Unit 8: Ecology — Quick Review

Population growth

Exponential: $dN/dt = rN$ . Unlimited, J-shaped.
Logistic: $dN/dt = rN(K - N)/K$ . S-shaped, plateaus at carrying capacity K.
Fastest growth rate at $N = K/2$ .

r vs. K strategists

r-selected	K-selected
Many offspring, little care	Few offspring, much care
Short-lived, fast maturation	Long-lived, slow maturation
Unstable environments	Stable environments

Survivorship curves

Type I — low early mortality (mammals).
Type II — constant mortality (birds).
Type III — high early mortality (fish, insects).

Community interactions

Competition (–/–), predation/parasitism (+/–), mutualism (+/+), commensalism (+/0).
Competitive exclusion: identical niches can't coexist.
Niche partitioning: divide resources to coexist.
Fundamental niche = potential; realized = actual.

Keystone species

Small biomass, outsized impact. Sea otters, sea stars, wolves. Removal triggers trophic cascades.

Succession

Primary — bare rock (slow). Secondary — disturbance with soil left (faster).
Intermediate disturbance hypothesis: diversity peaks at moderate disturbance.

Ecosystem energy

10% rule: ~90% of energy is lost between trophic levels.
Energy flows one-way; matter cycles.
NPP = GPP − respiration.

Four biogeochemical cycles

Cycle	Reservoir	Disruption
Water	Oceans, atmosphere	Dams, irrigation
Carbon	Atmosphere, ocean, biomass, fossil fuels	Fossil-fuel burning → climate change
Nitrogen	Atmosphere (N₂)	Fertilizer runoff → eutrophication
Phosphorus	Rocks	Mining, detergents

Biomes

Defined by climate: tropical rainforest (wet/warm), desert (dry), tundra (cold), boreal (cold/conifer), grassland (dry summers), etc.

Behavior

Innate (genetic) vs. learned (experience).
Imprinting, habituation, associative learning, insight.
Kin selection (Hamilton's rule: $rB > C$ ) explains altruism.

Human impacts

Invasive species, habitat fragmentation, climate change, pollution / eutrophication, overharvesting, sixth mass extinction.

💡 Exam Tip: On any food-web FRQ, trace effects 2–3 trophic levels away — graders reward cascade reasoning.

Key Terms

Carrying capacity (K) — max sustainable population.
Trophic cascade — change at one level propagating through the web.
Niche — role + environment + resources used.
Eutrophication — excess nutrients → algal bloom → hypoxia.
Keystone species — disproportionate impact relative to abundance.

Must-Know for the Exam

Solve a logistic growth problem given $r$ , $N$ , $K$ .
Identify a survivorship curve from a graph.
Explain competitive exclusion with an example.
Trace a trophic cascade (top-down OR bottom-up) through ≥3 levels.
Describe how humans disrupt at least one biogeochemical cycle.
Distinguish primary from secondary succession.
Apply Hamilton's rule to an altruism scenario.

💡 Exam Tip: When a graph shows population decline, check x- and y-axes carefully: is the decline caused by crossing K (density-dependent), a disturbance event (density-independent), or a trophic cascade? Named mechanism earns the point.

AP Biology Unit 8: Ecology