Levels of organisation
Ecology studies how organisms interact with each other and their environment. From small to large:
- Organism: one individual.
- Population: individuals of one species in an area.
- Community: all the populations of different species in an area.
- Ecosystem: the community plus the non-living (abiotic) factors such as water, soil and sunlight.
- Biosphere: all ecosystems on Earth.
Energy flow
Energy enters most ecosystems as sunlight and moves through trophic levels:
- Producers (autotrophs) capture energy by photosynthesis.
- Primary consumers (herbivores) eat producers; secondary consumers eat herbivores; and so on.
- Decomposers (fungi, bacteria) break down dead matter and return nutrients to the soil.
Energy flows one way and is lost as heat at every step. As a rule of thumb, only about 10 percent of the energy at one level reaches the next. That is why food chains are short and why there are fewer top predators than herbivores.
Worked example
A meadow's plants capture 10,000 kJ of energy. How much is available to a hawk that eats snakes, which eat mice, which eat plants?
- Plants (producers): 10,000 kJ
- Mice (primary consumers): about 1,000 kJ
- Snakes (secondary consumers): about 100 kJ
- Hawks (tertiary consumers): about 10 kJ
Only about 0.1 percent of the plant energy reaches the hawk. The numbers are approximations, but the pattern explains why eating lower on the food chain supports more people per hectare.
Population growth
- Exponential growth happens with unlimited resources: the population multiplies by a constant factor each generation, producing a J-shaped curve.
- Logistic growth is limited. Growth slows as the population nears the carrying capacity (K), the largest population the environment can sustain, giving an S-shaped curve.
Species interactions
- Competition (both harmed).
- Predation and herbivory.
- Mutualism: both benefit, as with bees and flowers.
- Commensalism: one benefits, the other is unaffected.
- Parasitism: one benefits at the host's expense.
Succession
After a disturbance, communities change in a predictable sequence. Primary succession starts on bare rock or new land with no soil, where pioneer species such as lichens begin soil formation. Secondary succession follows a disturbance, such as a fire, that leaves soil intact, and is faster.
Nutrient cycles
Unlike energy, matter is recycled. In the carbon cycle, producers take in CO₂, consumers and decomposers return it through respiration, and burning fossil fuels moves carbon stored for millions of years into the atmosphere as CO₂. The rising level of CO₂ is a main driver of the enhanced greenhouse effect.