AP Biology - Big Ideas from Unit #8: Ecology

Comprehensive Quizlet for AP Biology Unit #8: Ecology, aligned to the 2020 (May 2025) and 2025 (May 2026+) CED – Topics 8.1–8.7. 🌎 Covers ecological responses, energy flow, population dynamics, community interactions, biodiversity, and human disruptions. ✅ Includes content recall, EVALUATE prompts, and scenario-based application, with justification and SP alignment (SP 1.A, 1.B, 1.C, 2.A, 2.D, 3.C, 4.B, 5.A, 5.D, 6.C, 6.D, 6.E). Built for clarity, mathematical precision, and exam-level reasoning.

How do organisms respond to changes in their environment and why?

They respond via behavior and physiological mechanisms. They respond to maintain homeostasis.

Stimulus

External or Internal or Combination of Signals that causes a response in an organism. The stimulus can be anything (ex. changes in weather, scent, presence of another organism, sound, etc...)

Signaling Behavior - What is it? What does it produce?

Communication between organisms that changes behavior. Produces changes in behaviors of other organisms, which can result in differential reproductive success.

5 types of Communication Mechanisms in Animals

1) Visual (See it)

2) Audible (Hear it)

3) Tactile (Touch it)

4) Electrical (Sense electric fields)

5) Chemical Signals (Smell it - pheromones)

Communication Mechanism: Visual

Using light, color, body posture, or movement to convey information.

Ex. Peacocks displaying feathers to attract mates; wolves baring teeth to threaten.

THINK: See it

Communication Mechanism: Audible

Using sound waves or vocalizations to communicate across distances.

Ex. Bird songs during mating season; alarm calls in monkeys warning of predators; wolves howling.

THINK: Hear it

Communication Mechanism: Tactile

Using physical contact to convey messages or reinforce social bonds.

Ex. term-5Bees doing the waggle dance to communicate food source locations; dogs nuzzling for bonding.

THINK: Touch it

Communication Mechanism: Electrical

Using electric fields to sense the environment or communicate.

Ex. Electric fish (e.g., knife fish) emitting electric pulses to locate mates or prey.

THINK: Sense electrical fields.

Communication Mechanism: Chemical

Releasing and detecting chemical signals (pheromones) to communicate.

Ex. Ants leaving pheromone trails to food; moths releasing sex pheromones to attract mates; Cheetah peeing on a tree to mark territory.

THINK: Smell it.

4 uses of Communication Mecchanisms

1) Indicate dominance (THINK: "assert dominance")

2) Find food

3) Establish territory claims ( THINK: "this is my territory, now! Back off.")

4) Ensure reproductive success (THINK: Finding Mates)

In regardes to responses to the environment (behaviors), Identify what Natural Selection favors.

Natural Selection favors behaviors that increase survival and reproductive success. Essentially, behaviors that increase evolutionary fitness.

Innate Behavior

Genetically controlled; can occur without prior experience/training. Behavior determined by genetics.

Learned Behavior

Behavior developed as result of experience or learned from oberservation. Learned behavior is behavior that's learned.

Differentiate between Innate and Learned Behaviors

Innate Behavior: Behavior genetically determined.

Learned Behavior: Behavior learned from experience or observation.

Cooperative Behavior

a) Identify what cooperative behavior is and provide examples.

b) Justify the usage of cooperative behaviors by organisms.

a) Cooperative Behavior is teamwork between organisms of the same species. Individuals of the same species work together to increase survival and reproductive success.

Examples include wolf pack hunting and bee colony work.

b) Organisms use cooperative behaviors to increase individual fitness and population survival.

THINK: Cooperative Behavior = Same species individuals working together to increase evolutionary fitness; relevant as it increases individual fitness and overall population survival.

Ex. Wolf Pack Hunting

Aposematism

Evolved warning traits: can be markings, behaviors, and/or chemicals. Used to discourage predation.

Ex. Coral Snakes (Distinct RYB pattern to indicate venom presence to predators), Skunk (Arches back, raising its tail, turning its back, and stopping feet before spraying).

Mammals establish territories using scent marking. Identify the type of communication mechanism used.

The communication mechanism used by mammals to establish territories through scent marking is chemical communication.

Taxis

Directed movement toward (+) or away (-) from a stimulus. Movement must be in a clear direction relative to the stimulus.

Ex. Fish swimming toward higher oxygen concentration; A moth flying toward a light source (positive taxis).

THINK: Moving directionally, directional movement.

Kinesis

Random movement in response to a stimulus, characterized by changes in speed or activity rate rather than direction. Movement is random and only the activity level changes, not direction.

Ex. Pill bugs moving faster in dry areas; Woodlice moving randomly but more rapidly in dry environments.

THINK: Random movement--change in speed and/or activity rate, NOT direction.

Differentiate between Taxis and Kinesis

Taxis: Specific directional movement (towards or away from stimulus).

Kinesis: Random movement, change in speed/activity rate, NOT direction (as a result of stimulus).

Migration

Seasonal long-distance movement of organisms triggered by environmental cues. Movement is seasonal and often involves inherited cues like the position of the sun or magnetic fields.

Ex. Monarch butterflies migrating to Mexico; Salmon swimming upstream to reproduce.

Hibernation

Dormancy during cold seasons involving slowed metabolism and reduced physiological activity.

Ex. Bears hibernating during winter; Ground squirrels lowering metabolism during winter months.

THINK: Hide from the Cold.

Estivation

Dormancy during hot and dry seasons to survive drought and extreme heat.

Ex. Desert frogs burrowing underground; Lungfish estivating in dried-up ponds during drought.

THINK: Hibernation but in summer -- Escape the Heat.

Differentiate between Hibernation and Estivation

Hibernation: Hide from the Cold.

Estivation: Hibernation but in summer -- Escape the Heat.

Circadian Rhythm

Internal biological clock with a ~24-hour cycle influenced by environmental light-dark patterns.

Ex. Sleep-wake cycles in humans.

Evolution of Behavior

Behaviors evolve through natural selection when they increase an organism's fitness (survival and reproduction). Behaviors that enhance survival and reproduction are favored and passed on genetically.

Misconception Check: Taxis is any random movement.

Clarification: Taxis is directed movement toward or away from a stimulus. Random movement without direction is kinesis.

Misconception Check: Kinesis is movement toward a stimulus.

Clarification: Kinesis is non-directional; it is a change in the rate of activity or speed in response to a stimulus.

Misconception Check: Migration is short-term movement.

Clarification: Migration is long-distance, seasonal movement, often driven by environmental cues and usually genetically programmed.

Misconception Check: Hibernation and estivation occur for the same reason.

Clarification: Hibernation happens during cold seasons to survive low temperatures; estivation happens during hot/dry seasons to survive extreme heat and drought.

Misconception Check: All behavioral responses are learned.

Clarification: Many behaviors are innate (genetically hardwired) and do not require learning, such as taxis, migration, and reflexes.

Misconception Check: Organisms only respond to external stimuli.

Clarification: Organisms respond to both internal and external stimuli to maintain homeostasis and ensure survival.

Misconception Check: Fight-or-flight is a learned behavior.

Clarification: Fight-or-flight is an innate physiological response to a perceived threat, involving rapid systemic changes like increased heart rate.

Misconception Check: Plants do not exhibit behavioral responses

Clarification: Plants do show behavioral responses to stimuli, like phototropism (growth toward light) and photoperiodism (response to length of day/night).

Misconception Check: All communication between organisms is verbal.

Clarification: Communication can occur through visual, auditory, tactile, chemical, and electrical signals, not just sound.

Misconception: Natural selection does not act on behaviors.

Clarification: Natural selection does act on behaviors because behaviors that improve survival and reproduction are more likely to be passed on.

Misconception Check: Fitness only includes survival.

Clarification: No! Fitness includes both survival and REPRODUCTIVE SUCCESS.

Misconception Check: Behaviors are not genetically controlled

Clarification: Innate Behaviors (genetically controlled) and Learned Behaviors (behaviors learned, not genetically controlled) exist.

Misconception Check: Communication between organisms is done only through external cues.

Clarification: We also use internal cues, like the release of hormones, to communicate information within our bodies or other organisms.

Ecology

Scientific study of interactions between organisms and the environment.

Organismal Ecology

How an organism's structure, function, physiology, and behavior meet challenges posed by its environment.

Population

Groups of individuals of the same species living in an area.

Population Ecology

Analyzing factors affecting population size -- how and why change over time.

Community

Group of populations of different species in an area.

Community Ecology

How interactions between species affect community structure and organization.

Ecosystem

Community of organisms in an area and physical factors organisms interact with.

Ecosystem Ecology

Energy flow and chemical cycling between organisms and environment.

Landscape Ecology

Factors controlling energy, materials, and organisms across multiple ecosystems.

Biosphere

Global ecosystem.

Global Ecology

How regional energy and material exchange influences functioning and distribution of organisms across the biosphere (the world!).

Biotic Factors

Living factors (ex. animals, plants).

Abiotic Factors

Non-living factors (ex. water, sunlight).

Ecosystems in Equillibrium

Ecosystem is in balance, number of each type of organism stays constant. Note that ecosystems can be different sizes.

Climate

Long-term prevailing weather conditions in area.

Food chain

Series of organisms that transfer energy in form of food to each other in a single directions. Multiple food chains in an ecosystem.

Trophic Level

Organism's position in a food chain based on energy transfer; feeding levels.

Primary Producer

Autotrophs (convert light energy into chemical energy). Trophic level that supports all the others.

Autotrophs

Organisms that make their own food through photosynthesis or chemosynthesis.

Primary Consumers

First organisms to get energy from producers. Herbivores.

Secondary Consumers

Organisms that eat primary consumers. Carnivores.

Tertiary Consumers

Organisms that eat secondary consumers. Carnivores that eat other carnviores.

Heterotrophs

Organisms in trophic levels above autotrophs. They get their food from other organisms.

Quarternary Consumers

Organisms that eat animals in Tertiary level and below. Not very common.

Apex predator

Top of the food chain; no natural predators.

Herbivore

Organisms that only eat producers (plants).

Ex. Deer.

THINK: Herbivores = Plant Eater

Omnivores

Organisms that eat both primary producers (plants) and consumers (animals).

Ex. Bears.

THINK: Ominivores = Plant + Meat Eater

Carnivores

Organisms that eat consumers (other animals).

Ex. Snakes

THINK: Carnivores = Meat Eater

What happens when organisms in any trophic level die?

When organisms die in any trophic level, they're eaten by scavengers and reamins are broken up by decomposers.

What determines an organism's trophic level?

An organism's trophic level is the level above the highest food level it eats.

What is energy used for in organisms?

Energy is used to maintain homeostasis, support growth, drive reproduction, and power cellular processes.

Homeostasis

The regulation of internal conditions to maintain a stable environment necessary for survival and cellular function.

THINK: Homeostasis = Stability for Survival.

Where does life on Earth initially obtain energy?

Most life on Earth obtains energy initially from the sun through photosynthesis. Some life forms, particularly in extreme environments, obtain energy from inorganic chemical sources through chemosynthesis.

THINK: Sunlight powers most ecosystems (photosynthesis!); inorganic chemicals power a few (chemosynthesis!).

CED (From CollegeBoard): "Autotrophs capture energy from physical or chemical sources in the environment...

(a) Photosynthetic organisms capture energy present in sunlight.

(b) Chemosynthetic organisms capture energy from small inorganic molecules present in their environment, and this process can occur in the absence of oxygen"

Explain how the activities of autotrophs enable the flow of energy within an ecosystem

CED (From CollegeBoard): "Autotrophs capture energy from physical or chemical sources in the environment...

(a) Photosynthetic organisms capture energy present in sunlight.

(b) Chemosynthetic organisms capture energy from small inorganic molecules present in their environment, and this process can occur in the absence of oxygen"

THINK: Photosynthesis & Chemosynthesis

Differentiate between the 2 types of Autotrophs

1) Photosynthetic Organisms: Capture Sunlight

2) Chemosynthetic Organisms: Capture energy from small, inorganic molecules present in their environment with or without oxygen.

THINK: Photosynthetic = Capture Energy from Sun; Chemosynthetic = Capture Energy from Inorganic Chemical Molecules

Explain how the activities of heterotrophs enable the flow of energy within an ecosystem -- Where do heterotrophs get their energy from?

CED (From CollegeBoard): "Heterotrophs capture energy present in carbon compounds produced by other organisms.

Heterotrophs may metabolize carbohydrates, lipids, and proteins as sources of energy by hydrolysis."

Note that they don't metabolize nucleic acids, but metabolize the other biological macromolecules.*

*This concept connects us to Unit #1, when we're discussing biological macromolecules (2020 1.3-1.5, 2025 1.3-1.7).

Who performs Photosynthesis? Do all organisms do photosynthesis?

Photosynthesis is performed by (a) plants, (b) photosynthetic bacteria, (c) algae!

"All organisms do Cellular Respiration, but not all do photosynthesis."

Explain how endotherms maintain body temperature.

Endotherms use thermal energy (generated by metabolism) to maintain homeostatic body temperatures.

Ex. change in heart rate, fat storage, shivering - muscular contractions.

CED (from CollegeBoard): "Endotherms use thermal energy generated by metabolism to maintain homeostatic body temperatures."

THINK: Uses thermal energy (internal system) generated by metabolism to maintain homeostatic body temperatures .

Explain how ectotherms maintain body temperature

Ecotherms lack efficient internal mechanisms to regulate and maintain homeostatic body temperatures. Thus, they rely on behaviors to regulate temperature.

Ex. Moving into or out of sunshine.

CED (from CollegeBoard): "Ectotherms lack efficient internal mechanisms for maintaining body temperature, though they may regulate their temperature behaviorally by moving into the sun or shade or by aggregating with other individuals."

THINK: No internal system to regulate homeostatic body temperatures, regulates body temperature via behavior.

Semelparity

Reproduction where an organism produces all offspring in 1 event. It is known as big bang reproduction.

THINK: "All offspring produced in one shot--big bang reproduction).

Iteroparity

Reproduction where organisms produce offspring over many years. They reproduce several times. It is known as repeated reproduction.

THINK: Repeated reproduction. New offspring several times in organism's life.

Explain how two critical factors influence whether a species will evolve toward semelparity or iteroparity.

CED (from CollegeBoard): "Different organisms use various reproductive strategies in response to energy availability."

Life history traits like semelparity and iteroparity are shaped by natural selection to maximize reproductive success under different environmental constraints. High offspring mortality and low adult survival favor semelparity, while stable environments with higher survival rates favor iteroparity.

Key takeaway:

(a) Organisms that produce many offspring at a time (r-selected) are LESS energy-efficient and tend to be in UNSTABLE environments as resources aren't readily available and the environment experiences frequent change.

(b) Organisms that produce few offpsiring at a time (K-selected) are MORE energy-efficient and tend to be in STABLE environments.

THINK:

(a) Unstable Environment, LESS Energy-Efficient --> Semelparity (Big Bang Reproducton).

(b) Stable Environment, MORE Energy-Efficient --> Iteroparity (Repeated Reproduction).

K-selection

Selection for advantageous traits at high density. Operates in populations living at density near carrying capacity.

r-selection

Selection for maximizing reproductive success in low desity environments. Operates in population densities well below carrying capacity; little individual competition.

Metabolic Rate

Amount of energy used by an organism over a specific amount of time.

Relationship between Metabolic Rate and Size of the Organism

Inverse Relationship. ⬆️ Metabolic Rate = ⬇️ Organism Size.

Big organisms have low metabolic rates. Small organisms have high metabolic rates.

2025 CED (May 2026+ EXAMS) ONLY: Carbon Cycle

The Carbon Cycle describes the movement of carbon among the atmosphere, biosphere (living organisms), oceans, and geosphere (soil, rocks).

Key processes:

(a) Photosynthesis removes carbon dioxide (CO₂) from the atmosphere.

(b) Cellular respiration releases CO₂ back into the atmosphere.

(c) Decomposition of organisms returns carbon to soil and air.

(d) Combustion of fossil fuels releases stored carbon into the atmosphere.

CED (from CollegeBoard): "The carbon cycle involves recycling carbon atoms through Earth's biosphere into organisms as carbohydrates and back into the atmosphere as carbon dioxide (CO₂). Atthehighestlevelsoforganization,the carboncyclecanbesimplifiedintofour parts: photosynthesis, cellular respiration, decomposition, and combustion."

THINK: Carbon = Energy and Biomass Backbone; Photosynthesis in, Respiration and Combustion out.

2025 CED (May 2026+ EXAMS) ONLY: Nitrogen Cycle

The Nitrogen Cycle describes how nitrogen moves between the atmosphere, soil, and living organisms.

Key processes:

(a) Nitrogen fixation: Bacteria convert atmospheric N₂ into usable forms (ammonia, ammonium).

(b) Nitrification: Soil bacteria convert ammonia into nitrates (NO₃⁻) and nitrites (NO₂⁻).

(c) Assimilation: Plants absorb nitrates and incorporate them into organic molecules (proteins, nucleic acids).

(d) Ammonification: Decomposition releases ammonia back into the soil.

(e) Denitrification: Bacteria convert nitrates back into atmospheric N₂.

CED (from CollegeBoard): "The nitrogen cycle involves several steps, includingnitrogenfixation,assimilation, ammonification,nitrification,anddenitrification. These steps are performed by microorganisms in the soil. The largest reservoir of nitrogen is theatmosphere.Innitrogenfixation,nitrogen gas (N2) isfixedintoammonia (NH3), which ionizestoammonium(NH4 +) by acquiring hydrogen ions from the soil solution."

THINK: Nitrogen = Essential for Amino Acids, DNA, ATP; Bacteria drive this cycle!

2025 CED (May 2026+ EXAMS) ONLY: Hydrologic (Water) Cycle

The Water Cycle describes the continuous movement of water among the atmosphere, surface water, groundwater, and living organisms.

Key processes:

(a) Evaporation: Water changes from liquid to vapor, entering the atmosphere.

(b) Transpiration: Plants release water vapor into the air.

(c) Condensation: Water vapor cools and forms clouds.

(d) Precipitation: Water falls to Earth's surface (rain, snow, sleet).

(e) Infiltration: Water soaks into soil and becomes groundwater.

(f) Runoff: Water flows over land into bodies of water.

CED (from CollegeBoard): "The hydrologic (water) cycle involves water movement and storage within the hydrosphere. Reservoirs include oceans, surface water, the atmosphere, and living organisms. Processes include evaporation, condensation, precipitation, and transpiration."

THINK: Water = Earth's Life Conveyor

2025 CED (May 2026+ EXAMS) ONLY: Phosphorus Cycle

The Phosphorus Cycle describes the movement of phosphorus through rocks, soil, water, and living organisms.

Key points:

(a) Weathering of rocks releases phosphate (PO₄³⁻) into soil and water.

(b) Plants absorb phosphates; consumers obtain phosphorus through food.

(c) Decomposition returns phosphorus to soil or sediments.

(d) Phosphorus does NOT cycle through the atmosphere significantly.

CED (from CollegeBoard): "The phosphorus cycle involves weathering rocks releasing phosphate (PO₄³⁻) into soil and groundwater. Producers take in phosphate, which is incorporated into biological molecules; consumers eat producers, transferring phosphate to animals. Phosphorus returns to the soil via decomposition of biomass, or excretion. Phosphate can also be incorporated back into the environment via decomposition of decaying organic matter."

THINK: Phosphorus = DNA, ATP, Bones; Rock-based, No Atmospheric Gas Phase.

2025 CED (May 2026+ EXAMS) ONLY: How do human activities impact the Carbon, Nitrogen, Phosphorus, and Hydrologic (Water) cycles?

(a) Carbon Cycle: Burning fossil fuels releases excess carbon dioxide (CO₂) into the atmosphere, intensifying the greenhouse effect and contributing to climate change.

(b) Nitrogen Cycle: Excess fertilizer use introduces large amounts of reactive nitrogen into ecosystems, leading to eutrophication, hypoxia, and disruption of aquatic ecosystems.

(c) Phosphorus Cycle: Fertilizer runoff increases phosphorus levels in aquatic systems, causing excessive algal growth (algal blooms) and resulting in oxygen depletion (hypoxia).

(d) Hydrologic (Water) Cycle: Urbanization increases surface runoff and decreases infiltration into groundwater. Deforestation reduces transpiration rates. Both activities alter the natural flow of water and can increase flood risks and degrade water quality.

THINK: Burning → Climate change; Fertilizer → Eutrophication + Dead Zones; Urbanization → Runoff, Flooding, Water Pollution.

2025 CED (May 2026+ EXAMS) ONLY: Explain how energy flows and matter cycles through trophic levels. (2025 CED 8.2.B).

(a) Energy flows one way through ecosystems — from producers to consumers to decomposers — and is lost as heat at each transfer.

(b) Matter cycles between the environment and organisms through biogeochemical cycles (carbon, nitrogen, phosphorus, water).

(c) Each cycle demonstrates conservation of matter and relies on abiotic and biotic reservoirs (e.g., atmosphere, soil, organisms).

(d) Energy flow and matter cycling are essential for life and ecosystems' stability.

THINK: Energy = One-way flow → Lost as heat; Matter = Cycles back and forth; Life depends on both.

What does a (+) NET GAIN IN ENERGY result in?

A net gain in energy results in energy storage or growth.

THINK: +NET Energy = Energy Storage or Growth

What does a (-) NET LOSS IN ENERGY result in?

A net loss in energy results in loss of mass and possibly death.

THINK: -NET Energy = Loss of Mass and Possibly Death

What do changes in energy availability result in? (2)

1) Changes in energy availability result in changes in population size.

2) Changes in energy availability can result in disruptions to an ecosystem.

Know that...

(a) A change in energy resources (e.g, sunlight) can affect the amount and size of trophic levels.

(b) Changes in the producer level can affect the amount and size of other trophic levels.

Relationship between Population Size and Going up Trophic Levels

Inverse Relationship. ⬆️ Trophic Levels = ⬇️ Population Size.

Population size decreases as you go up trophic levels due to the 10% energy transfer rule between levels and the other 90% is lost as heat or used for growth.

10% Energy Rule

Only 10% of the energy in one level moves up to the next level, while 90% is lost due to heat/energy. Recall that Energy Transfer is inefficient.

Detrivitores AKA Decomposers

Consumers that get their energy from nonliving organic material (detritus).

Ex. Prokaryotes (Bacteria), Fungi

Detritus

Nonliving Organic Material (ex. Dead organism remains, feces, fallen leaves). Note they're often consumed by secondary and tertiary consumers.

Importance of Detrivitores/Decomposers in Chemical Cycling

Decomposers convert organic matter from all trophic levels to inorganic compounds usable by primary producers.

Law of Conservation of Energy

Energy cannot be created nor destroyed -- only transformed.