Rucete ✏ AP Environmental Science In a Nutshell
1. The Living World: Ecosystems
This unit explores the structure, function, and dynamics of ecosystems, focusing on energy flow, species interactions, biogeochemical cycles, and ecosystem productivity. Understanding these principles is essential for analyzing environmental changes and ecological balance.
1.1 Introduction to Ecosystems
Biological Populations and Communities
• Organism: individual living entity
• Species: group capable of interbreeding and producing fertile offspring
• Population: group of same-species organisms in a specific area
• Community: interacting populations of different species
Ecological Niches
• A niche describes an organism’s role, habitat, and interactions in its ecosystem
• Includes adaptations, food sources, and position in food web
Generalist vs. Specialist Species
• Generalists: broad diet and habitat range; adaptable (e.g., humans, cockroaches)
• Specialists: narrow niche; sensitive to change (e.g., pandas, koalas)
Types of Species Interactions
• Amensalism: one harmed, one unaffected (e.g., black walnut tree)
• Commensalism: one benefits, one unaffected (e.g., remora on shark)
• Competition: intraspecific or interspecific; drives evolution
• Mutualism: both benefit (e.g., bees and flowers)
• Parasitism: one benefits, one harmed (e.g., fleas, tapeworms)
• Predation: predator hunts and kills prey (e.g., lions, anteaters)
• Saprotrophism: decomposers feed on dead matter (e.g., fungi, vultures)
Law of Tolerance
• Species thrive within specific ranges of abiotic and biotic factors
Limiting Factors
• Factors like light, nutrients, and temperature can restrict population growth
Predator–Prey Dynamics
• Populations cycle due to food availability and consumption rates
Resource Partitioning
• Morphological: different structures for same resource
• Spatial: different habitat zones
• Temporal: active at different times
1.2 Terrestrial Biomes
Overview of Biomes
• Biomes are global ecosystems defined by vegetation and climate
• Temperature and precipitation are key factors
Deserts
• Less than 20 inches of rain/year; extreme daily temperature swings
• Plants: succulents with water storage, waxy leaves, vertical growth
• Animals: nocturnal, burrowers, low surface area (e.g., kangaroo rats)
• Threats: global warming, grazing, salinization, off-road activity
• Solutions: rain-retaining grooves, crop rotation, water-efficient use
Forests
• Cover one-third of land; most plant biomass and primary productivity
Tropical Rainforests
• Near equator; high rainfall and biodiversity; poor soil
• Evergreen trees with large leaves, shallow roots
• Threats: logging, mining, invasive species, slash-and-burn
• Solutions: selective harvesting, sustainable practices, education
Temperate Deciduous Forests
• Four seasons, rich soil, broadleaf trees
• Threats: acid rain, habitat loss, invasive species
• Solutions: renewable energy, reforestation, invasive removal
Temperate Coniferous Forests
• Found in coastal/inland regions with mild winters or dry climates
• Trees: pine, spruce, fir; animals: thick fur, hibernation
• Threats: logging, acid rain, habitat disruption
• Solutions: forest certification, legal protection, indigenous rights
Taiga (Boreal Forest)
• Largest terrestrial biome; cold, nutrient-poor soil, peat accumulation
• Conifer trees dominate; animals: moose, lynxes, bears
• Threats: oil exploration, hunting, warming, pests
• Solutions: park creation, pest control, development limits
Grasslands
• Savannas: scattered trees, tropical; seasonal rain and fire
• Temperate grasslands: prairies/steppes; rich soil, extreme seasons
• Threats: agriculture, poaching, erosion, monocultures
• Solutions: windbreaks, crop rotation, wetland restoration
Tundra
• Arctic and alpine regions; low precipitation, permafrost, short growing season
• Plants: low, clumped, snow-insulated; animals: migratory, fat-insulated
• Threats: melting permafrost, mining, pollution, invasive species
• Solutions: climate action, protected zones, industrial limits
1.3 Aquatic Biomes
General Features
• Aquatic biomes include marine, freshwater, and Antarctic ecosystems.
• Organisms obtain nutrients directly from water, aided by filter feeding and high dispersal ability.
• Water provides buoyancy, stable temperatures, UV protection, and reduces the need for structural support.
Antarctic
• Coldest region on Earth with low precipitation and extremely dry air.
• Phytoplankton blooms in summer support krill, which are key to the Antarctic food web.
• Threats: climate change, invasive species, tourism, and potential fishing and mineral exploitation.
• Solutions: invasive species monitoring, climate mitigation, and strict treaty enforcement.
Marine Biomes
• Cover ~75% of Earth’s surface, with 3% salinity; drive rainfall, oxygen production, and CO₂ absorption.
• Ocean circulation (e.g., Great Conveyor Belt) distributes heat globally and affects climate systems.
Ocean Zones
• Littoral Zone: nearshore; organisms must tolerate tidal changes, wave action, and predators.
• Neritic Zone: edge of continental shelf; well-lit, stable, highly productive.
• Photic Zone: sunlit upper layer; supports 90% of marine life, including plankton and nekton.
Coral Reefs
• Formed by polyps with symbiotic algae (zooxanthellae); need shallow, clear, warm waters.
• Types: fringing reefs, barrier reefs, and atolls.
• Threats: bleaching from ocean acidification and warming, overfishing, and pollution.
• Solutions: reduce plastic use, enforce trade bans, address climate change.
Lakes
• Formed by glacial, volcanic, tectonic, or river processes; may be natural or artificial.
• Zones:
• Littoral: shallow, near shore, rooted plants.
• Limnetic: open water, light-penetrated.
• Profundal: deep, dark, low-oxygen zone.
• Benthic: lake bottom with sediment-dwelling organisms.
• Euphotic: upper light zone where photosynthesis occurs.
Lake Types
• Oligotrophic: deep, cold, clear, nutrient-poor, oxygen-rich (e.g., Lake Superior).
• Mesotrophic: moderate depth and nutrients, supports diverse fish (e.g., Lake Ontario).
• Eutrophic: shallow, warm, nutrient-rich, murky, low oxygen (e.g., Lake Erie).
Lake Stratification & Turnover
• Summer: warm epilimnion above cool hypolimnion; separated by thermocline.
• Turnover (spring/fall): mixes oxygen and nutrients throughout layers.
Wetlands
• Covered with water part of the year; support hydrophilic vegetation and rich biodiversity.
• Ecological services: flood control, carbon storage, erosion reduction, groundwater recharge, nurseries for fish/shellfish.
• Threats: agriculture, development, pollution, invasive species, grazing, groundwater pumping.
• Solutions: buffer zones, invasive species control, native vegetation planting, reduce stormwater runoff.
Rivers and Streams
• Flow downhill; nutrients influenced by vegetation, erosion, and surrounding terrain.
• Zones:
• Source Zone: cold, clear, high-oxygen headwaters (e.g., trout habitat).
• Transition Zone: slower, warmer, more sediment and nutrients.
• Floodplain Zone: murky, nutrient-rich, empties into estuaries.
Riparian Areas
• Bordering bodies of water, rich in moisture-loving plants like willows and cottonwoods.
• Provide habitats, erosion control, and water filtration.
1.4 The Carbon Cycle
Overview of the Carbon Cycle
• Carbon is essential for all life and cycles through the biosphere, atmosphere, hydrosphere, and geosphere.
• Key biological molecules containing carbon include carbohydrates, proteins, fats, and nucleic acids (DNA/RNA).
• Atmospheric CO₂ makes up less than 1% of the atmosphere (~420 ppm), but plays a major role in climate regulation.
Carbon Sinks and Reservoirs
• Oceans: absorb atmospheric CO₂, support phytoplankton and calcium carbonate formation in marine organisms.
• Sedimentary rocks: limestone (CaCO₃) is the largest long-term carbon reservoir.
• Terrestrial biosphere: forests store large amounts of carbon in plant tissue and soil.
• Soil organic matter: includes stable forms like calcium carbonate and decomposing plant litter.
Biological Processes
• Photosynthesis: plants fix carbon from CO₂ to form glucose.
6CO₂ + 6H₂O + sunlight → C₆H₁₂O₆ + 6O₂
• Cellular respiration: releases CO₂ back into atmosphere.
C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O + energy
Geochemical and Physical Processes
• Weathering and erosion of rocks release carbon.
• Carbon precipitates into deep ocean as dead tissue or shells.
• CO₂ dissolves into oceans, increasing acidity and reducing carbonate ion availability.
• Volcanic eruptions, soil disturbance (plowing, mining), and decay release CO₂ or methane (CH₄).
Ocean Acidification
• CO₂ + H₂O → H₂CO₃ → H⁺ + HCO₃⁻
• Increased H⁺ lowers pH, reducing carbonate (CO₃²⁻) and harming organisms like corals and shellfish.
Human Impact on the Carbon Cycle
• Fossil fuel burning and deforestation release long-stored carbon into the atmosphere.
• Imbalance since the Industrial Revolution: more carbon is released than sequestered.
• Impacts include global warming, ocean acidification, sea-level rise, and extreme weather.
Largest Carbon Sinks (in descending order)
• Marine sediments & sedimentary rocks (~75 million Gt)
• Oceans (~40,000 Gt)
• Fossil fuel deposits (~4,000 Gt)
• Soil organic matter (~1,500 Gt)
• Atmosphere (from 578 Gt in 1700 to 766 Gt in 2000)
• Terrestrial plants (~580 Gt)
1.5 The Nitrogen Cycle
Overview of Nitrogen
• Nitrogen makes up 78% of Earth's atmosphere and is essential for amino acids, proteins, and nucleic acids (DNA/RNA).
• Found in soil, oceans, and living organisms.
• Often a limiting nutrient in ecosystems, especially for plant growth and productivity.
Human Impact on the Nitrogen Cycle
• Human activities such as fertilizer use, fossil fuel combustion, wastewater, and agriculture have altered nitrogen flow.
• Excess nitrogen can lead to eutrophication, water acidification, and toxicity to aquatic life.
Key Biological Processes
Nitrogen Fixation
• Converts atmospheric nitrogen (N₂) into usable forms like ammonia (NH₃) or nitrates (NO₃⁻).
• Performed by nitrogen-fixing bacteria (e.g., rhizobium in legume root nodules), lightning, or industrial methods (Haber-Bosch).
Nitrification
• Converts ammonia (NH₃) to nitrites (NO₂⁻) and then to nitrates (NO₃⁻), the form most usable by plants.
• Carried out by specialized nitrifying bacteria.
Assimilation
• Plants absorb NH₄⁺, NH₃, and NO₃⁻ through roots and use them to build proteins and nucleic acids.
• Nitrogen moves through food chains when herbivores and carnivores consume plants and animals.
Ammonification
• Decomposers (bacteria and fungi) break down dead organisms and wastes into ammonia (NH₃) or ammonium ions (NH₄⁺).
Denitrification
• Anaerobic bacteria convert nitrates (NO₃⁻) back into nitrogen gas (N₂) and nitrous oxide (N₂O), returning it to the atmosphere.
Environmental Effects
• Fertilizer runoff into waterways causes algal blooms, oxygen depletion, and fish kills (eutrophication).
• Nitrous oxide is a potent greenhouse gas contributing to climate change.
1.6 The Phosphorus Cycle
Overview of Phosphorus
• Phosphorus is essential for DNA, RNA, ATP, and cell membranes (phospholipids).
• Unlike nitrogen and carbon, phosphorus does not have a gaseous form under normal conditions and rarely enters the atmosphere.
• The cycle is slow and primarily sedimentary, occurring between land, water, and sediments.
Major Reservoirs
• Rocks, minerals, and sediments are the largest phosphorus stores.
• Released by weathering, erosion, and mining activities.
Biological Importance
• Plants absorb phosphate (PO₄³⁻) from soil; animals obtain it by consuming plants or other animals.
• Decomposers return phosphorus to the soil when organisms die or excrete waste.
Key Processes
• Weathering: releases phosphate from rocks into soil and water.
• Assimilation: uptake of phosphate by plants and incorporation into organic molecules.
• Decomposition: returns organic phosphorus to inorganic form in the soil.
• Sedimentation: phosphorus settles into sediment in aquatic systems and forms new rock over geologic time.
Human Impact on the Phosphorus Cycle
• Mining phosphate rocks for fertilizers and detergents accelerates the cycle.
• Runoff from agriculture leads to water pollution, causing algal blooms and eutrophication.
• Phosphorus accumulation in water bodies can lead to oxygen depletion and biodiversity loss.
1.7 The Hydrologic (Water) Cycle
Overview of the Water Cycle
• The hydrologic cycle describes the continuous movement of water through the atmosphere, lithosphere, hydrosphere, and biosphere.
• It is driven by solar energy and gravity, with no chemical transformation of water.
• Water plays a critical role in climate regulation, weathering, erosion, nutrient transport, and supporting life.
Key Processes
• Evaporation: liquid water changes into vapor due to heat, primarily from oceans and lakes.
• Transpiration: water vapor released from plant leaves (collectively with evaporation called evapotranspiration).
• Condensation: vapor forms clouds through cooling and atmospheric saturation.
• Precipitation: water returns to Earth as rain, snow, sleet, or hail.
• Infiltration: water soaks into soil and recharges groundwater.
• Runoff: excess surface water flows into rivers, lakes, and oceans.
Storage Locations
• Atmosphere (as vapor), surface water (lakes, rivers, oceans), glaciers and ice caps, groundwater aquifers, and organisms.
Human Impact on the Water Cycle
• Withdrawing groundwater for agriculture and consumption reduces aquifer levels.
• Deforestation decreases transpiration and increases runoff and erosion.
• Urbanization increases impervious surfaces, reducing infiltration and increasing flash floods.
• Climate change alters precipitation patterns, glacier melting, and evaporation rates.
1.8 Primary Productivity
Definition of Primary Productivity
• Primary productivity refers to the rate at which autotrophs (mainly plants and algae) convert solar energy into chemical energy via photosynthesis.
• It determines the energy available to all other organisms in an ecosystem (consumers and decomposers).
Types of Productivity
• Gross Primary Productivity (GPP): total amount of energy captured by producers through photosynthesis.
• Net Primary Productivity (NPP): energy remaining after producers use some for respiration (NPP = GPP – respiration).
• NPP represents the energy available for herbivores and higher trophic levels.
Factors Affecting Productivity
• Light availability, temperature, nutrient levels, and water availability strongly influence productivity.
• Tropical rainforests and estuaries have high NPP due to favorable climate and nutrient availability.
• Deserts, tundras, and open oceans generally have low NPP.
Importance in Ecosystems
• NPP indicates ecosystem health and potential for supporting biodiversity.
• High NPP allows for more complex and stable food webs.
• Low NPP limits energy flow and reduces species richness.
1.9 Trophic Levels
Definition of Trophic Levels
• Trophic levels refer to the hierarchical levels in a food chain that represent the flow of energy and nutrients.
• Each level consists of organisms that share the same function in the food web and obtain energy in similar ways.
Main Trophic Levels
• Primary Producers (Autotrophs): convert solar energy into chemical energy through photosynthesis (e.g., plants, algae).
• Primary Consumers: herbivores that eat producers (e.g., rabbits, insects).
• Secondary Consumers: carnivores that eat herbivores (e.g., snakes, frogs).
• Tertiary Consumers: carnivores that eat other carnivores (e.g., hawks, lions).
• Decomposers and Detritivores: break down dead organisms and recycle nutrients (e.g., fungi, bacteria, vultures).
Energy Transfer Between Levels
• Only about 10% of energy is transferred from one trophic level to the next; the rest is lost as heat (see 1.10).
• This energy loss limits the number of trophic levels in an ecosystem (typically 4–5).
Ecological Pyramids
• Pyramid of Energy: shows energy loss at each level; always upright.
• Pyramid of Biomass: shows total dry mass of organisms at each level; usually upright but can be inverted in aquatic systems.
• Pyramid of Numbers: shows the number of organisms at each level; shape varies by ecosystem.
1.10 Energy Flow and the 10% Rule
Overview of Energy Flow
• Energy enters ecosystems through sunlight and is converted into chemical energy by producers.
• Energy then flows from one trophic level to the next through consumption (herbivory, predation).
• The flow of energy is one-way—energy is not recycled like matter.
The 10% Rule
• Only about 10% of the energy from one trophic level is transferred to the next level.
• The remaining 90% is lost as heat due to metabolism, movement, growth, and reproduction.
• For example, if plants capture 1,000 kcal, only ~100 kcal is available to herbivores, and ~10 kcal to primary carnivores.
Implications of Energy Loss
• Limits the length of food chains—typically no more than 4–5 trophic levels.
• Explains why top predators are fewer in number and require large territories to find sufficient food.
• Highlights the efficiency of eating lower on the food chain (e.g., plant-based diets require less energy input).
Second Law of Thermodynamics
• States that in energy transformations, some energy is always lost as heat and becomes less useful.
• Explains why energy quality decreases as it moves through an ecosystem.
1.11 Food Chains and Food Webs
Food Chains
• A food chain is a linear sequence that shows how energy and nutrients flow from one organism to another.
• Starts with a producer (e.g., grass) → primary consumer (e.g., rabbit) → secondary consumer (e.g., snake) → tertiary consumer (e.g., hawk).
• Each step in the chain represents a different trophic level.
• Food chains are simple models and do not show complexity of real ecosystems.
Food Webs
• A food web is a network of interconnected food chains showing how energy flows through an ecosystem.
• Food webs provide a more realistic representation of feeding relationships.
• Organisms may occupy more than one trophic level depending on their diet.
Stability and Complexity
• Complex food webs tend to be more stable and resilient to disturbances.
• Keystone species play a crucial role in maintaining the structure and balance of food webs (e.g., sea otters, wolves).
Disruptions to Food Webs
• Removal or decline of key species can cause trophic cascades and ecosystem collapse.
• Human activities (e.g., pollution, overfishing, habitat destruction) often disrupt food webs.