Ecology and Ecosystems
How Organisms Interact With Their Environment
Energy Flow in Ecosystems
Ecosystems are structured by trophic levels:
- Producers (autotrophs): plants, algae — convert sunlight into chemical energy via photosynthesis
- Primary consumers: herbivores eating producers
- Secondary/tertiary consumers: carnivores
- Decomposers: fungi and bacteria breaking down dead organic matter, returning nutrients
Energy transfer between levels is inefficient — typically only ~10% of energy is passed up each trophic level (the rest lost as heat, metabolism, waste). This is why food chains rarely exceed 5 levels and why large carnivores are rare.
Ecological Interactions
Organisms interact in many ways:
- Predation: predator benefits (+), prey harmed (−)
- Competition: both harmed (−/−); interspecific (between species) or intraspecific (within species)
- Mutualism: both benefit (+/+): mycorrhizae, coral-algae
- Commensalism: one benefits, other unaffected (+/0)
- Parasitism: parasite benefits, host harmed (+/−)
Keystone species have disproportionately large effects on ecosystem structure relative to their abundance (e.g., sea otters controlling urchin populations, wolves in Yellowstone).
The 10% Rule
On average, only 10% of energy stored in one trophic level is available to the next. This limits food chain length and explains why: 10,000 kg of plant matter → ~1,000 kg herbivores → ~100 kg secondary consumers → ~10 kg top predators. Eating lower on the food chain is more energy-efficient.
Which ecological interaction is described as (+/+) — both species benefit?
Mutualism (+/+) benefits both parties. Examples: bees and flowers (pollination), mycorrhizal fungi and plant roots, clownfish and anemones.
Correct answer: Mutualism
Why do food chains rarely exceed 5 trophic levels?
The ~10% energy transfer rule means that by trophic level 5, only 0.01% of the original producer energy remains — insufficient to sustain a viable population.
Correct answer: Energy is lost at each level so little remains for higher consumers
Describe what a keystone species is and give one example of how its removal affects its ecosystem.
Keystone species maintain ecosystem balance far beyond what their numbers suggest.
Correct answer: A keystone species has a disproportionately large effect on ecosystem structure relative to its biomass. Example: When wolves were extirpated from Yellowstone, elk populations grew and overgrazed riparian vegetation, causing stream bank erosion, loss of beaver habitat, and reduced biodiversity. Wolf reintroduction (1995) reversed these effects — a 'trophic cascade'.