ACT Science: Biodiversity and Ecosystem Stability—Food Webs and Energy Flow

Biodiversity is the variety of species in an ecosystem. Ecosystems with high biodiversity are generally more stable and resilient. Why? (1) Redundancy: If one species declines, others can fill its role. (2) Complexity: More species create more food web connections, buffering against disruption. (3) Genetic diversity: More genes in a population mean better adaptation. On the ACT, you won't calculate biodiversity precisely, but you'll understand: (1) How organisms interact in food webs, (2) How energy flows from producers (plants) to consumers (animals), (3) How ecosystem changes (species loss, invasive species) affect stability. Example: If a forest loses 50% of its tree species, food sources shrink, herbivores decline, predators suffer. But if 5 tree species remain and they can all feed herbivores, the ecosystem may recover faster. High biodiversity = more stable ecosystem. Low biodiversity = fragile ecosystem vulnerable to collapse.

Energy flow rule: Only ~10% of energy transfers from one trophic level to the next. Producers have 10,000 units; herbivores get 1,000; carnivores get 100. This is why ecosystems support fewer top predators than herbivores.

Misconception 1: Ecosystems are in perfect balance. False. They're dynamic; balance is temporary. Misconception 2: All species are equally important. False. Keystone species (like sea otters) have outsized impact; losing them destabilizes ecosystems. Misconception 3: Removing a species only affects its immediate predators/prey. False. Food web effects ripple through the ecosystem. Remove plants → herbivores starve → carnivores starve. Misconception 4: Biodiversity always means more species. False. It means variety across trophic levels, genetic diversity, and habitat diversity. Ten species of herbivores aren't as valuable as one herbivore, one carnivore, one plant species. Misconception 5: Invasive species never integrate. False. Some invasive species become part of the ecosystem; the issue is rapid, destabilizing change. Remember: Ecosystems are interconnected webs, not independent chains.

Checklist: (1) Identify the food web. (2) Trace energy flow (who eats whom). (3) Predict impact of change (species loss, invasion). (4) Consider ecosystem stability (diverse = stable).

Ecosystem 1: Rainforest with 1000+ species (very high biodiversity). Stability: High. Reason: Massive redundancy; loss of one species usually doesn't collapse food webs. Ecosystem 2: Monoculture (single crop, zero wild diversity). Stability: Very low. Reason: Pest outbreaks can wipe out entire harvest; no natural predators to control pests; no genetic variation. Ecosystem 3: Temperate forest with 50 tree species, multiple herbivores and carnivores. Stability: Moderate-high. Reason: Reasonable diversity; if one tree declines, herbivores shift to others. Ecosystem 4 (hypothetical): What if sea otters disappear from a kelp forest? Sea urchin population explodes (predator removed). Urchins eat kelp. Kelp forest collapses. Fish dependent on kelp habitat vanish. Predators dependent on fish starve. Answer: Losing one keystone species cascades through the ecosystem. For each scenario, rate stability (high/moderate/low) and explain how biodiversity supports stability.

Daily drill: Choose a local ecosystem. List species. Create a simple food web. Predict impact of removing one species. Repeat for different species.

About 2-3 ecology questions per ACT Science section involve food webs, energy flow, or biodiversity. These test whether you understand ecosystems as interconnected systems, not just collections of species. If you grasp food web concepts and cascade effects, you answer these confidently. Students who think systemically about ecosystems score 2-3 points higher on ecology questions because they predict ripple effects others miss.

Spend 2-3 days on ecosystem ecology. Draw food webs, trace energy, predict consequences of change. By test day, ecosystem questions will feel manageable and you'll demonstrate sophisticated biological thinking.