Unit 1: The Living World: Ecosystems (AP Environmental Science)

1.1 Introduction to Ecosystems

• Ecosystems: living (biotic) and non-living (abiotic) components and their interactions (energy flow, matter cycling).

• Definitions:

  • Individual: single functioning organism

  • Population: group of same species individuals

  • Community: multiple species in same space/time

  • Ecosystem: integrates abiotic/biotic factors

  • Biome: multiple ecosystems, same climate

  • Biosphere: Earth region with living organisms

  • Species: group reproductively isolated

• Core concepts: Biotic vs. Abiotic, Trophic interactions, energy flow, biogeochemical cycles, dynamic nature of ecosystems.

• Importance: sustain systems, support biodiversity, provide ecosystem services.

1.2 Terrestrial Biomes

• Enduring Understanding: Ecosystems result from biotic/abiotic interactions; terrestrial biomes linked to climate.

• Essential knowledge: Biomes have characteristic plant/animal communities adapted to climate.

• Major biomes: taiga, temperate rainforests, temperate seasonal forests, tropical rainforests, shrubland, temperate grassland, savanna, desert, tundra.

• Distribution drivers: climate, geography, latitude, altitude, nutrients, soil; dynamic with climate change.

• Climate drivers: annual precipitation and temperature shape biome boundaries and productivity.

1.3 Aquatic Biomes

• Categories: FRESHWATER (rivers/streams, lakes/ponds, wetlands) and MARINE (estuaries, oceans, coral reefs, marshes).

• Key factors: salinity, flow, depth, temperature, turbidity, nutrient availability, light.

• Algae in marine biomes: crucial for oxygen production and CO₂ uptake.

• Light: Photosynthesis affected by light penetration (red vs blue wavelengths) and depth.

1.4 The Carbon Cycle

• Concept: Movement of carbon (atoms/molecules) between sources and sinks.

• Major sinks: ocean (largest), biosphere, sediments, limestone, coral reefs, soil, atmosphere.

• Main sources: burning fossil fuels, burning biomass, respiration, animal agriculture, deforestation.

• Photosynthesis: \text{CO}2 + \text{H}2\text{O} + \text{light} \rightarrow \text{C}6\text{H}{12}\text{O}6 + \text{O}2 (rapid CO₂ uptake)

• Cellular respiration: \text{C}6\text{H}{12}\text{O}6 + \text{O}2 \rightarrow \text{CO}2 + \text{H}2\text{O} + \text{energy} (rapid CO₂ release)

• Burial: Slow geological storage. Extraction & Combustion: Burning fossil fuels releases CO₂ faster than burial can store it.

• Ocean carbon exchange: Ocean absorbs CO₂, increasing uptake leads to ocean acidification.

• Key concept: Carbon sinks vs sources; rapid fossil fuel burning increases atmospheric CO₂.

1.5 The Nitrogen Cycle

• Overview: Movement of nitrogen; atmosphere (N₂) is major reservoir.

• Major forms: N₂, NH₄⁺ (ammonium), NO₂⁻ (nitrite), NO₃⁻ (nitrate).

• Key processes:

  • Nitrogen fixation: N₂ → NH₃/NH₄⁺ (bioavailable) by bacteria.

  • Nitrification: NH₃/NH₄⁺ → NO₂⁻ → NO₃⁻ (aerobic bacteria).

  • Ammonification: organic nitrogen → NH₃/NH₄⁺ by decomposers.

  • Assimilation: Plant uptake of NO₃⁻ or NH₄⁺.

  • Denitrification: NO₃⁻ → N₂ (gas) under anaerobic conditions.

• Ecological significance: Limiting factor for growth (DNA, amino acids); excess nitrogen causes eutrophication (algal blooms, hypoxic zones).

• Human impacts: Fertilizer use, runoff, pollution.

1.6 The Phosphorus Cycle

• Key point: No atmospheric component.

• Major reservoirs: Sedimentary rocks (largest), soil, land biota.

• Sources: Fertilizers (NPK), rock phosphates, living organisms.

• Limiting factor: Often limiting nutrient; slow transfer from ocean to land.

• Biological roles: Cell membranes (phospholipids), ATP, DNA/RNA backbone.

• Environmental note: Can contribute to eutrophication; moves slower than nitrogen.

1.7 The Hydrologic (Water) Cycle

• Basics: Sun-powered movement of water (solid, liquid, gas) between sources/sinks.

• Major reservoirs: Oceans (primary), ice caps, groundwater.

• Key processes: Evaporation, Transpiration (evapotranspiration), Condensation, Precipitation, Infiltration, Runoff, Seepage.

• Freshwater distribution: Glaciers/ice caps (~68.7%), groundwater (~0.9%), surface water (~3.0%). Oceans ~96.5% of all water.

• Cohesion (water-water attraction) and Adhesion (water-other substance attraction) cause Capillary action.

1.8 Primary Productivity

• Primary productivity: Rate of solar energy conversion to organic matter via photosynthesis.

• Gross Primary Productivity (GPP): Total photosynthesis rate.

• Net Primary Productivity (NPP): Energy stored as biomass after respiration (R): \text{NPP} = \text{GPP} - \text{R}

• Units: \text{kcal m}^{-2}\text{yr}^{-1}

• Light limitation: Red light absorbed quickly in water; blue light penetrates deeper.

• Implications: Higher productivity supports greater biodiversity and energy for higher trophic levels.

1.9 Trophic Levels

• Concept: Position in food chain/web by feeding mode.

• Levels: Producers (autotrophs), Primary consumers (herbivores), Secondary consumers (carnivores eating herbivores), Tertiary consumers (carnivores eating carnivores), Quaternary consumers.

• Decomposers: Critical nutrient recyclers.

1.10 Energy Flow and the 10% Rule

• Principle: Energy moves up trophic levels; only a fraction is transferred.

• 10% Rule: Approx. 10% of usable energy from one trophic level transfers to the next; rest lost as heat. \text{E}{\text{n}} \approx 0.1 \times \text{E}{\text{n-1}}

• Consequence: Biomass and energy decrease at higher trophic levels (pyramid shape).

1.11 Food Chains and Food Webs

• Food chain: Linear energy flow.

• Food web: Interlocking network of food chains; more realistic model.

• Biogeochemical cycles: Integral to matter (food webs) and energy flow.

• Trophic cascades: Predators regulate prey, affecting lower trophic levels (e.g., wolves in Yellowstone).

Ecological Concepts, Interactions, and Scenarios

• Biotic interactions:

  • Competition (intra-/interspecific)

  • Predation

  • Mutualism (both benefit)

  • Commensalism (one benefits, other unaffected)

  • Herbivory and parasitism

• Resource partitioning: Species use different resources or the same resource differently (temporal/spatial niche differentiation).

Major Biome Distribution and Climate Forcing

• Regional climate forces (latitude, altitude, ocean currents, wind, topography) shape biome distribution.

Practical Case Studies and Ecosystem Processes

• Insect outbreaks: Mountain pine beetle outbreaks linked to climate change, forest management; cause tree death, increased fire risk, carbon release.

• Pollinator declines: Honeybee Colony Collapse Disorder (CCD) affects pollination services, crop yields, agricultural economics.

• Sustainable management: Selective logging, hydroelectric mitigation (flow management, fish passage).

Aquatic Ecology: Freshwater and Marine Systems

• River Continuum Concept (RCC): Energy inputs shift along river: headwaters (CPOM, shredders) to large rivers (phytoplankton, FPOM).

• Flood Pulse Concept: Inundation/drought controls lateral exchange, sustaining productivity.

• Estuaries/Coastal Wetlands: Variable, ecologically/economically important (nutrient cycling, habitat, storm protection).

Coral Reefs and Ocean Chemistry

• Coral reef loss: Reduces shoreline protection, habitat, biodiversity; alters oxygen dynamics.

• Ocean acidification: CO₂ + H₂O ⇌ H₂CO₃ ⇌ H⁺ + HCO₃⁻ ⇌ H⁺ + CO₃²⁻. Increased CO₂ (carbonic acid) reduces carbonate ions, impairs calcification in corals.

Key Equations and Concepts (Summary)

• Photosynthesis: \text{CO}2 + \text{H}2\text{O} + \text{light} \rightarrow \text{C}6\text{H}{12}\text{O}6 + \text{O}2

• Cellular respiration: \text{C}6\text{H}{12}\text{O}6 + \text{O}2 \rightarrow \text{CO}2 + \text{H}2\text{O} + \text{energy}

• Carbon balance: Fossil fuel burning increases atmospheric CO₂.

• 10% Rule: \text{E}{\text{n}} \approx 0.1 \times \text{E}{\text{n-1}

• NPP: \text{NPP} = \text{GPP} - \text{R}

• Phosphorus cycle: Limiting nutrient, no atmospheric reservoir; major sinks in rocks/sediments.

• Nitrogen cycle transformations:

  • Nitrogen fixation: \text{N}2 \rightarrow \text{NH}3/\text{NH}_4^+

  • Nitrification: \text{NH}3/\text{NH}4^+ \rightarrow \text{NO}2^- \rightarrow \text{NO}3^-

  • Denitrification: \text{NO}3^- \rightarrow \text{N}2

  • Ammonification: organic N → \text{NH}_4^+

  • Assimilation: plants take up \text{NO}3^- or \text{NH}4^+

• Hydrologic cycle processes: Evaporation, evapotranspiration, condensation, precipitation, infiltration, runoff, groundwater flow; cohesion/adhesion enable capillary action.

Practical Implications and Ethical Considerations

• Climate change impacts: Conservation, land use, adaptation.

• Eutrophication: Balancing agriculture and water quality via fertilizer regulations.

• Hydroelectric development: Balancing renewable energy with ecosystem protection (fish passage, flow regimes).

• Insect/pollinator health: Interactions of disturbance, climate, management affect ecosystem services; ethical land-use practices.