AP Biology Unit 8 Ecology: Understanding Energy Movement in Ecosystems

Primary producers (autotrophs)

Organisms that build organic molecules (e.g., glucose) from inorganic sources (e.g., CO₂ and water), forming the base of most ecosystems’ energy supply.

Photosynthesis

Process (common in plants, algae, cyanobacteria) that captures sunlight and converts it into chemical energy stored in organic molecules.

Chemosynthesis

Production of organic molecules using chemical energy from inorganic compounds (e.g., at deep-sea hydrothermal vents) rather than sunlight.

Primary production

The rate at which producers convert energy into biomass; sets the energy budget for the entire ecosystem.

Gross Primary Productivity (GPP)

Total chemical energy captured by producers in a given area and time (total photosynthetic/chemosynthetic capture).

Net Primary Productivity (NPP)

Energy stored as new producer biomass after producers meet metabolic needs; the portion available to consumers and decomposers.

Respiration (R)

Energy producers use for cellular respiration to make ATP for metabolism; reduces energy available for growth/biomass.

NPP equation

Relationship showing available biomass energy: NPP = GPP − R.

Limiting factors (productivity controls)

Environmental constraints that restrict photosynthesis/chemosynthesis and therefore primary productivity (e.g., light, temperature, water, nutrients).

Light availability

A limiting factor for productivity; decreases with water depth and under dense canopy, reducing photosynthesis.

Temperature (as a productivity factor)

Influences enzyme activity; very low temperatures slow photosynthesis and primary productivity.

Water availability

Often the major limiting factor for primary productivity in terrestrial ecosystems.

Nutrients (nitrogen and phosphorus)

Commonly limiting resources (especially in aquatic systems and many soils) that constrain primary productivity.

Trophic level

An organism’s feeding position in a food chain or food web, defined by energy transfer (not by “rank” or complexity).

Primary consumer

Consumer that eats producers (herbivore); obtains energy directly from producer biomass.

Secondary consumer

Consumer that eats primary consumers; receives energy one trophic step removed from producers.

Tertiary consumer (and higher)

Consumer that eats secondary consumers; typically has less available energy due to cumulative losses at each transfer.

Food chain

A single, linear pathway of energy transfer (producer → herbivore → carnivore).

Food web

Network showing multiple feeding relationships and energy pathways; more realistic than a single chain and allows organisms to occupy multiple roles.

Detritus

Dead organic matter and waste that form a major pathway of energy flow separate from the grazing (plant → herbivore) pathway.

Detritivores

Organisms that ingest detritus (dead material/waste), helping move energy through detrital food webs.

Decomposers

Fungi and bacteria (especially) that chemically break down complex organic molecules into simpler compounds, releasing energy as heat via respiration and freeing nutrients.

Trophic efficiency

Fraction of energy at one trophic level that becomes incorporated into the next trophic level’s biomass; often around ~10% but highly variable.

Pyramid of energy

Diagram of energy available at each trophic level per area per time (e.g., kJ/m²/year); always upright because energy is lost as heat at each transfer.

Pyramid of biomass

Diagram of the mass of living tissue at each trophic level; can be inverted in some aquatic systems where producers (e.g., phytoplankton) have low standing biomass but high turnover/productivity.