AP Environmental Science Unit 1 Notes: How Ecosystems and Biomes Work

Ecosystem

A system formed by interactions between a community of living organisms and the nonliving environment; emphasizes energy flow, matter cycling, and organism–environment relationships.

Biotic factors

Living (or once-living) components of an ecosystem, such as plants, animals, fungi, bacteria, and detritus.

Abiotic factors

Nonliving physical and chemical components of an ecosystem, such as sunlight, temperature, water, soil, nutrients, salinity, pH, and dissolved oxygen.

Community

All the interacting populations (living organisms) in a given area.

Habitat

The place where an organism lives (its “address”).

Ecological niche

An organism’s role in the ecosystem (its “job”), including how it gets energy, resource use, tolerated conditions, timing of activity, and interactions with other species.

Trophic level

A feeding level in an ecosystem (e.g., producer, primary consumer, secondary consumer) used to describe how energy moves through organisms.

Producer (autotroph)

An organism that captures energy (usually from sunlight via photosynthesis) and builds biomass, forming the base of most food webs.

Consumer (heterotroph)

An organism that obtains energy by eating producers and/or other consumers.

Decomposer

An organism (often bacteria or fungi) that breaks down dead organic matter and wastes, helping recycle nutrients back into the environment.

10% rule

A rough guideline that only a small fraction (often ~10%) of energy becomes biomass available to the next trophic level; most is used for life processes and lost as heat.

Gross Primary Productivity (GPP)

The total rate at which producers capture energy via photosynthesis (total energy captured).

Net Primary Productivity (NPP)

The energy producers store as biomass after subtracting their own respiration; the energy available to consumers. (NPP = GPP − R)

Food web

A realistic network of interconnected feeding relationships showing multiple pathways of energy transfer in an ecosystem.

Trophic cascade

An indirect, multi-level effect that occurs when a change at one trophic level (often top predators) alters populations and biomass at other levels.

Limiting factor

A resource or environmental condition that restricts population growth or ecosystem productivity (e.g., water in deserts, phosphorus in many freshwater systems).

Density-dependent factor

A limiting factor whose effects increase as population density increases (e.g., competition, disease).

Density-independent factor

A factor that affects populations regardless of density (e.g., drought, hurricanes, wildfires).

Disturbance

A discrete event that disrupts an ecosystem (e.g., fire, storms, pest outbreaks, logging).

Resilience

How quickly an ecosystem recovers after a disturbance.

Biome

A large geographic region characterized by a particular climate and the dominant plant communities adapted to that climate (especially temperature and precipitation).

Dissolved oxygen (DO)

Oxygen gas mixed in water that aquatic organisms need; influenced by temperature (cold water holds more DO) and by decomposition/oxygen demand.

Estuary

A highly productive coastal ecosystem where freshwater mixes with ocean water (brackish), with strong nutrient input and tidal mixing; often serves as nursery habitat.

Upwelling

The rise of deep, cold, nutrient-rich water to the surface, fueling phytoplankton growth and supporting productive fisheries.

Eutrophication

Nutrient enrichment (often nitrogen and/or phosphorus) that increases algal growth; when algae die, decomposition raises oxygen demand and can lead to low-oxygen “dead zones” or fish kills.