Ecology and Evolution Review

Trophic Levels

1. Producers (autotrophs): Plants, algae — convert sunlight to chemical energy
2. Primary consumers (herbivores): Eat producers
3. Secondary consumers (carnivores): Eat herbivores
4. Tertiary consumers: Top predators
5. Decomposers: Break down dead organisms, recycle nutrients

Energy Transfer

• Only about 10% of energy is passed to the next trophic level
• The rest is lost as heat through respiration
• This is why food chains rarely exceed 4-5 levels
• Energy pyramid: Producers have the most energy; top predators have the least

Biogeochemical Cycles

• Carbon cycle: Photosynthesis (removes CO₂) ↔ Respiration (releases CO₂); fossil fuels; decomposition
• Nitrogen cycle: N₂ fixation → ammonium → nitrites → nitrates → assimilation by plants; denitrification returns N₂
• Water cycle: Evaporation → condensation → precipitation → collection

Population Growth Models

• Exponential growth: \(\frac{dN}{dt} = rN\) — unlimited resources, J-shaped curve
• Logistic growth: \(\frac{dN}{dt} = rN\left(\frac{K-N}{K}\right)\) — limited resources, S-shaped curve
  • \(K\) = carrying capacity
  • Growth rate is highest at \(N = K/2\)

Factors Affecting Population Size

• Density-dependent: Competition, predation, disease (effects increase with population density)
• Density-independent: Natural disasters, weather, human activities

Species Interactions

• Competition (−/−): Both species harmed
• Predation (+/−): Predator benefits, prey harmed
• Mutualism (+/+): Both benefit (e.g., bees and flowers)
• Commensalism (+/0): One benefits, other unaffected
• Parasitism (+/−): Parasite benefits, host harmed

Darwin's Four Postulates

1. Variation exists within populations
2. Some variation is heritable
3. More offspring are produced than can survive (overproduction)
4. Individuals with favourable traits have higher fitness (survival and reproduction)

Evidence for Evolution

• Fossil record: Shows gradual changes over time
• Comparative anatomy: Homologous structures (same origin, different function)
• Molecular biology: DNA/protein sequence similarities
• Biogeography: Distribution of species across regions

Types of Selection

• Directional: Favours one extreme phenotype
• Stabilising: Favours intermediate phenotype
• Disruptive: Favours both extremes

Q: A food web shows: Grass → Grasshopper → Frog → Snake → Hawk. If a disease kills most frogs, predict the short-term effect on grasshopper and snake populations.

A:

• Grasshoppers: Population would increase because their main predator (frogs) is reduced
• Snakes: Population would decrease because their food source (frogs) is reduced
• Secondary effects: Increased grasshoppers may reduce grass; snakes may switch to other prey if available

• Energy flows through trophic levels with ~10% efficiency per level
• Population growth follows exponential or logistic models depending on resources
• Species interact through competition, predation, mutualism, commensalism, and parasitism
• Natural selection requires variation, heritability, overproduction, and differential fitness
• Be prepared to trace effects through food webs and predict population changes