AP Environmental Science Unit 1

An ecosystem consists of living (biotic) organisms interacting with nonliving (abiotic) factors through energy transfers and biogeochemical cycling.

Ecosystem Structure and Interactions
Levels of Organization

  • Organism: An individual living entity.

  • Species: Organisms that share genetic similarity and can interbreed in nature to produce fertile offspring.

  • Population: All individuals of a single species living in a defined area.

  • Community: All interacting populations of different species in an area.

  • Ecosystem: The biological community plus its physical, abiotic environment.

Niche, Interactions, and Dynamics

  • Habitat vs. Niche: Habitat is the physical address where an organism lives; niche is its ecological role, including resource use and species interactions.

  • Generalists vs. Specialists: Generalists occupy broad niches and tolerate wide environmental gradients; specialists occupy narrow niches and are vulnerable to environmental disruption.

  • Species Interactions:

    • Mutualism: Both species benefit.

    • Commensalism: One species benefits; the other is unaffected.

    • Parasitism: One species benefits at the expense of the host.

    • Amensalism: One species is harmed while the other is unaffected.

    • Competition: Intraspecific (within same species) or interspecific (between different species).

  • Resource Partitioning: Competitors divide resources spatially, temporally, or morphologically to reduce competition.

  • Tolerance and Carrying Capacity:

    • Law of Tolerance: Survival and distribution depend on staying within environmental limits.

    • Carrying Capacity: The maximum sustained population size supported by available resources.

    • Limiting Factors: Density-dependent (competition, disease) vs. density-independent (storms, drought, fire).

Energy Flow and Matter Cycling

  • Energy Flow: Unidirectional solar energy input degrades to heat as entropy increases (Second Law of Thermodynamics).

  • Matter Cycling: Atoms cycle continuously between living organisms and physical reservoirs.

  • Decomposers and Detritivores: Break down dead organic material, returning nutrients to primary producers.

Trophic Efficiency and Productivity

  • 10% Rule: Approximately 10%10% of energy is passed to the next trophic level; 90%90% is lost to metabolic heat and waste.

  • Bioaccumulation vs. Biomagnification: Bioaccumulation occurs within a single organism over its lifespan; biomagnification increases pollutant concentration at higher trophic levels.

  • Primary Productivity Equations: NPP=GPP−RNPP=GPP−R

    • Gross Primary Productivity (GPPGPP): Total chemical energy captured by primary producers via photosynthesis.

    • Net Primary Productivity (NPPNPP): Energy remaining as biomass after producer respiration (RR).

Biogeochemical Cycles
Hydrologic (Water) Cycle

  • Powered by solar radiation through evaporation, transpiration, condensation, precipitation, infiltration, percolation, and runoff.

  • Ocean reservoirs hold 97%97% of Earth's water; freshwater (3%3%) is primarily locked in ice sheets and deep aquifers.

Carbon Cycle

  • Major sinks: Sedimentary rocks, deep ocean waters, fossil fuels, soils, and terrestrial plants.

  • Key processes: Photosynthesis absorbs CO2CO2​; cellular respiration, decomposition, and combustion release CO2CO2​.

  • Excess anthropogenic CO2CO2​ drives atmospheric warming and ocean acidification.

Nitrogen Cycle

  • Atmosphere is the main reservoir (78%78% as N2N2​ gas).

  • Sequential steps:

    1. Nitrogen Fixation: N2→NH3/NH4+N2​→NH3​/NH4+​ via nitrogen-fixing bacteria (Rhizobium) or lightning.

    2. Nitrification: NH4+→NO2−→NO3−NH4+​→NO2−​→NO3−​ via aerobic bacteria.

    3. Assimilation: Plants absorb NO3−NO3−​ and NH4+NH4+​ to synthesise organic compounds.

    4. Ammonification: Decomposers convert nitrogenous waste to NH4+NH4+​.

    5. Denitrification: Anaerobic bacteria reduce NO3−NO3−​ back into N2N2​ gas.

Phosphorus Cycle

  • Lacks a significant atmospheric gas phase; cycles slowly via weathering of sedimentary rock.

  • Frequently the limiting nutrient in freshwater ecosystems; runoff triggers cultural eutrophication.

Terrestrial and Aquatic Biomes
Terrestrial Biomes

Defined mainly by temperature and precipitation:

  • Deserts: Low rainfall; specialized succulents and nocturnal adaptations.

  • Tropical Rainforests: High rainfall and uniform temperatures; highest NPP and biodiversity; nutrient-poor soils.

  • Temperate Deciduous Forests: Four distinct seasons; rich, fertile soil enriched by decaying leaves.

  • Boreal Forests (Taiga): Long, cold winters; dominated by evergreen conifers; slow organic decomposition.

  • Tundra: Cold climate with low precipitation; presence of permafrost restricts tree growth.

Aquatic Biomes

Organized by salinity, depth, light, and dissolved oxygen:

  • Freshwater Systems:

    • Lakes: Zoned into Littoral (rooted plants), Limnetic (open lit surface), Profundal (deep, dark), and Benthic (sediment bottom). Feature seasonal turnover in temperate zones.

    • Wetlands: Marshes and swamps providing natural flood mitigation, water filtration, and groundwater recharge.

  • Marine Systems:

    • Estuaries: Highly productive brackish waters where rivers meet the sea.

    • Coral Reefs: Biodiverse shallow-water marine ecosystems reliant on zooxanthellae endosymbionts.

    • Open Ocean: Vast total volume; low productivity per unit area, but significant global NPP share.

Ecosystem Dynamics, Biodiversity, and Services
Community Dynamics

  • Trophic Cascades: Top-down or bottom-up direct and indirect population regulation across multiple trophic levels.

  • Keystone Species: Organisms with a disproportionately large impact on ecosystem structure relative to their biomass.

  • Indicator Species: Sensitive organisms whose population health reflects environmental conditions.

Biodiversity Measures

  • Genetic Diversity: Heritable variation within a population.

  • Species Richness: Total count of different species present.

  • Species Evenness: Relative distribution of individuals among species.

  • Habitat Fragmentation: Division of continuous habitats into smaller patches, enhancing edge effects.

Ecosystem Services

  1. Provisioning: Direct harvestable products (e.g., food, timber, freshwater).

  2. Regulating: Ecosystem control of climate, disease, and water purification.

  3. Supporting: Essential processes underlying all services (e.g., soil formation, nutrient cycling).

  4. Cultural: Non-material aesthetic, recreational, and spiritual benefits.