Q: Explain the 10% rule and why food chains rarely exceed 4-5 levels.
A: At each trophic level, only about 10% of energy passes to the next level. The other 90% is lost as heat through cellular respiration, waste products, and movement energy. Starting with 10,000 kcal of plant energy: primary consumers get 1,000 kcal, secondary consumers get 100 kcal, tertiary consumers get just 10 kcal. This is why large carnivores are rare and ecosystems can support far more plant-eaters than meat-eaters. It also explains why eating lower on the food chain (e.g. plant-based diet) is more energy-efficient for the planet.
Q: What are the HIPPO threats to biodiversity, and which is most significant?
A: HIPPO: Habitat loss (H) — by far the leading cause, as species need habitat to survive; Invasive species (I) — outcompete, predate, or introduce disease to native species; Pollution (P) — DDT, mercury, plastic, nitrogen runoff; Population growth (P) — human population amplifies all other threats by requiring more land, resources, and waste absorption; Overharvesting (O) — fishing, hunting, logging beyond sustainable levels. Habitat loss accounts for roughly 70-85% of species at risk. The current extinction rate is 100-1000x the natural background rate, suggesting a 6th mass extinction is underway.
Q: Compare exponential (J-curve) and logistic (S-curve) population growth.
A: Exponential growth: population grows at a constant rate per individual — the more individuals, the faster it grows. Produces a J-shaped curve. Occurs when resources are unlimited (introduced species, bacteria in fresh media). Cannot continue indefinitely. Logistic growth: growth rate slows as population approaches carrying capacity (K) — the maximum population the environment can sustainably support. Produces an S-shaped curve. Regulated by density-dependent factors (food competition, disease, predation) that intensify as population grows. Most real populations show logistic growth with fluctuations.
Q: What is the CHNOPS mnemonic and why are biogeochemical cycles important?
A: CHNOPS = Carbon, Hydrogen, Nitrogen, Oxygen, Phosphorus, Sulfur — the six essential elements making up all living matter that cycle between organisms and the environment. Key cycles: Carbon cycle (photosynthesis fixes CO2; respiration and decomposition release it — disrupted by fossil fuels). Nitrogen cycle (N2 fixed by bacteria → NH3 → NO3- → taken up by plants → returned by decomposition; denitrification returns N2 to atmosphere). Phosphorus cycle: no atmospheric phase — moves through rock weathering, soil, water, organisms. Disrupting cycles causes eutrophication, climate change, and soil degradation.
Q: What is the difference between a food chain and a food web, and what is a trophic cascade?
A: A food chain is a linear sequence of who eats whom (grass → rabbit → fox). A food web is a complex network of all feeding relationships in an ecosystem — more realistic since most species eat multiple things and are eaten by multiple predators. A trophic cascade occurs when changes to one trophic level cause effects that ripple through the food web. Classic example: wolf reintroduction to Yellowstone reduced elk overgrazing → willows and aspens recovered → beaver populations grew → streams changed → songbirds increased. Removing apex predators (overfishing sharks, hunting wolves) destabilizes entire ecosystems.