Ecology and Ecosystem Dynamics
Grade 12 · Science · Study Guide
Understand how living organisms interact with each other and their environment, and how energy and matter flow through ecosystems.
Ecology is the scientific study of interactions between organisms and their environment. Ecosystems consist of biotic (living) components and abiotic (non-living) components like temperature, water, and nutrients. Energy flows through ecosystems in one direction — from the sun through producers (autotrophs like plants) to consumers (heterotrophs) at successive trophic levels. Each level of the food chain transfers only about 10% of energy to the next (the 10% rule), which explains why ecosystems can support far more plants than large carnivores.
Biogeochemical cycles describe how matter (carbon, nitrogen, water, phosphorus) cycles repeatedly through the biotic and abiotic components of ecosystems. The carbon cycle includes photosynthesis (CO₂ → organic carbon), respiration (organic carbon → CO₂), and the burning of fossil fuels (releasing stored carbon). The nitrogen cycle converts atmospheric nitrogen (N₂) into biologically usable forms through nitrogen fixation by bacteria, nitrification, assimilation, and denitrification.
Population ecology examines how populations grow and are regulated. Exponential growth (J-curve) occurs when resources are unlimited. Logistic growth (S-curve) occurs when populations approach the carrying capacity (K) — the maximum population size an environment can sustain. Limiting factors can be density-dependent (competition, predation, disease — stronger effects at high density) or density-independent (floods, fires, temperature extremes). Community ecology studies interactions between species: predation, competition, mutualism, commensalism, and parasitism.
| Interaction type | Species A | Species B | Example |
|---|---|---|---|
| Mutualism | + | + | Bees pollinate flowers; both benefit |
| Commensalism | + | 0 | Barnacles on whales; whale unaffected |
| Parasitism | + | − | Tapeworm in host; host harmed |
| Predation | + | − | Lion eats zebra |
| Competition | − | − | Two plant species competing for sunlight |
Key terms
- Trophic level
- A position in a food chain; producers are level 1, herbivores level 2, primary carnivores level 3, etc.
- 10% rule
- Only about 10% of energy is transferred from one trophic level to the next; 90% is lost as heat.
- Carrying capacity (K)
- The maximum population size that an environment can sustain indefinitely given available resources.
- Nitrogen fixation
- Conversion of atmospheric N₂ into ammonia (NH₃) by bacteria; makes nitrogen available to plants.
- Keystone species
- A species that has a disproportionately large effect on its ecosystem relative to its abundance.
- Ecological succession
- The gradual process by which ecosystems change and develop over time, often after a disturbance.
- Mutualism
- A symbiotic relationship where both species benefit (e.g., bees and flowers, mycorrhizal fungi and roots).
Flashcards
- 10% rule (energy transfer)
- ~10% of energy passes to next trophic level; ~90% lost as heat through respiration and metabolism
- Carrying capacity (K)
- Maximum sustainable population size in an environment; population growth slows as it approaches K
- Nitrogen cycle
- Fixation (N₂→NH₃) → nitrification (NH₃→NO₃⁻) → assimilation → denitrification (NO₃⁻→N₂)
- Keystone species
- Species with outsized ecological impact; removal causes major ecosystem disruption (e.g., sea otters, wolves)
- Primary succession
- Ecological succession on bare rock or newly exposed land with no soil; begins with pioneer species
- Secondary succession
- Succession after disturbance where soil remains; faster than primary succession
Practice questions
If a plant contains 1,000 kcal of energy, how much energy is typically available to a secondary consumer?
- 1,000 kcal
- 100 kcal
- 10 kcal
- 1 kcal
Show answer
Answer: 10 kcal
Applying the 10% rule twice: plants (1,000 kcal) → primary consumers (100 kcal) → secondary consumers (10 kcal). Each transfer loses ~90% as metabolic heat.
Carrying capacity is the minimum population size an ecosystem can support.
Show answer
Answer: False
Carrying capacity (K) is the MAXIMUM population size that an environment can sustain, not the minimum. Population growth slows as the population approaches K.
The conversion of atmospheric nitrogen to ammonia by bacteria is called nitrogen ___.
Show answer
Answer: fixation
Nitrogen fixation by bacteria (like Rhizobium in legume root nodules) converts inert N₂ gas into ammonia (NH₃), which plants can absorb and use to build proteins.
Which relationship benefits one species while the other is unaffected?
- Mutualism
- Parasitism
- Commensalism
- Competition
Show answer
Answer: Commensalism
In commensalism, one species benefits (+) and the other is neither helped nor harmed (0). Example: barnacles attach to whales for transportation but don't affect the whale.
Abiotic factors in an ecosystem include sunlight, temperature, and water availability.
Show answer
Answer: True
Abiotic factors are non-living physical and chemical components of an ecosystem including sunlight, temperature, water, soil pH, salinity, and wind — all of which influence which organisms can survive there.
Fun facts
- When wolves were reintroduced to Yellowstone in 1995, they triggered a 'trophic cascade' — changing elk behavior, allowing riverside vegetation to recover, which stabilized riverbanks and even altered river flow.
- The Amazon rainforest produces its own rainfall — trees transpire so much water that they create the clouds that rain back on the forest.
- A single teaspoon of healthy soil contains more microorganisms than there are people on Earth.
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