📚Study Guide: Ecology
Unit 8: Ecology
Ecology examines the interactions between organisms and their environments, scaling from individual organisms to the entire biosphere. This unit is particularly important because it connects biological concepts to global challenges like climate change, biodiversity loss, and resource management. Students must master population ecology, including exponential and logistic growth models, density-dependent and density-independent limiting factors, and life history strategies (r-selected vs. K-selected species). Community ecology explores interspecific interactions--competition, predation, herbivory, parasitism, mutualism, and commensalism--and how these interactions shape community structure over time through ecological succession. Ecosystem ecology requires understanding energy flow (10% energy transfer between trophic levels) and biogeochemical cycles (carbon, nitrogen, phosphorus, water). The AP exam frequently presents data on population dynamics, asks students to interpret survivorship curves, or requires analysis of how human activities disrupt nutrient cycles. Conservation biology and the impacts of invasive species, habitat fragmentation, and pollution are also prominent topics. Success in this unit depends on the ability to interpret graphs, apply mathematical models, and synthesize information about complex biological systems interacting with human society.
Key Concepts
- Population Growth Models: Exponential growth (dN/dt = rN) occurs when resources are unlimited, producing a J-shaped curve. Logistic growth (dN/dt = rN[(K-N)/K]) incorporates carrying capacity (K), producing an S-shaped curve. Population growth rate depends on birth rate, death rate, immigration, and emigration.
- Density-Dependent vs. Density-Independent Factors: Density-dependent factors (competition, predation, disease, waste accumulation) intensify as population density increases. Density-independent factors (natural disasters, climate extremes) affect populations regardless of density.
- Life History Strategies: r-selected species (insects, weeds) produce many offspring with little parental investment, thriving in unstable environments. K-selected species (elephants, whales) produce few offspring with high parental investment, thriving near carrying capacity in stable environments.
- Community Interactions: Competition (-/-), predation (+/-), herbivory (+/-), parasitism (+/-), mutualism (+/+), commensalism (+/0). Competitive exclusion principle states that two species competing for the same limiting resource cannot coexist indefinitely.
- Ecological Succession: Primary succession occurs on bare rock with no soil; pioneer species (lichens, mosses) initiate soil formation. Secondary succession occurs after a disturbance removes vegetation but leaves soil intact. Succession progresses toward a climax community.
- Energy Flow and Trophic Levels: Producers (autotrophs) convert solar energy to chemical energy. Consumers (heterotrophs) transfer energy through food chains/webs. Only about 10% of energy is transferred between trophic levels; the rest is lost as heat through respiration.
- Biogeochemical Cycles: Carbon cycle (photosynthesis, respiration, combustion, decomposition). Nitrogen cycle (nitrogen fixation, ammonification, nitrification, denitrification). Phosphorus cycle (weathering of rocks, uptake by plants, decomposition).
Vocabulary
- Carrying Capacity (K): The maximum population size that a particular environment can sustain indefinitely given available resources.
- Niche: The role and position a species has in its environment, including all biotic and abiotic interactions. Fundamental niche is the full potential range; realized niche is the actual occupied range due to competition.
- Trophic Level: The position an organism occupies in a food chain (producer, primary consumer, secondary consumer, tertiary consumer, decomposer).
- Keystone Species: A species that has a disproportionately large effect on its environment relative to its abundance (e.g., sea otters controlling sea urchin populations).
- Biodiversity: The variety of life in a particular habitat or ecosystem, measured as species richness (number of species) and species evenness (relative abundance).
- Eutrophication: Excessive nutrient enrichment (usually nitrogen and phosphorus) of water bodies, leading to algal blooms, oxygen depletion, and dead zones.
Processes and Diagrams to Know
- Survivorship Curves: Type I (low mortality early, high in old age--humans), Type II (constant mortality--birds, rodents), Type III (high mortality early, low later--trees, marine invertebrates).
- Carbon Cycle Diagram: Be able to trace carbon through photosynthesis, respiration, decomposition, fossil fuel combustion, and ocean absorption.
- Food Web Diagram: Identify producers, primary consumers, secondary consumers, and decomposers; indicate energy flow direction with arrows.
Experimental Designs
- Mark-Recapture Method: Estimating population size using the formula N = (M x C) / R, where M = marked initially, C = total captured second time, R = marked recaptured.
- Quadrat Sampling: Using square plots to estimate plant population density and distribution in a habitat.
Common Mistakes
- Confusing Biotic and Abiotic Factors: Biotic factors are living or once-living components (predators, food, disease). Abiotic factors are non-living (temperature, water, sunlight).
- Assuming Food Webs Are Linear: Food webs are complex networks. Removing one species can have cascading effects (trophic cascades) throughout the ecosystem.
- Forgetting Energy is Lost at Each Trophic Level: The 10% rule means that biomass and energy decrease dramatically at higher trophic levels. Top predators require vast territories.
- Confusing Primary and Secondary Succession: Primary succession starts with NO soil (bare rock); secondary succession starts with soil already present after a disturbance like fire.
AP Exam Strategies
- Read Graphs Carefully: In population growth graphs, identify whether the curve is J-shaped (exponential) or S-shaped (logistic) and note the carrying capacity line.
- Calculate Energy Transfer: If producers have 10,000 kcal, primary consumers have ~1,000 kcal, secondary consumers ~100 kcal. Show your calculations.
- Connect Human Activities to Cycles: When discussing nutrient cycles, explicitly link fossil fuel combustion to increased atmospheric CO2, or fertilizer runoff to aquatic eutrophication.
- Use Mathematical Models: For mark-recapture, write the formula and substitute values. For logistic growth, explain what happens when N approaches K (growth rate approaches zero).
Real-World Applications
- Climate Change Mitigation: Understanding the carbon cycle informs strategies like reforestation, which increases carbon sequestration in biomass and soil.
- Fisheries Management: Logistic growth models help set sustainable catch limits to prevent population collapse (e.g., Atlantic cod fisheries).
- Invasive Species Control: Knowledge of competitive exclusion and trophic interactions guides management of invasives like zebra mussels or kudzu vine.