AP Environmental Science Unit 1 — The Living World: Ecosystems
1. Ecosystems, Biotic Factors, and Abiotic Factors
Ecosystem
An ecosystem is all the living and nonliving things in an area and how they interact.
Biotic Factors
Biotic factors are the living parts of an ecosystem.
Examples:
Plants
Animals
Bacteria
Fungi
Abiotic Factors
Abiotic factors are the nonliving parts of an ecosystem.
Examples:
Sunlight
Water
Temperature
Soil
Air
Rocks
2. Biogeochemical Cycles
Biogeochemical Cycle
A biogeochemical cycle is the movement of nutrients and elements through living and nonliving parts of Earth.
Examples:
Carbon cycle
Nitrogen cycle
Phosphorus cycle
Water cycle
Sink
A sink is a place that stores a substance.
Example:
Oceans store large amounts of carbon.
Source
A source is something that releases a substance.
Example:
Burning fossil fuels releases carbon dioxide.
3. Carbon Cycle
Carbon moves between the atmosphere, organisms, soil, oceans, and fossil fuels.
Photosynthesis
Plants take in CO₂ from the atmosphere and use sunlight to make food.
CO₂ → Plants
Cellular Respiration
Plants and animals break down food and release CO₂.
Plants/Animals → CO₂
Decomposition
Decomposers break down dead organisms and waste. Carbon moves into the soil and can eventually return to the atmosphere.
Dead organisms → Decomposition → Soil/CO₂
Combustion
Burning fossil fuels releases stored carbon as CO₂.
Fossil fuels → Burning → CO₂
Why does burning fossil fuels increase atmospheric carbon?
Fossil fuels contain carbon that has been stored underground for millions of years. Burning them releases this carbon as CO₂ into the atmosphere.
Simple Carbon Cycle
CO₂ in atmosphere → Photosynthesis → Plants → Animals → Respiration → CO₂
Dead organisms → Decomposition → Soil
Fossil fuels → Combustion → CO₂
4. Photosynthesis and Cellular Respiration
Photosynthesis Equation
6CO₂ + 6H₂O + sunlight → C₆H₁₂O₆ + 6O₂
Carbon dioxide + water + sunlight → glucose + oxygen
Cellular Respiration Equation
C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O + energy
Glucose + oxygen → carbon dioxide + water + energy
Major Carbon Sink
The oceans are a major carbon sink because they store large amounts of carbon.
Major Carbon Source
A major human-caused carbon source is burning fossil fuels.
5. Nitrogen Cycle
Most nitrogen is found in the atmosphere as N₂.
Plants cannot directly use atmospheric N₂, so nitrogen must be changed into usable forms.
Nitrogen Fixation
Nitrogen fixation changes atmospheric N₂ into NH₃/NH₄⁺.
Usually done by nitrogen-fixing bacteria.
N₂ → NH₃/NH₄⁺
Nitrification
Bacteria change:
NH₃/NH₄⁺ → NO₂⁻ → NO₃⁻
Ammonia/ammonium → nitrite → nitrate
Assimilation
Plants take up nitrate (NO₃⁻) or other usable nitrogen compounds and use them to make proteins and other molecules.
Plants → Animals when animals eat plants
Ammonification / Mineralization
Decomposers break down dead organisms and waste and return nitrogen to the soil as NH₃/NH₄⁺.
Dead organisms/waste → NH₃/NH₄⁺
Denitrification
Bacteria change NO₃⁻ back into atmospheric N₂.
NO₃⁻ → N₂
Major Nitrogen Reservoir
The atmosphere is the major reservoir of nitrogen.
Simple Nitrogen Cycle
N₂ atmosphere
↓
Nitrogen fixation
↓
NH₃/NH₄⁺
↓
Nitrification
↓
NO₂⁻
↓
NO₃⁻
↓
Assimilation by plants
↓
Animals
↓
Death/waste
↓
Ammonification
↓
NH₃/NH₄⁺
NO₃⁻ → Denitrification → N₂
6. Phosphorus Cycle
Phosphorus is mainly found in rocks and sediments.
Phosphorus Cycle
Rocks → Weathering → Phosphate in soil/water → Plants → Animals → Death/waste → Decomposition → Soil → Sediments → Rocks
Major Phosphorus Reservoir
Rocks and sediments
Why is phosphorus often a limiting nutrient?
Phosphorus does not have a major atmospheric form and is released slowly from rocks through weathering.
7. Hydrologic (Water) Cycle
The water cycle describes how water moves through Earth’s atmosphere, land, oceans, and organisms.
Evaporation
Liquid water → water vapor
The Sun provides energy for evaporation.
Condensation
Water vapor cools and becomes tiny water droplets, forming clouds.
Precipitation
Water falls from clouds as:
Rain
Snow
Sleet
Hail
Runoff
Water flows over land into rivers, lakes, and oceans.
Infiltration
Water enters the soil.
Groundwater
Water stored underground.
Major Water Reservoir
The oceans contain most of Earth’s water.
Simple Water Cycle
Sun
↓
Evaporation
↓
Condensation
↓
Clouds
↓
Precipitation
↓
Runoff/Infiltration
↓
Rivers/Groundwater
↓
Ocean
↓
Evaporation
8. Species Interactions
Mutualism (+/+)
Both species benefit.
Example:
Bee + flower
The bee gets food and the flower gets pollinated.
Parasitism (+/−)
One species benefits while the other is harmed.
Example:
Tick + dog
The tick gets food while the dog is harmed.
Predator-Prey (+/−)
A predator hunts and eats prey.
Example:
Wolf + deer
Wolf = predator
Deer = prey
Interspecific Competition
Competition between different species.
Example:
Lions and hyenas competing for food.
“Interspecific” = different species.
Intraspecific Competition
Competition between members of the same species.
Example:
Two deer competing for food.
“Intraspecific” = same species.
9. Fundamental vs. Realized Niche
Fundamental Niche
The full range of conditions and resources a species could use if there were no competition or other limitations.
Think:
“Where could it live?”
Realized Niche
The conditions and resources a species actually uses because of competition, predators, and other factors.
Think:
“Where does it actually live?”
10. Climatograms and Biomes
Climatogram
A graph showing climate conditions over time.
Temperature = line
Precipitation = bars
Desert
Very little precipitation
Can be hot or cold
Dry conditions
Tundra
Very cold
Low precipitation
Short growing season
Permafrost is common
Tropical Rainforest
Warm year-round
Very high precipitation
High biodiversity
High productivity
Temperate Deciduous Forest
Moderate precipitation
Four seasons
Trees lose leaves during colder months
Chaparral / Shrubland
Hot, dry summers
Mild, wet winters
Shrubs and small trees
Fires are common
11. Primary Productivity
Primary Productivity
The rate at which producers create biomass/chemical energy.
GPP — Gross Primary Productivity
The total amount of energy captured by producers through photosynthesis.
NPP — Net Primary Productivity
The energy left for plant growth after plants use some energy for cellular respiration.
Formula
NPP = GPP − Respiration
NPP is the energy available to consumers.
12. Terrestrial Primary Productivity
Lowest Primary Productivity
Deserts
They have very little water, so plants cannot grow much.
Highest Primary Productivity
Tropical rainforests
They have:
Lots of sunlight
Lots of water
Warm temperatures
Long growing seasons
13. Food Webs and Trophic Levels
Producer
Makes its own food, usually through photosynthesis.
Example:
Grass
Primary Consumer
Eats producers.
Examples:
Grasshopper
Rabbit
Mouse
Secondary Consumer
Eats primary consumers.
Examples:
Frog
Some snakes
Tertiary Consumer
Eats secondary consumers and is often near the top of the food chain.
Examples:
Hawk
Fox
Omnivore
Eats both plants and animals.
Example:
Fox or mouse
Example Food Web
Grass → Grasshopper → Frog → Snake → Hawk
Grass → Rabbit → Fox → Hawk
Grass → Mouse → Snake
Grasshopper → Mouse → Fox
Important Food Web Rule
Arrows point toward the organism receiving the energy.
Example:
Grass → Rabbit
means the rabbit gets energy from the grass.
14. 10% Rule and Energy Transfer
10% Rule
Only about 10% of energy is transferred from one trophic level to the next.
About 90% is lost, mostly as heat and through metabolism.
Example
Plants = 10,000 units
Primary consumers = 1,000 units
Secondary consumers = 100 units
Tertiary consumers = 10 units
Energy Pyramid
Top:
Tertiary consumers — least energy
Secondary consumers
Primary consumers
Bottom:
Producers — most energy
Ultimate Energy Source
The Sun is the ultimate source of energy for most ecosystems.
15. Specialists vs. Generalists
Specialist
A species with a narrow niche that depends on specific resources or conditions.
Example:
Panda
Pandas have a specialized diet centered heavily around bamboo.
Generalist
A species with a broad niche that can use many different resources and habitats.
Example:
Raccoon
Raccoons can eat many types of food and live in many environments.
Easy Way to Remember
Specialist = few resources
Generalist = many resources
16. Laws of Thermodynamics
First Law of Thermodynamics
Energy cannot be created or destroyed.
It can only be transformed from one form to another.
Example:
Sunlight → chemical energy in plants
Second Law of Thermodynamics
Every energy transfer results in some energy being lost as heat.
Energy transfers are never 100% efficient.
This helps explain the 10% rule.
17. Yellowstone Wolves, Keystone Species, and Trophic Cascades
Wolves were reintroduced to Yellowstone in 1995.
Why were wolves reintroduced?
Wolves were reintroduced partly to help control the elk population and reduce overgrazing.
What happened?
Wolves ↑
↓
Elk ↓ / elk behavior changed
↓
Less overgrazing
↓
Plants ↑
↓
More food and habitat for other organisms
Keystone Species
A keystone species is a species that has a very large effect on its ecosystem compared with its population size.
Removing a keystone species can cause major ecosystem changes.
Why are wolves a keystone species?
Wolves affect elk populations and behavior, which changes plant growth and affects other organisms.
Trophic Cascade
A trophic cascade happens when a change at one trophic level causes changes at other trophic levels.
How are keystone species and trophic cascades related?
A keystone species can cause a trophic cascade because changing its population can affect multiple trophic levels.
Example:
Wolves ↑ → Elk ↓ → Plants ↑ → Other organisms benefit
18. Eutrophication
What Causes Eutrophication?
Too many nutrients, especially nitrogen and phosphorus, enter a body of water.
Common sources:
Fertilizer runoff
Sewage
Animal waste
What Happens?
Excess nitrogen/phosphorus
↓
Algal bloom
↓
Algae die
↓
Decomposers break down algae
↓
Decomposers use oxygen
↓
Dissolved oxygen decreases
↓
Fish and other aquatic organisms may die
Cultural Eutrophication
Eutrophication caused or greatly increased by human activities.
Key Idea
Too many nutrients → algae grow → decomposition increases → oxygen decreases → aquatic life suffers
MUST-KNOW CONNECTIONS
Energy Flow
Sun → Producers → Primary Consumers → Secondary Consumers → Tertiary Consumers
Only about 10% of energy moves to the next level.
Carbon Connection
Photosynthesis removes CO₂ from the atmosphere.
Respiration and combustion add CO₂ to the atmosphere.
Nitrogen Connection
N₂ → Fixation → NH₃/NH₄⁺ → Nitrification → NO₃⁻ → Plants
NO₃⁻ → Denitrification → N₂
Keystone Species Connection
Keystone species → major ecosystem effects → trophic cascade
Eutrophication Connection
Fertilizer → Nitrogen/Phosphorus → Algal bloom → Decomposition → Low oxygen → Fish deaths
Productivity Connection
GPP = total energy captured
NPP = energy left after respiration
NPP = GPP − respiration
Niche Connection
Fundamental niche = where a species could live
Realized niche = where a species actually lives