Energy Movement Through Ecosystems (AP Environmental Science Unit 1)

Primary productivity

The rate at which primary producers convert solar energy (or chemical energy in some ecosystems) into chemical energy stored in biomass.

Primary producers

Organisms (mainly plants, algae, and some bacteria) that make organic molecules from inorganic sources and serve as the “energy gateway” into ecosystems.

Gross primary productivity (GPP)

The total rate at which producers capture energy (usually via photosynthesis) and store it as chemical energy.

Net primary productivity (NPP)

The rate at which producers store energy as new biomass available to consumers after subtracting energy used for producer respiration.

Producer respiration (R)

Energy producers use for their own cellular processes (maintenance, growth, transport), reducing the energy available as new biomass.

Limiting factor

A resource in shortest supply relative to demand that restricts photosynthesis, growth, or primary productivity.

Light penetration (photic zone depth)

How deeply sunlight reaches in water; it helps determine how much photosynthesis can occur in aquatic ecosystems.

Nutrient runoff

Movement of nutrients (often nitrogen and phosphorus) from land into water, which can artificially increase producer growth in aquatic systems.

Algal bloom

Rapid increase in algae (often driven by nutrient pollution) that can disrupt aquatic ecosystems.

Oxygen depletion

A drop in dissolved oxygen in water, often occurring after algal blooms when decomposition and respiration consume oxygen.

Trophic level

An organism’s feeding position in an ecosystem—where it gets its energy (producer, consumer levels, decomposers/detritivores).

Autotroph

A primary producer that makes its own organic molecules from inorganic sources (e.g., plants, algae, cyanobacteria).

Primary consumer (herbivore)

A consumer that eats producers (plants/algae).

Secondary consumer

A consumer that eats primary consumers.

Tertiary consumer

A consumer that eats secondary consumers (and may be a top predator in some food chains).

Decomposer

Organisms (especially fungi and bacteria) that break down dead organic matter and wastes, recycling nutrients back into the ecosystem.

Detritivore

Organisms (e.g., earthworms and many aquatic invertebrates) that feed on detritus—dead organic matter and wastes.

Detrital pathway

The flow of energy through dead organic matter and waste (detritus) to detritivores and decomposers, rather than through the grazing pathway.

Ecological pyramids

Models that show how energy, biomass, or number of organisms changes across trophic levels.

Pyramid of energy

A diagram showing energy available at each trophic level per unit area per unit time; always upright because energy is lost as heat at each transfer.

Pyramid of biomass

A diagram showing the total mass of living organic matter at each trophic level; can be unusual in aquatic systems due to low standing biomass but high producer turnover.

Pyramid of numbers

A diagram showing the number of organisms at each trophic level; can be inverted (e.g., one tree supporting many insects), making it less informative than energy pyramids.

Energy flow

The one-way movement of energy through ecosystems: enters (usually as sunlight), moves through feeding, and is ultimately lost as heat via respiration (energy is not recycled).

10% rule

A rule of thumb that about 10% of energy at one trophic level is transferred to the next level as new biomass; most is lost as heat or enters detrital material.

Trophic cascade

Ripple effects across trophic levels caused by a change at one level (e.g., removing a top predator increases herbivores, which can decrease producers).