Populations & Communities: every key term you need (+ practice quiz)

More AP Environmental Science guides

• Earth Systems
• Biodiversity & Conservation
• Energy Flow & Nutrient Cycles
• Land Use & Sustainability

Energy Flow

Transfer of energy through ecosystem; from sun → producers → consumers; 10% efficiency between trophic levels.

Trophic Level

Position in food chain: producers (1), primary consumers (2), secondary consumers (3), tertiary consumers (4).

Producer

Autotroph that captures solar energy and converts to chemical energy; plants, algae, photosynthetic bacteria.

Consumer

Heterotroph eating other organisms; primary eats plants, secondary eats primary, tertiary eats secondary.

Decomposer

Organism breaking down dead matter (bacteria, fungi); returns nutrients to soil.

Biomass

Total dry weight of living organisms; decreases at higher trophic levels (10% rule).

Bioaccumulation

Toxin accumulation in organism over time; concentration increases with age.

Biomagnification

Toxin concentration increases at higher trophic levels; top predators most contaminated.

Nutrient Cycle

Movement of elements through organisms and environment; carbon, nitrogen, phosphorus, water cycles.

Carbon Cycle

Carbon movement: atmosphere → producers → consumers → decomposers → atmosphere; fossil fuels interrupt.

Nitrogen Cycle

Nitrogen fixation (N₂→NH₃), nitrification, assimilation, denitrification; bacteria essential.

Phosphorus Cycle

Phosphorus cycling through rock weathering, soil, organisms; no gaseous form; limiting nutrient in freshwater.

Water Cycle

Evaporation, transpiration, condensation, precipitation, infiltration, runoff; driven by solar energy.

NPK

Nitrogen, phosphorus, potassium; essential plant macronutrients; fertilizer components.

Limiting Factor

Resource or condition limiting population growth; often least abundant relative to need.

Primary Productivity

Rate of biomass production by producers; GPP (gross) minus respiration = NPP (net).

Gross Primary Productivity

Total energy fixed by producers through photosynthesis.

Net Primary Productivity

Energy available to consumers after respiration costs; actual growth of producers.

Eutrophication

Nutrient enrichment causing excess algae growth, oxygen depletion, dead zones in water.

Dead Zone

Water area with insufficient dissolved oxygen; kills aquatic life; caused by algae decomposition.

Logistic growth

S-shaped population curve where growth slows as N approaches carrying capacity K; growth rate = rN(1 - N/K).

Exponential growth

J-shaped growth under unlimited resources; population doubles at a constant interval, N = N0·e^(rt).

Overshoot and dieback

Population exceeds K, degrades resources, then crashes; can lower K permanently (reindeer on St. Matthew Island).

Density-dependent factor

Limiting factor whose effect strengthens with crowding: competition, predation, disease, waste accumulation.

Density-independent factor

Limiting factor unrelated to population size: floods, fires, hurricanes, extreme cold.

Type I, II, III survivorship

Type I: high survival until old age (humans); Type II: constant mortality (birds); Type III: massive juvenile death (fish, oysters).

r-selected species

Many offspring, little parental care, early maturity, short life; thrive in disturbed environments (dandelions, mosquitoes).

K-selected species

Few offspring, heavy parental care, long life, populations near K; slow to recover (elephants, whales).

Rule of 70

Doubling time (years) ≈ 70 ÷ percent growth rate; a 2% growth rate doubles a population in about 35 years.

Total fertility rate (TFR)

Average number of children a woman bears in her lifetime; replacement level is about 2.1 in developed nations.

Demographic transition model

Four/five stages: high birth and death → falling death → falling birth → low both → possible decline; tracks industrialization.

Age-structure diagram

Population pyramid by age and sex; wide base predicts rapid growth (population momentum), columnar shape means stability.

Population momentum

Growth continues even after TFR falls to replacement because a large cohort of young people is still entering reproductive years.

Infant mortality rate

Deaths under age 1 per 1,000 live births; strongly linked to sanitation, healthcare, and women's education, and inversely to TFR.

Resource partitioning

Competing species divide a niche (different feeding heights, times, prey sizes) to reduce competition and coexist.

Competitive exclusion principle

Two species cannot occupy the identical niche indefinitely; one outcompetes the other or niches diverge.

Mutualism vs. commensalism vs. parasitism

Both benefit (+/+); one benefits, other unaffected (+/0); one benefits at other's expense (+/−).