60-second revision
- 'Ecosystem' coined by A. G. Tansley (1935); 'ecology' by Ernst Haeckel.
- Only about 10% of energy passes to the next trophic level (Lindeman's 10 percent law).
- Pyramid of energy is always upright; biomass can be inverted in the sea; numbers can be inverted on a tree.
- Lichens are pioneers on bare rock; succession ends in a climax community.
- Ecotone = transition zone between ecosystems; edge effect = more species there.
- Biomagnification: DDT and mercury concentrate up the food chain.
- Rhizobium fixes nitrogen in legume root nodules; denitrifying bacteria return it to the air.
What an ecosystem is
An ecosystem is a functional unit of nature in which living organisms interact with each other and with their physical surroundings. The term was coined by A. G. Tansley in 1935. Ecosystems range from a small pond to a forest or ocean; the largest natural units of similar climate and vegetation, like the tundra, taiga or tropical rainforest, are called biomes.
| Component | Includes |
|---|---|
| Abiotic | Sunlight, temperature, water, soil, air, minerals |
| Producers (autotrophs) | Green plants, algae, phytoplankton — make food by photosynthesis |
| Consumers (heterotrophs) | Herbivores (primary), carnivores (secondary, tertiary), omnivores |
| Decomposers | Bacteria and fungi — break down dead matter and release nutrients |
Food chains, webs and energy flow
A food chain shows who eats whom; interlinked chains form a food web, which makes an ecosystem more stable. The grazing food chain begins with green plants (grass > grasshopper > frog > snake > eagle). The detritus food chain begins with dead organic matter and is very important in forests, where most energy flows through decomposers.
Energy flows in one direction — from the Sun to producers to consumers — and is lost as heat at every step; it is not recycled. Nutrients, in contrast, are recycled. According to Lindeman's 10 percent law, only about 10% of the energy at one trophic level is passed on to the next. For example, if plants fix 50,000 kJ, herbivores get about 5,000 kJ, primary carnivores about 500 kJ and secondary carnivores about 50 kJ. This is why food chains rarely have more than four or five links.
Ecological pyramids
| Pyramid | Shape | Exception |
|---|---|---|
| Energy | Always upright | None — energy always decreases upward |
| Numbers | Usually upright | Inverted in a tree ecosystem (one tree, many insects, many more parasites) |
| Biomass | Usually upright on land | Inverted in the ocean, where a small standing crop of phytoplankton supports a larger mass of zooplankton and fish |
Productivity
Gross primary productivity (GPP) is the total rate of photosynthesis; net primary productivity (NPP) is GPP minus what plants use in respiration, i.e. what is available to consumers. Tropical rainforests, coral reefs and estuaries are among the most productive ecosystems; deserts and the open ocean are among the least productive per unit area.
Ecological succession
- Primary succession begins on a surface with no soil, such as bare rock or new lava. Lichens are the pioneers, followed by mosses, herbs, shrubs and finally trees.
- Secondary succession occurs where a community has been removed but soil remains — after a fire, flood or abandoned farm — and is faster.
- The final, relatively stable community in balance with the climate is the climax community.
- Hydrarch succession starts in water (pond to forest); xerarch starts in dry areas (rock to forest).
Key terms
| Term | Meaning |
|---|---|
| Habitat | The place where an organism lives |
| Niche | The functional role of a species — what it eats, when it is active, how it uses resources |
| Ecotone | Transition zone between two ecosystems, e.g. mangroves, estuaries, forest–grassland edge; shows the edge effect |
| Keystone species | Has a disproportionately large effect on its community, e.g. top predators |
| Flagship species | Charismatic species used to attract support for conservation, e.g. tiger, giant panda |
| Indicator species | Reveals environmental conditions, e.g. lichens are sensitive to air pollution |
| Biomagnification | Rising concentration of non-degradable toxins (DDT, mercury) at higher trophic levels |
Biogeochemical cycles
Nutrients move between living and non-living parts of the ecosystem. In the nitrogen cycle, atmospheric nitrogen is fixed by lightning and by bacteria — free-living ones like Azotobacter and symbiotic ones like Rhizobium in legume root nodules; ammonia is converted to nitrites and nitrates (nitrification); and denitrifying bacteria return nitrogen to the air. In the carbon cycle, plants absorb CO2 in photosynthesis, while respiration, decomposition and burning of fossil fuels release it. Oceans and forests act as major carbon sinks. Phosphorus has a sedimentary cycle with no significant gaseous phase.
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