AP Environmental Science Unit 1 Review: The Living World – Ecosystems
Mastering the foundational concepts in AP Environmental Science Unit 1 is the single most important step toward earning a qualifying score on the exam. This unit, titled The Living World: Ecosystems, establishes the vocabulary, cycles, and ecological principles that every subsequent unit—from biodiversity to pollution to global change—builds upon. A thorough AP Environmental Science Unit 1 review requires more than memorizing definitions; it demands an understanding of how energy moves, how matter cycles, and how ecosystems respond to change.
Energy Flow and Trophic Levels
The first law of thermodynamics states that energy cannot be created or destroyed, while the second law dictates that entropy increases during every transfer. In ecology, this explains why energy flows one way through an ecosystem: entering as sunlight, captured by producers, and eventually lost as heat at every trophic transfer Not complicated — just consistent. But it adds up..
Some disagree here. Fair enough The details matter here..
Producers (autotrophs) form the base of every food web. Through photosynthesis, they convert solar energy into chemical energy (glucose). Consumers (heterotrophs) obtain energy by eating other organisms. Primary consumers are herbivores, secondary consumers eat herbivores, and tertiary consumers eat secondary consumers. Decomposers (bacteria and fungi) and detritivores (earthworms, vultures) break down dead organic matter, returning nutrients to the environment.
A critical concept for the exam is the 10% Rule (Lindeman’s trophic efficiency rule). Worth adding: on average, only about 10% of the energy biomass from one trophic level is transferred to the next. That's why the remaining 90% is lost primarily as metabolic heat, used for cellular respiration, or left as unconsumed waste (bones, fur, cellulose). This inefficiency explains why food chains rarely exceed four or five levels and why biomass pyramids typically narrow sharply toward the apex.
Gross Primary Productivity (GPP) is the total rate of photosynthesis. Net Primary Productivity (NPP) is the energy remaining after producers use some for their own respiration (NPP = GPP – R). NPP represents the energy actually available to consumers. Ecosystems like tropical rainforests and estuaries have high NPP due to abundant sunlight, water, and nutrients, while deserts and open oceans have low NPP.
Biogeochemical Cycles: Matter Recycles
Unlike energy, matter cycles within a closed system (Earth). You must be able to diagram and describe the major reservoirs and fluxes for the Water, Carbon, Nitrogen, and Phosphorus cycles Small thing, real impact. Still holds up..
The Water Cycle (Hydrologic Cycle)
Driven by solar energy, this cycle involves evaporation, transpiration (evapotranspiration combined), condensation, precipitation, infiltration, and runoff. Human impacts include groundwater overdraft (aquifer depletion), deforestation reducing transpiration, and urbanization increasing impervious surfaces and flash flooding.
The Carbon Cycle
Carbon moves between the atmosphere (CO₂), oceans (dissolved CO₂, carbonates), fossil fuels, biomass, and sedimentary rock. Photosynthesis and respiration are the biological drivers. Combustion of fossil fuels and deforestation are the primary anthropogenic fluxes adding excess CO₂ to the atmosphere, driving climate change. The ocean acts as a massive carbon sink, but increased absorption leads to ocean acidification, threatening calcifying organisms.
The Nitrogen Cycle
Nitrogen gas (N₂) makes up 78% of the atmosphere but is biologically unavailable to most organisms. Nitrogen fixation (by lightning or specialized bacteria like Rhizobium in legume root nodules) converts N₂ to ammonia (NH₃) or ammonium (NH₄⁺). Nitrification converts ammonium to nitrite (NO₂⁻) then nitrate (NO₃⁻), the form most plants absorb. Assimilation incorporates nitrogen into proteins and DNA. Ammonification (decomposition) returns nitrogen to ammonium. Denitrification (by anaerobic bacteria) returns N₂ to the atmosphere.
Human intervention via the Haber-Bosch process (synthetic fertilizer production) has doubled the global fixed nitrogen supply. This causes eutrophication—nutrient overload in waterways leading to algal blooms, decomposition-driven hypoxia (dead zones), and biodiversity loss Worth keeping that in mind..
The Phosphorus Cycle
Phosphorus has no significant atmospheric reservoir. It cycles slowly from phosphate rock $\rightarrow$ weathering $\rightarrow$ soil/water $\rightarrow$ organisms $\rightarrow$ decomposition $\rightarrow$ sedimentation $\rightarrow$ geological uplift. It is often the limiting nutrient in freshwater aquatic systems. Mining phosphates for fertilizer and detergent runoff accelerates this cycle, also contributing heavily to eutrophication Took long enough..
Ecosystem Structure and Dynamics
Biomes and Climate
Terrestrial biomes are defined primarily by temperature and precipitation (climatographs). You must know the characteristics of major biomes: Tropical Rainforest, Temperate Deciduous Forest, Boreal Forest (Taiga), Temperate Grassland, Chaparral, Desert, Tundra, and Savanna. Aquatic biomes are categorized by salinity (freshwater vs. marine), depth (photic vs. aphotic zones), and flow (lotic/running vs. lentic/standing).
Ecological Succession
Primary succession occurs on bare rock or sand where no soil exists (e.g., after glacial retreat or volcanic eruption). Pioneer species like lichens and mosses chemically weather rock to form soil. Secondary succession occurs where soil remains intact after a disturbance (fire, farming, hurricane). It proceeds much faster. Both processes move toward a climax community—a relatively stable, diverse endpoint—though modern ecology recognizes that frequent disturbances often prevent a true "final" state Simple, but easy to overlook..
Species Interactions
- Competition (-/-): Intraspecific (same species) or interspecific (different species). Competitive Exclusion Principle states two species cannot occupy the exact same niche indefinitely. Resource partitioning and character displacement allow coexistence.
- Predation (+/-): Drives coevolution (camouflage, mimicry, speed). Keystone species (e.g., sea otters, wolves) exert disproportionate control on community structure.
- Symbiosis: Close, long-term interactions.
- Mutualism (+/+): Pollinators/flowers, coral/zooxanthellae, mycorrhizae/roots.
- Commensalism (+/0): Epiphytes on trees, cattle egrets eating insects stirred by grazing.
- Parasitism (+/-): Ticks, tapeworms, mistletoe. Parasites rarely kill hosts immediately.
Survivorship Curves and Reproductive Strategies
- Type I (K-selected): High survival until old age (humans, elephants). Few offspring, high parental care, stable environments near carrying capacity (K).
- Type III (r-selected): High mortality young, few survive to adulthood (fish, insects, dandelions). Many offspring, no parental care, unstable environments, high biotic potential (r).
- Type II: Constant mortality rate (birds, rodents).
Carrying Capacity (K) is the maximum population size an environment can sustain. Logistic growth (S-curve) models this reality, incorporating environmental resistance. Overshoot and die-off occur when populations exceed K temporarily Most people skip this — try not to..
Practice Application: FRQ Style Thinking
To solidify this AP Environmental Science Unit 1 review, practice applying concepts to Free Response Question (FRQ) scenarios.
Scenario: A developer clears a temperate deciduous forest to build a housing subdivision with manicured lawn
