Unit 1 – The Living World: Ecosystems

What is an Ecosystem?

An ecosystem is a biological community of interacting organisms and their physical environment. It includes all living (biotic) and non-living (abiotic) components in a particular area that work together as a functional unit. Ecosystems can range in size from a small pond to an entire forest, and they are the fundamental units of organization in environmental science.

Understanding ecosystems is crucial for the AP Environmental Science exam because it forms the foundation for studying energy flow, nutrient cycling, and environmental interactions throughout the course.

Biotic Factors

Biotic factors are all the living components in an ecosystem. These include all organisms that directly or indirectly affect each other and the environment. Biotic factors can be classified into three main categories:

• Producers (Autotrophs): Organisms that make their own food through photosynthesis or chemosynthesis. Examples include plants, algae, and phytoplankton. Producers form the base of every food chain by converting solar energy into chemical energy stored in organic compounds.
• Consumers (Heterotrophs): Organisms that obtain energy by consuming other organisms. They are further divided into herbivores (plant-eaters), carnivores (meat-eaters), omnivores (eat both plants and animals), and detritivores (feed on dead organic matter).
• Decomposers: Specialized organisms (bacteria and fungi) that break down dead organic matter and waste products, returning nutrients to the soil. Decomposers play a critical role in nutrient cycling and closing biogeochemical loops.

Abiotic Factors

Abiotic factors are all the non-living physical and chemical components of an ecosystem that influence living organisms. These factors determine which species can survive in a particular environment and shape ecosystem characteristics. Key abiotic factors include:

• Sunlight: Primary energy source for most ecosystems; drives photosynthesis and influences temperature
• Temperature: Affects metabolic rates, species distribution, and determines biome classification
• Water: Essential for all life; availability determines ecosystem productivity and species composition
• Soil: Provides nutrients, anchorage for plants, and habitat for many organisms
• Nutrients: Chemical elements (nitrogen, phosphorus, carbon) required for organism growth
• pH: Acidity or alkalinity of soil and water affects organism survival and nutrient availability
• Salinity: Salt concentration in water determines which aquatic organisms can survive
• Oxygen: Necessary for aerobic respiration; levels vary in aquatic ecosystems

Understanding Biomes

Terrestrial biomes are large geographical regions on Earth characterized by distinct climate conditions, particularly temperature and precipitation patterns. These two factors are the primary determinants of biome classification and dictate which plant and animal species can survive in each region. The same biome can occur in geographically distant areas with similar climates.

For the AP Environmental Science exam, you must understand the distinguishing features of each major terrestrial biome, including climate data, characteristic vegetation, and typical biodiversity levels.

🌴 Tropical Rainforest

Location: Near the equator (Amazon Basin, Congo Basin, Southeast Asia)

Characteristics: Highest biodiversity of all terrestrial biomes. Year-round plant growth with no distinct seasons. Dense canopy layers (emergent, canopy, understory, forest floor). Nutrient-poor soil because nutrients are rapidly cycled. Broad-leaved evergreen trees with drip tips for water runoff.

Exam Focus: Highest NPP (Net Primary Productivity) of terrestrial biomes due to abundant sunlight and water. Threatened by deforestation for agriculture and logging.

🏜️ Desert

Location: 30° N and 30° S latitude (Sahara, Mojave, Gobi, Atacama)

Characteristics: Lowest precipitation of all biomes. Low biodiversity with highly adapted species. Sparse vegetation including cacti, succulents, and drought-resistant shrubs. Plants have adaptations like CAM photosynthesis, water storage, extensive root systems, and reduced leaf surface area. Animals are often nocturnal to avoid extreme daytime heat.

Exam Focus: Formed by rain shadow effects and descending air at 30° latitude. Lowest NPP of terrestrial biomes. Vulnerable to desertification from overgrazing and climate change.

🌾 Temperate Grassland (Prairie)

Location: Interior of continents (North American Great Plains, Eurasian Steppe, Pampas)

Characteristics: Dominated by grasses and herbaceous plants with few to no trees. Deep, fertile soil (mollisols) rich in organic matter. Frequent fires that prevent tree establishment and maintain grassland. Large grazing animals like bison, antelope, and prairie dogs. Strong seasonal temperature variation.

Exam Focus: Often converted to agriculture (breadbaskets of the world). Fire is a natural disturbance that maintains the ecosystem. Susceptible to soil erosion when plowed.

🍂 Temperate Deciduous Forest

Location: Eastern North America, Western Europe, Eastern Asia

Characteristics: Trees lose leaves seasonally (deciduous) to conserve water during cold winters. Four distinct seasons with clear seasonal changes. Dominated by broad-leaved hardwood trees (oak, maple, beech, hickory). Rich, fertile soil from annual leaf litter decomposition. High biodiversity with stratified vegetation layers.

Exam Focus: Moderate NPP. Trees are deciduous to reduce water loss when soil freezes. Many areas have been cleared for human development and agriculture.

🌲 Taiga/Boreal Forest

Location: Northern Canada, Scandinavia, Russia (between 50-60°N latitude)

Characteristics: Largest terrestrial biome by area. Dominated by coniferous (cone-bearing) evergreen trees like spruce, fir, and pine. Needle-like leaves reduce water loss and shed snow. Long, harsh winters and short growing seasons. Low biodiversity compared to forests at lower latitudes. Acidic, nutrient-poor soil.

Exam Focus: Coniferous trees are adapted to cold with needle leaves and flexibility. Important for timber industry and carbon storage. Vulnerable to climate change impacts.

❄️ Tundra

Location: Arctic regions above 60°N (northern Alaska, Canada, Russia, Greenland)

Characteristics: Coldest biome with permafrost (permanently frozen subsoil). No trees; vegetation limited to low-growing plants (mosses, lichens, grasses, small shrubs). Very short growing season (6-10 weeks). Minimal decomposition due to cold temperatures. Low biodiversity. Animals include caribou, arctic foxes, polar bears, and migratory birds.

Exam Focus: Permafrost prevents deep root penetration and tree growth. Lowest NPP of all biomes. Highly vulnerable to climate change; thawing permafrost releases methane (greenhouse gas).

🦁 Savanna (Tropical Grassland)

Location: Sub-Saharan Africa, South America, Australia

Characteristics: Grassland with scattered individual trees (not dense enough to form canopy). Distinct wet and dry seasons. Frequent fires during dry season maintain grassland by preventing tree dominance. Large herds of grazing mammals (zebras, wildebeest, elephants). Predators include lions, cheetahs, and hyenas.

Exam Focus: Fire and grazing are essential disturbances that maintain savanna structure. Seasonal rainfall patterns drive migration of herbivores.

💡 Exam Tip: For the AP exam, memorize the temperature and precipitation ranges for each biome. Practice using climate diagrams (temperature and precipitation graphs) to identify biomes. Remember: temperature and precipitation are the two PRIMARY factors determining biome distribution!

Overview of Aquatic Biomes

Aquatic biomes are ecosystems characterized by the presence of water and classified primarily by salinity (salt concentration), depth, water flow, and distance from shore. Aquatic biomes cover approximately 75% of Earth's surface and are divided into two main categories: freshwater and marine.

Key factors affecting aquatic biomes:

• Salinity: Freshwater (<1% salt) vs. Marine (~3.5% salt)
• Depth: Determines light penetration (photic vs. aphotic zones)
• Turbidity: Water clarity affects photosynthesis rates
• Nutrient availability: Influences productivity
• Temperature: Affects dissolved oxygen and metabolic rates

💧 Freshwater Biomes

Streams and Rivers

Characteristics: Flowing freshwater systems. Rivers are larger and carry more water than streams. Water is constantly moving, preventing thermal stratification. High oxygen content due to water turbulence and mixing.

Headwaters (source): Cold, clear, fast-flowing, high oxygen, rocky substrate

Downstream: Warmer, slower flow, higher turbidity, more nutrients, muddy substrate

Lakes and Ponds

Characteristics: Standing (lentic) freshwater systems. Lakes are larger and deeper than ponds. Stratified into distinct zones based on light penetration and depth.

Lake Zones (important for exam!):

• Littoral Zone: Shallow water near shore where sunlight reaches bottom; most photosynthesis occurs here; emergent and submerged plants present
• Limnetic Zone: Open water surface area; extends to depth of light penetration; dominated by phytoplankton (algae) and zooplankton
• Profundal Zone: Deep water below light penetration (aphotic); too dark for photosynthesis; low oxygen; decomposers and adapted fish
• Benthic Zone: Bottom sediments across all depths; detritus accumulates; inhabited by bottom-dwellers (benthic organisms) like insect larvae, worms, and bacteria

Lake Productivity Classification: Oligotrophic (low nutrients, clear water, low productivity) → Mesotrophic (moderate) → Eutrophic (high nutrients, murky water, high productivity, algal blooms)

Freshwater Wetlands

Characteristics: Areas where soil is saturated with water for all or part of the year. Include marshes, swamps, and bogs.

Ecological Importance: Extremely high productivity. Filter pollutants and excess nutrients from water. Provide flood control by absorbing excess water. Critical habitat for amphibians, waterfowl, and many plant species. Wetlands are among the most productive ecosystems on Earth.

Exam Focus: Wetlands are often called "nature's kidneys" because they filter water. Many have been drained for agriculture and development, leading to habitat loss and increased flooding.

🌊 Marine Biomes

Oceans (Open Water)

Characteristics: Saltwater ecosystems covering ~70% of Earth's surface. High salinity (~3.5%). Divided into zones by depth and distance from shore.

Depth Zones:

• Photic Zone (Euphotic): Upper ~200m where sunlight penetrates; photosynthesis occurs; high productivity; phytoplankton thrive
• Aphotic Zone: Below ~200m; no sunlight; no photosynthesis; organisms rely on detritus falling from above or chemosynthesis at hydrothermal vents

Horizontal Zones:

• Pelagic Zone: Open water column away from shore; dominated by swimming organisms (fish, whales, jellyfish)
• Benthic Zone: Ocean floor at all depths; organisms adapted to pressure, cold, darkness

Coral Reefs

Location: Shallow, warm tropical and subtropical waters (typically between 30°N and 30°S latitude)

Characteristics: Highest biodiversity of marine ecosystems (often called "rainforests of the sea"). Built by coral animals that secrete calcium carbonate (CaCO₃) skeletons. Corals have mutualistic relationship with zooxanthellae (photosynthetic algae) that provide up to 90% of coral's energy through photosynthesis. Clear, warm water (20-30°C), low turbidity, and stable salinity required.

Exam Focus: Threatened by coral bleaching (caused by warming ocean temperatures), ocean acidification, pollution, and physical damage. Coral bleaching occurs when stressed corals expel their zooxanthellae, losing color and energy source.

Estuaries

Characteristics: Areas where freshwater from rivers meets and mixes with saltwater from the ocean. Salinity varies with tides, river flow, and location. Experience regular tidal fluctuations.

Ecological Importance: Among the most productive ecosystems on Earth due to high nutrient input from rivers. Serve as nurseries for many commercially important fish and shellfish species. Filter pollutants before they reach open ocean. Support diverse communities including salt marsh grasses, mangroves (in tropics), oysters, crabs, and waterfowl.

Exam Focus: Estuaries are highly productive but vulnerable to pollution from upstream sources. Many have been filled or dredged for development. Mangroves in tropical estuaries provide coastal protection from storms and erosion.

⚠️ Common Pitfall: Don't confuse photic/aphotic zones with pelagic/benthic zones! Photic/aphotic refer to light availability (vertical), while pelagic/benthic refer to water column vs. bottom (horizontal). They describe different dimensions of aquatic ecosystems.

Overview

The carbon cycle describes the movement of carbon atoms through Earth's atmosphere, biosphere, hydrosphere, and geosphere. Carbon is the fundamental building block of all organic molecules (carbohydrates, proteins, lipids, nucleic acids) and is essential for all life. The carbon cycle has both fast (biological) and slow (geological) components.

Major Carbon Reservoirs: Atmosphere (as CO₂), oceans (dissolved CO₂ and carbonate), fossil fuels (coal, oil, natural gas), rocks and sediments (limestone), soil organic matter, living organisms (biomass).

Key Processes in the Carbon Cycle

Human Impacts on the Carbon Cycle

• Fossil Fuel Combustion: Releases ancient carbon stored underground for millions of years, rapidly increasing atmospheric CO₂ levels
• Deforestation: Reduces photosynthesis capacity and releases stored carbon from trees when burned or decomposed
• Agriculture: Tilling soil releases soil carbon; livestock produce methane (CH₄), a potent greenhouse gas
• Climate Change: Elevated CO₂ levels trap heat (greenhouse effect), warming Earth's climate

💡 Exam Tip: Remember that photosynthesis and cellular respiration are opposite processes! Photosynthesis removes CO₂ from the atmosphere (carbon sink), while respiration, decomposition, and combustion return CO₂ to the atmosphere (carbon sources). Be able to write and recognize the chemical equations for both processes.

Overview

The nitrogen cycle describes the movement of nitrogen through the atmosphere, soil, water, and living organisms. Nitrogen is essential for all life as a component of proteins, DNA, RNA, and ATP. The atmosphere is 78% nitrogen gas (N₂), but most organisms cannot use N₂ directly because of its strong triple bond. Specialized bacteria are required to convert N₂ into biologically usable forms.

Major Nitrogen Reservoirs: Atmosphere (as N₂ gas - largest reservoir), soil (organic matter, NH₄⁺, NO₃⁻), living organisms (proteins, nucleic acids), groundwater and surface water.

Key Processes in the Nitrogen Cycle

Human Impacts on the Nitrogen Cycle

• Synthetic Fertilizers: Haber-Bosch process produces ammonia-based fertilizers, massively increasing nitrogen available to crops but causing runoff pollution
• Eutrophication: Excess nitrogen (from fertilizers, sewage) causes algal blooms in water bodies, leading to oxygen depletion (dead zones) when algae die and decompose
• Air Pollution: Combustion of fossil fuels produces nitrogen oxides (NOₓ) that contribute to smog and acid rain
• Greenhouse Gas: Agricultural practices increase N₂O emissions, a greenhouse gas ~300x more potent than CO₂

💡 Exam Tip: Remember the order: Fixation → Nitrification → Assimilation → Ammonification → Denitrification. Know which bacteria are involved in each step! Nitrogen fixation and denitrification involve the atmosphere (N₂), while the middle steps occur in soil. Legumes + Rhizobium = nitrogen fixation is a frequently tested concept!

Overview

The phosphorus cycle describes the movement of phosphorus through rocks, soil, water, and living organisms. Phosphorus is essential for life as a component of DNA, RNA, ATP (cellular energy), and phospholipids (cell membranes). Unlike carbon and nitrogen, phosphorus has NO significant atmospheric component - it does not form stable gaseous compounds. This makes the phosphorus cycle the slowest of the biogeochemical cycles.

Major Phosphorus Reservoirs: Rocks and sediments (largest reservoir), soil, ocean sediments, living organisms (biomass), dissolved in water (smallest reservoir).

Key Processes in the Phosphorus Cycle

Human Impacts on the Phosphorus Cycle

• Mining: Humans extract phosphate rock to produce fertilizers, disrupting long-term geological storage
• Agricultural Runoff: Excess phosphorus fertilizers wash into waterways, causing severe eutrophication and algal blooms
• Detergents: Phosphate-containing detergents contribute to water pollution (many now banned)
• Wastewater: Sewage and animal waste release high concentrations of phosphorus into water systems
• Limited Supply: Unlike nitrogen, phosphorus cannot be synthesized; we rely on mining finite phosphate rock deposits

⚠️ Common Pitfall: Remember that phosphorus is the ONLY major biogeochemical cycle without an atmospheric component! It moves through rock → soil → organisms → water → sediments → rock. The lack of atmospheric phase makes this cycle extremely slow and means phosphorus is often a limiting nutrient in ecosystems.

Overview

The hydrologic cycle (water cycle) describes the continuous movement of water through Earth's atmosphere, surface, and subsurface in its three phases: liquid, solid (ice), and gas (water vapor). The cycle is driven by solar energy and gravity. Water is essential for all life and plays a critical role in climate regulation, nutrient transport, and ecosystem function.

Major Water Reservoirs: Oceans (97% of Earth's water), ice caps and glaciers (2%), groundwater, lakes and rivers (0.6%), atmosphere (0.001%), soil moisture, living organisms.

Key Processes in the Hydrologic Cycle

Human Impacts on the Hydrologic Cycle

• Deforestation: Reduces transpiration and infiltration, increases runoff and erosion
• Urbanization: Impervious surfaces (concrete, asphalt) prevent infiltration, increasing runoff and flood risk
• Agriculture: Irrigation depletes groundwater faster than recharge; soil compaction reduces infiltration
• Dams and Reservoirs: Alter natural water flow, increase evaporation from large surface areas
• Climate Change: Alters precipitation patterns, increases evaporation, accelerates glacier melting

💡 Exam Tip: Know the difference between infiltration (water soaking INTO soil), percolation (water moving THROUGH soil downward), and runoff (water flowing OVER land surface). Understand how human activities like urbanization and deforestation disrupt natural water cycle processes. Be able to define watershed/drainage basin!

Overview

Primary productivity is the rate at which producers (autotrophs) convert solar energy into chemical energy stored in organic compounds (biomass) through photosynthesis. It represents the foundation of ecosystem energy flow and determines how much energy is available to support all other organisms in the food chain. Primary productivity is typically measured in units of energy per area per time (e.g., kcal/m²/year or g/m²/year).

Understanding primary productivity is crucial for the AP Environmental Science exam because it explains ecosystem carrying capacity, food web structure, and environmental limitations on life.

Gross Primary Productivity (GPP)

Gross Primary Productivity (GPP) is the total rate at which producers capture and store energy through photosynthesis in a given area over a specific time period. It represents ALL the energy fixed by photosynthesis before any is used by the producers themselves.

Net Primary Productivity (NPP)

Net Primary Productivity (NPP) is the rate at which energy is stored in plant biomass after producers have used some energy for their own cellular respiration (R). NPP represents the energy actually available to consumers (herbivores) and is what supports the rest of the food chain.

Key Point: NPP is analogous to a business's profit - it's what remains after operating expenses (respiration) are subtracted from revenue (GPP).

Factors Affecting Primary Productivity

Several environmental factors limit or enhance primary productivity in ecosystems:

• Sunlight: Essential for photosynthesis; more light generally means higher productivity. In aquatic systems, light penetration decreases with depth (photic vs. aphotic zones).
• Water Availability: Terrestrial productivity limited by precipitation. Deserts have low NPP; tropical rainforests have high NPP.
• Temperature: Affects enzyme activity and metabolic rates. Warm temperatures generally increase productivity (within tolerance range).
• Nutrients: Nitrogen (N) and phosphorus (P) are often limiting nutrients - whichever is in shortest supply limits productivity. In aquatic systems, nutrient availability can limit phytoplankton growth.
• CO₂ Concentration: Raw material for photosynthesis; rarely limiting in natural systems but increased levels can enhance productivity.

💡 Exam Tip: You MUST be able to calculate NPP using the formula NPP = GPP - R. Remember: only about 1% of solar energy hitting Earth is captured by producers (GPP), and only 40% of GPP becomes NPP. The rest is lost to respiration. NPP determines how many consumers an ecosystem can support!

Overview

Trophic levels represent the hierarchical feeding positions in a food chain, showing how energy and matter flow through an ecosystem. Each level indicates the number of energy transfers an organism is away from the original solar energy source. Trophic levels help us understand ecosystem structure, energy flow, and population dynamics.

The word "trophic" comes from the Greek word trophos, meaning "feeding" or "nourishment."

The Five Trophic Levels

⚠️ Common Pitfall: Don't confuse trophic levels with population sizes! Higher trophic levels typically have SMALLER populations and LESS biomass than lower levels, even though individual organisms may be larger. This is due to energy loss at each transfer.

Overview

Energy flows through ecosystems in one direction - from the sun → producers → consumers → decomposers. Unlike nutrients, which are recycled, energy is not recycled. At each trophic level, a large amount of energy is lost as heat through cellular respiration, movement, and other metabolic processes. This energy loss follows the laws of thermodynamics and fundamentally shapes ecosystem structure.

The 10% Rule is one of the most important concepts in AP Environmental Science for understanding energy transfer between trophic levels.

The 10% Rule Explained

Why is 90% of Energy Lost?

• Cellular Respiration: Organisms use most consumed energy to power life processes (movement, growth, reproduction, maintaining body temperature). This energy is released as heat and lost to the environment.
• Incomplete Consumption: Not all organisms at a lower level are eaten. Many die and are decomposed instead.
• Incomplete Digestion: Not all parts consumed are digested and absorbed. Indigestible parts (bones, fur, cellulose) are excreted as waste.
• Laws of Thermodynamics: Second law states that energy transformations are inefficient - some energy is always lost as heat (entropy increases).

Consequences of the 10% Rule

• Limited Food Chain Length: Most food chains have only 4-5 trophic levels because insufficient energy reaches higher levels to support viable populations.
• Biomass Pyramids: Total biomass (living matter) decreases at each successive trophic level, creating a pyramid shape when graphed.
• Energy Pyramids: Total energy available decreases at each trophic level, always forming a true pyramid (never inverted).
• Population Sizes: Higher trophic levels support fewer individuals. There are more producers than herbivores, more herbivores than carnivores.
• Human Food Production: Eating lower on the food chain (plants instead of meat) is more energy-efficient. It takes ~10 kg of grain to produce 1 kg of beef.

💡 Exam Tip: The 10% Rule is HEAVILY tested on the AP exam! Practice calculations where you multiply or divide by 0.1 (10%) to find energy at different trophic levels. Remember: energy flows ONE WAY (not recycled), biomass and population size generally decrease with each level, and this explains why food chains are short!

Food Chains

A food chain is a linear sequence showing how energy and matter flow from one organism to another through feeding relationships. Each step in a food chain represents a trophic level. Food chains show a simplified, single pathway of energy transfer.

Key Characteristics of Food Chains:

• Linear and simplified representation
• Always begin with a producer (autotroph)
• Arrows show direction of energy flow (what eats what)
• Typically limited to 4-5 levels due to energy loss

Food Webs

A food web is a complex network of interconnected and overlapping food chains that shows all feeding relationships in an ecosystem. Food webs are more realistic representations because most organisms eat multiple types of food and are eaten by multiple predators.

Why Food Webs are Important:

• Show Complexity: Reveal intricate feeding relationships and energy pathways
• Demonstrate Interconnections: Show how species depend on multiple food sources
• Explain Stability: More connections = more stable ecosystem. If one species declines, predators have alternative food sources.
• Predict Impacts: Help predict consequences of removing or adding species

Types of Food Chains

⚠️ Common Pitfall: In food chains and webs, arrows point in the direction of ENERGY FLOW (from food source to consumer), NOT the direction of "who eats who." So "Grass → Rabbit" means energy flows from grass to rabbit (rabbit eats grass). Never draw arrows backward!

📚 Study Strategies

Practice drawing all four biogeochemical cycles from memory. Create flashcards for biome characteristics. Work through multiple 10% Rule problems. Use mnemonic devices for nitrogen cycle bacteria (e.g., "Nitrifying Nitrosomonas and Nitrobacter"). Compare food chains to food webs in diagrams.