Ecology practice games — free for AP Biology.
This unit covers population ecology, community ecology, ecosystems and biodiversity — essential concepts for AP Biology. Use our interactive study games to test your understanding, or review questions in traditional format below.
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This unit covers population ecology, community ecology, ecosystems and biodiversity — essential concepts for AP Biology. Use our interactive study games to test your understanding, or review questions in traditional format below.
Key Concepts Breakdown
1 Population Ecology
Students must understand how populations grow (exponential vs. logistic), the concept of carrying capacity (K), and the factors that regulate population size. You must be able to interpret and calculate population growth using r (intrinsic growth rate) and apply the logistic growth equation. Density-dependent and density-independent limiting factors are frequently tested.
Key Points
- Exponential growth: dN/dt = rN — occurs when resources are unlimited; produces a J-curve
- Logistic growth: dN/dt = rN[(K-N)/K] — growth slows as N approaches K; produces an S-curve
- Density-dependent factors (predation, disease, competition) intensify as population density increases; density-independent factors (storms, drought) do not
- Life history strategies: r-selected species (many offspring, little parental care) vs. K-selected species (few offspring, high parental investment)
A deer population of 200 lives in a habitat with K = 500 and r = 0.4. Calculate dN/dt.
Plug into the logistic equation: dN/dt = 0.4 × 200 × [(500 - 200)/500] = 0.4 × 200 × 0.6 = 48 deer/year. Notice that if N were 500 (at K), the term [(K-N)/K] equals zero and growth stops. This is the hallmark of logistic growth and a common exam calculation.
2 Community Ecology
Students must understand interspecific interactions (competition, predation, mutualism, commensalism, parasitism) and their effects on population size and community structure. Succession (primary and secondary) and the roles of keystone species are heavily tested. Know how species interactions shape community composition over time.
Key Points
- Interspecific competition follows the competitive exclusion principle: two species competing for identical niches cannot coexist indefinitely; resource partitioning allows coexistence via niche differentiation
- Predator-prey cycles are coupled oscillations — prey population peaks precede predator population peaks (classic: lynx and hare data)
- Keystone species have a disproportionately large effect on community structure relative to their biomass (e.g., sea otters controlling sea urchin populations)
- Primary succession begins on bare substrate (no soil); secondary succession follows disturbance where soil remains — both end at a climax community
Sea otters are removed from a kelp forest ecosystem. Predict the cascade of effects on the community.
Sea otters are a keystone predator of sea urchins. Without otters, sea urchin populations increase unchecked (density-dependent regulation removed). Sea urchins overgraze kelp, collapsing the kelp forest. Species dependent on kelp for habitat (fish, invertebrates) decline dramatically. This trophic cascade illustrates how removing one species restructures the entire community.
3 Ecosystems
Students must be able to trace energy flow through trophic levels and understand that energy is lost (~90%) at each level (10% rule). Biogeochemical cycles — especially carbon, nitrogen, and phosphorus — are essential. Know the difference between gross primary productivity (GPP) and net primary productivity (NPP), and be able to calculate energy available at any trophic level.
Key Points
- Only ~10% of energy transfers between trophic levels; the rest is lost as heat via cellular respiration
- NPP = GPP − Respiration; NPP represents energy available to primary consumers
- The nitrogen cycle requires prokaryotes at key steps: nitrogen fixation (N₂ → NH₃), nitrification (NH₃ → NO₃⁻), and denitrification (NO₃⁻ → N₂)
- Decomposers (bacteria and fungi) are critical — they return nutrients from dead organic matter to inorganic form, completing biogeochemical cycles
A grassland ecosystem has a GPP of 8,000 kcal/m²/yr and plant respiration uses 3,200 kcal/m²/yr. How much energy is available to primary consumers? To secondary consumers?
NPP = GPP − Respiration = 8,000 − 3,200 = 4,800 kcal/m²/yr available to primary consumers. Applying the 10% rule, secondary consumers receive 10% of 4,800 = 480 kcal/m²/yr. This calculation type appears directly on AP exams; always start from NPP (not GPP) when determining consumer energy availability.
4 Biodiversity
Students must understand the three levels of biodiversity (genetic, species, ecosystem) and why each matters for ecosystem stability. Know the major causes of biodiversity loss (habitat destruction, invasive species, pollution, overexploitation, climate change — memorize as HIPCO) and the relationship between species diversity and ecosystem resilience. Island biogeography is tested in the context of habitat fragmentation.
Key Points
- Species richness and evenness together define community diversity; higher diversity generally increases ecosystem stability and productivity
- Island biogeography: species richness on islands reaches equilibrium where immigration rate equals extinction rate; larger and closer islands support more species
- Habitat fragmentation acts like 'islands' of habitat — smaller, more isolated fragments support fewer species and increase extinction risk
- Biodiversity hotspots are regions with exceptionally high species richness AND high rates of endemism that face significant habitat threat
Two forest fragments remain after logging: Fragment A is 100 km² and 2 km from the main forest; Fragment B is 10 km² and 20 km from the main forest. Which fragment will maintain higher species richness over time, and why?
Fragment A will maintain higher species richness because island biogeography predicts that larger islands (fragments) have lower extinction rates (more resources, larger populations) and closer islands have higher immigration rates (recolonization from the source). Fragment B is both smaller and more isolated, compounding the extinction risk. This is a direct application of the equilibrium model of island biogeography.
Questions, answered.
What is Ecology?
Ecology is Unit 8 of AP Biology, covering population ecology, community ecology, ecosystems and biodiversity.
How to study for AP Biology Unit 8?
Start with the Quick Summary above, review the Key Concepts, then test yourself with our interactive study games. Aim for 80%+ accuracy before moving on.
How many questions are in this unit?
This unit has 30+ review questions across 5 different game modes.