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AP Environmental Science | Unit 1 Notes

Unit 1: A Living World – Ecosystems

  • Environment Science is interdisciplinary; study covers both living and non-living components and their interactions.
  • Abiotic factors: non-living components of an ecosystem (e.g., soil, rocks, temperature, sunlight, pH, humidity, nutrients, precipitation).
  • Biotic factors: living components of an ecosystem (e.g., bacteria, protists, fungi, plants, animals). These reflect the six kingdoms of life.
  • Ecosystems are open systems: they receive inputs and produce outputs of matter and energy; balance is maintained via feedback loops.

Ecosystem Balance and Feedback Loops

  • Feedback loop: a circular process where a system’s output serves as input to the same system.
    • Positive feedback: output increases the response, which increases the output (amplifies over time). Examples are often context-specific and can be natural or human-caused (anthropogenic).
    • Negative feedback: reverses a change, restoring a “normal” homeostatic state.
  • Predator–prey interactions are classic negative feedbacks that regulate populations and contribute to cycle dynamics (boom-and-bust).
  • Humans alter natural feedback loops, potentially causing ripple effects and ecosystem disruption. This reflects humans not living in full harmony with nature.

The Tragedy of the Commons and Ecological Footprint

  • The Commons: shared, limited resources.
  • Tragedy: resources are depleted due to short-term, self-interested use in the absence of laws or enforcement.
  • Examples: common grazing lands, fisheries, clean air and water resources, etc.
  • Ecological Footprint: measures how much land and resources are required to support a given lifestyle.
  • Definitions:
    • Unsustainable: using more resources than Earth can replenish.
    • Sustainable: meeting present needs without compromising future generations’ ability to meet theirs.
  • Core idea: humanity’s consumption rate can outpace Earth’s capacity to renew its resources.

Introduction to Ecology

  • Ecosystem: all biotic (living) and abiotic (non-living) components of an environment and their interactions.
  • Scale: an ecosystem can be as small as a tidal pool or as large as a kelp forest depending on observer-defined boundaries.
  • Habitat: “where” an organism lives.
  • Niche: “how” an organism lives—its role, including interactions with biotic and abiotic factors.

Ecosystem Interactions

1) Predator–Prey: popualtions are regulated by each other; lead to negative feedback and boom-and-bust cycles.
2) Symbiotic Interactions: relationships between species (mutualism, commensalism, parasitism).
3) Competitive Interactions:

  • Limiting Resource/Factor: a resource in short supply that constrains growth or abundance.
  • Competition occurs in all ecosystems.
  • Intraspecific competition: within the same species.
  • Interspecific competition: between different species.
  • Competitive Exclusion Principle: two species in the same niche will compete for the same resources; one will outcompete the other.
  • Resource Partitioning: species occupy slightly different niches to reduce competition, improving survival.
  • Habitat Loss: reducing ecosystem diversity and resource availability increases niche overlap and competition, raising extinction risk.

Species Roles and Community Dynamics

  • Keystone species: a species whose removal drastically reduces ecosystem health.
  • Case Study: Grey Wolf in Yellowstone National Park.
    • Trophic Cascade: removal of top predators triggers reciprocal changes across trophic levels, altering ecosystem structure and nutrient cycling.
    • Reintroduction of wolves restored balance to biotic and abiotic factors; grey wolf serves as a keystone species in Yellowstone.
  • Ecosystem Engineers: species that modify their environment in ways that other organisms depend on; can be keystone species. (Examples referenced but not explicitly listed in this transcript.)
  • Indicator species: sensitive to environmental conditions; their presence/absence/health indicates ecosystem health.
  • Range of Tolerance: every species has an ecological range for survival; drastic environmental changes can cause extinction.
  • Generalist vs Specialist:
    • Generalist: broad niche, wide tolerance, e.g., raccoons, rats, crows; often invasive; adapt to changes.
    • Specialist: narrow niche, narrow range of tolerance, e.g., corals, frogs, pandas; often indicator species; more susceptible to extinction.
  • Invasive species tend to be generalists and can outcompete natives, reducing biodiversity and destabilizing ecosystems.

Community Ecology and Energy Foundations

  • Community: interactions among populations within a given location; primarily biotic factors.
  • All living things are cellular and require energy to function.
  • Cellular energy currency: ATP (adenosine triphosphate).
  • Solar energy is essential for life; without solar input, no cellular work (no life as we know it).
  • Two key biochemical reactions connect solar energy to cell work:
    • Photosynthesis: converts solar energy into chemical energy stored in sugars.
    • Cellular respiration: releases stored energy from glucose to produce ATP for cellular work.

Photosynthesis and Cellular Respiration

  • Photosynthesis (producer/autotroph):
    • Equation: 6CO2 + 6H2O
      ightarrow C6H{12}O6 + 6O2
    • Producers include cyanobacteria, phytoplankton, plants, and various protists.
    • Glucose stores energy but is not itself ATP; it acts as a store of potential energy that can be unlocked via cellular respiration to perform work.
  • Cellular respiration (consumers/heterotrophs):
    • Glucose + oxygen → carbon dioxide + water + ATP.
    • Plants also perform respiration to release energy from their stored sugars.
    • The two processes are essentially reverse in terms of the chemical equations (one stores energy, the other releases it).

Types of Consumers

  • Herbivores: feed on plants.
  • Omnivores: feed on both plants and animals.
  • Carnivores: feed on animals.
  • Detritivores: feed on detritus/dead material (e.g., worms, crabs, vultures).
  • Decomposers: chemically break down dead material (e.g., bacteria, fungi).

Energy Transfer and Trophic Structure

  • Energy Pyramid: producers have the most energy because they capture solar energy first.
  • Food Chains: linear sequences showing energy transfer from producers to apex predators.
  • Energy transfer is not equal to “who eats whom”; arrows denote energy flow.
  • Food chains can be longer than 4–5 organisms though energy diminishes rapidly with each transfer.
  • Detrital pathways and decomposers recycle energy by consuming dead matter from all trophic levels.
  • 10% Rule: only about 10% of energy at one trophic level is transferred to the next; the rest is lost as heat or used for metabolism.
    • Simple rule of thumb: if the producer has energy E, the next level has approximately 0.1E, the next 0.01E, etc.
    • Practice example: If grass has E = 849.3 ext{ kJ}, energy available to the next consumer is roughly E_{ ext{next}} \approx 0.1\times 849.3 = 84.93\text{ kJ}; two transfers would give roughly 8.493\text{ kJ}, etc.
  • Food Webs: more realistic than simple chains; organisms can occupy multiple trophic levels; removal of a species affects the entire community.
  • Biodiversity and ecosystem health are linked to more complex, resilient food webs.

Biogeochemical Cycles and the Water Cycle

  • Matter vs energy: Energy flows in one direction through ecosystems; matter cycles within.
  • Atoms recycle between the biosphere, atmosphere, hydrosphere, and lithosphere.
  • Source vs sink (reservoirs):
    • Source: releases more than it stores.
    • Sink/Reservoir: stores more than it releases.

The Water Cycle (Hydrological Cycle)

  • Water remains H2O throughout the cycle but changes state (solid, liquid, gas).
  • Driven by solar and heat energy; considered a relatively fast cycle; sources and sinks are less distinctive than in other cycles.
  • Major reservoirs: Ocean (largest), ice caps, groundwater (freshwater crucial for drinking and irrigation).
  • Major processes/sources:
    • Evaporation: liquid water to water vapor.
    • Transpiration: plant water loss from roots to atmosphere.
    • Condensation: water vapor to liquid droplets (cloud formation).
    • Precipitation: rain, snow, sleet, hail returning water to the surface.
  • Terminology around movement of water:
    • Surface Runoff: water moving over land to a watershed.
    • Infiltration: water entering soil; Percolation: water moving through soil/rock to groundwater reservoirs (aquifers, e.g., Ogallala Aquifer).
    • Watershed: land area draining all precipitation to a single point.
  • Human impacts on the water cycle include:
    • Impermeable surfaces (e.g., concrete) increase runoff, cause flooding, and reduce infiltration/recharge.
    • Urban heat islands (more heat in cities) due to reduced vegetation; solution includes plantings and greenspaces.
    • Dams alter watershed dynamics, block flow of water, sediment, and nutrients downstream; potential solution is dam removal.
    • Groundwater depletion, especially for irrigation; solutions include drip irrigation, regulatory limits, soil moisture management, and soil health improvements to retain water.
    • Warmer temperatures increase evaporation and precipitation variability; solution: switch to renewable energy.

The Carbon Cycle

  • Central to human impact and climate change due to CO2 and CH4 as greenhouse gases.
  • Carbon enters and leaves ecosystems through four major reservoirs and fluxes across the hydrosphere, lithosphere, and biosphere.
  • Major carbon sinks:
    1) Hydrosphere: oceans are the largest carbon sink; carbonate minerals are used by many marine organisms (e.g., crustaceans, mollusks, corals, sponges) to form shells.
    2) Lithosphere: second-largest reservoir; includes sedimentation (organic and inorganic), fossil fuel deposits (coal, oil, natural gas), soils, rocks (e.g., limestone, chalk), and permafrost.
    3) Biosphere: vegetation stores carbon in biomass via photosynthesis; carbon moves through trophic levels as organisms grow and die.
  • Natural sources include biological and geochemical processes; ice core data helps track historical greenhouse gas trends.
  • Human impacts on carbon cycles include:
    • Lithosphere depletion: fossil fuel extraction reduces long-term carbon storage.
    • Atmospheric accumulation of CO2 and CH4 due to combustion and other activities.
    • Ocean uptake of excess atmospheric CO2, leading to ocean warming and acidification.
  • Anthropogenic sources (examples and percentages provided):
    • Combustion of fossil fuels is the main contributor (about 74% of carbon emissions), affecting transportation (28%), electricity (28%), industry (22%), and agriculture (9%).

The Nitrogen Cycle

  • Nitrogen is essential for cells to make amino acids (proteins) and nucleotides (DNA).
  • The atmosphere’s nitrogen gas (N2) is highly abundant but unusable by most organisms without conversion (“fixing”).
  • Bacteria play a key role in nitrogen fixation, making nitrogen accessible to plants and, through the food web, to animals.
  • Step-by-step cycle:
    1) Nitrogen Fixation: Abiotic (lightning) and biotic (bacteria–plant mutualism) convert N2 to ammonia NH3.
    2) Ammonification: Bacteria convert organic nitrogen to ammonium NH4+; decomposition also releases ammonia.
    3) Nitrification: NH4+ is converted to nitrite NO2– and then to nitrate NO3–, which plants can uptake.
    4) Assimilation: Plants take up nitrates and convert them into amino acids and nucleotides; these pass to animals when plants are consumed.
    5) Denitrification: Nitrates NO3– are converted back to gaseous N2 and returned to the atmosphere.
  • Human impacts on the nitrogen cycle:
    • Soil erosion and degradation reduce soil nitrogen availability and disrupt nitrogen-fixing microbes.
    • Fertilizers introduce additional nitrogen and phosphorus; labeling on fertilizers lists the percent of each limiting factor contained, in order of limiting to least.
    • Nitrogen cycle disruption alters soil fertility and plant productivity across agricultural systems.
  • Carnivorous plants in nutrient-poor soils serve as a case study: they rely on decomposition of captured proteins to obtain nitrogen since soil nitrogen is limited.
  • Fertilizer runoff can cause eutrophication in water bodies, leading to algal blooms, hypoxic dead zones, and disruption of aquatic ecosystems.

The Phosphorus Cycle

  • Phosphorus is needed to form ATP (the energy currency) and nucleotides (DNA).
  • Unlike nitrogen, phosphorus has no atmospheric component; it moves primarily through the lithosphere, hydrosphere, and biosphere.
  • The cycle is slow because it largely involves geological processes and weathering of rocks.
  • Human impacts:
    • Fertilizer runoff leads to eutrophication and oxygen depletion in aquatic systems.

Terrestrial Biomes

  • Key climatic factors defining biomes: (1) Temperature and (2) Precipitation; they determine the distribution of biospheres.
  • Temperature is warmer at the equator and cooler toward the poles due to solar angle; precipitation patterns are driven by global wind circulation.

Major Terrestrial Biomes and Characteristics

  • Tropical Rainforest: highest precipitation; warm, highly stable climate; large, tall trees; high biodiversity; nutrient-poor soils (rapid decomposition and uptake by vegetation).
  • Hot Deserts: high temperatures with very low rainfall; adaptations for aridity.
  • Savannah/Tropical Grassland: warm year-round; periodic fires; distinct wet and dry seasons; large herbivores and predators.
  • Temperate Grasslands: cooler temperatures; seasonal rainfall; rich soils; prairies; lower biodiversity than tropical grasslands.
  • Temperate Rainforest: abundant year-round rainfall; cooler temperatures; old-growth forests (notably in higher latitudes).
  • Temperate Deciduous Forest: seasonal variation; trees like oak, hickory, elm, maple; moderate rainfall.
  • Coniferous Forest / Taiga / Boreal: long cold winters; conifer dominance; lower biodiversity relative to deciduous forests; shorter growing season.
  • Chaparral: coastal biome; hot, dry summers; mild, wet winters; fire-adapted shrubs.
  • Tundra: cold desert; very low precipitation; short growing season; permafrost; limited plant depth of roots; arctic and high mountains.
  • Note: some lists include terms like Dessert Rainforest, Tropical Savannah in shorthand; full study notes should align with course vocabulary.

Primary Productivity in Biomes

  • Primary Productivity (NPP) measures production by producers; producers supply energy to higher trophic levels.
  • Highest NPP: Tropical rainforest; lowest NPP: Desert; tundra is the second lowest.
  • Human Impacts on Biomes:
    1) Deforestation (e.g., Amazon for crops and cattle; Indonesia for palm oil) reduces biodiversity and releases CO2.
    2) Soil erosion from modern agricultural practices; tilling, compaction, overgrazing, salination; soil degradation requires conservation strategies like reduced disturbance and cover crops.
    3) Climate change shifts biomes and biodiversity; warming can extend growing seasons but disrupt migrations and increase fire frequency; permafrost thaw releases carbon; solutions include renewable energy, carbon taxes, and reforestation.

Aquatic Ecosystems and the Hydrosphere

  • Estuaries: transition zones between rivers (freshwater) and oceans (saltwater); often brackish; sheltered habitats (marshes, mangroves). The Florida Everglades is a major mangrove habitat.
  • Hydrosphere connectivity: watersheds integrate surface and groundwater; groundwater connects marine and freshwater systems.
  • Light availability and productivity: light is essential for photoautotrophs; phytoplankton are key pelagic producers; coral, kelp, emergent wetland plants also contribute to production.
  • Salinity gradients: from freshwater to estuarine to marine; salinity generally increases with depth; organisms have salinity tolerances.
  • pH: acidity; rainfall is slightly acidic (pH ~6.2), pure water is neutral (pH 7), seawater is slightly basic (pH ~8.6).
  • Temperature and depth: both decrease away from the equator and with depth; seasonal turnover (in lakes) redistributes nutrients.
  • Nutrients: productivity depends on light and nutrients; nutrients enter estuaries via land sediments or upwellings; phytoplankton blooms drive marine food webs when light and nutrients are sufficient.

Human Impacts on Aquatic Ecosystems

1) Run-off of sediments: increases turbidity, reduces light penetration, decreases photosynthesis, and can suffocate gills in fish.
2) Saltwater intrusion: over-extraction of groundwater causes saline water to move into aquifers; harms freshwater plants; creates positive feedback loops with further pumping.
3) Ocean acidification: CO2 dissolves to form carbonic acid, releasing hydrogen ions; carbonate availability decreases; affects calcifying organisms (shell-building species).
4) Climate change – warming oceans: contributes to coral bleaching and altered biodiversity; stronger storms; altered migration and seasonality.
5) Run-off of fertilizers causing eutrophication: nutrient surges trigger algal blooms; subsequent decomposition depletes dissolved oxygen, causing dead zones.
6) Dams: trap sediments and nutrients; affect downstream ecosystems and fish migration; can contribute to coastal erosion downstream; potential remedy is dam removal in some cases.
7) Destruction of mangroves: mangroves provide coastal protection, fish nurseries, and sediment absorption; loss increases erosion and flood risk, reducing biodiversity.
8) Overfishing: rapid declines in fish stocks due to efficient but poorly regulated fishing; bycatch damages non-target species; the tragedy of the commons underlies these dynamics.
9) Loss of keystone species: removal triggers trophic cascades and biodiversity loss (e.g., otters and kelp forests in California).
10) Invasive species: non-native species that outcompete natives and disrupt food webs; examples include the lionfish in the Florida Keys.

Practical and Synthesis Notes

  • Always connect biogeochemical cycles to ecosystem services: nutrient availability supports productivity and biodiversity.
  • Consider both natural dynamics (e.g., predator–prey cycles, climate variability) and human impacts (e.g., deforestation, pollution, climate change) when evaluating ecosystem health.
  • Use the 10% rule to estimate energy transfer across trophic levels and to understand why food webs are typically only 4–5 levels deep.
  • Recognize the importance of keystone species and ecosystem engineers in maintaining structural integrity and function of ecosystems.
  • When discussing biomes, relate climate factors (temperature and precipitation) to productivity and biodiversity outcomes.
  • In contemplating environmental solutions, prioritize sustainable practices: renewable energy, conservation, restoration (forests, wetlands, mangroves), soil improvement, and responsible water management.

Key formulas and constants to remember:

  • Photosynthesis: 6CO2 + 6H2O
    ightarrow C6H{12}O6 + 6O2
  • Cellular respiration (reverse of photosynthesis): C6H{12}O6 + 6O2
    ightarrow 6CO2 + 6H2O + ext{ATP}
  • 10% Rule for energy transfer between trophic levels: If the energy at one level is E0, the energy at the next level is approximately E1
    oughly E0 imes 0.1; successive levels follow En
    oughly E_0 imes (0.1)^n.
  • Water cycle processes: Evaporation, Transpiration, Condensation, Precipitation, Infiltration, Percolation (etc.).

Quick Glossary (from the transcript)

  • Abiotic factors, Biotic factors, Habitat, Niche, Keystone species, Trophic cascade, Ecosystem engineer, Indicator species, Range of tolerance, Generalist, Specialist, Detritivore, Decomposer, Autotroph, Heterotroph, Producer, Consumer, Omnivore, Carnivore, Herbivore, Phytoplankton, Zooplankton, Watershed, Aquifer, Eutrophication, Bycatch, Keystone cascade, Permafrost, Mangroves, Estuary, Upwelling, Salinity, pH, Dissolved oxygen, and more as encountered in the course materials.

Connections and Relevance

  • These concepts interlink: energy flow determines trophic structure; biogeochemical cycles regulate nutrient availability; biomes reflect climatic controls; human actions alter cycles, energy use, and land/water use, feeding back into climate and biodiversity.
  • Real-world implications include climate action, sustainable agriculture, fisheries management, deforestation policies, and habitat restoration; each is a lever for maintaining ecosystem health and resilience.