pond, cooled lava | Starts where life was destroyed: abandoned farmland, a burnt or cut forest, a flooded area |
| No soil at the start | Soil already present |
| Very slow (hundreds to thousands of years) | Faster |
Succession on land (xerarch) vs in water (hydrarch)
- Xerarch (on bare rock): lichens (pioneers; they secrete acids that break the rock) → mosses (bryophytes) → herbs → shrubs → forest (climax).
- Hydrarch (in water): phytoplankton (pioneers) → rooted submerged plants → floating plants → reed swamp → marsh meadow → scrub → forest (climax).
30-second revision
- NPP = GPP − R. Earth's NPP ≈ 170 billion tons; oceans ≈ 55 billion tons.
- Decomposition: fragmentation → leaching → catabolism → humification → mineralisation.
- Only 2–10% of PAR is captured; energy flow is unidirectional; 10% law.
- GFC starts with living plants; DFC with detritus.
- Energy pyramid is always upright. Biomass pyramid is inverted in the sea; number pyramid is inverted for a tree.
- Primary succession: no soil, slow; secondary: soil present, faster.
- Pioneers: lichens (xerarch), phytoplankton (hydrarch). Both end in a mesic climax.