The basics
What it covers: Responses to the environment; energy flow through ecosystems; population ecology and growth models; density-dependent and density-independent limiting factors; community ecology (species interactions); biodiversity; and ecosystem disruptions.
Exam weight: 10–15% of the AP Biology exam.
The big question: How do organisms, populations, and communities interact with each other and their environment, and what happens when those systems are disrupted?
Big Ideas covered: Evolution (BI 1), Energetics (BI 2), Systems Interactions (BI 4).
Key topics at a glance
Responses to the Environment
Behavioral responses (e.g., migration) and physiological responses (e.g., hibernation) that help organisms survive environmental challenges.
Energy Flow & the 10% Rule
Only about 10% of energy transfers between trophic levels — the rest is lost as metabolic heat. This is why food chains rarely exceed 4-5 levels.
Exponential vs. Logistic Growth
Exponential: J-shaped, unlimited resources. Logistic: S-shaped, growth slows as the population nears carrying capacity (K).
Density-Dependent Factors
Competition, predation, and disease — effects intensify as population density increases.
Density-Independent Factors
Natural disasters and extreme weather — limit population growth regardless of population density.
Species Interactions
Competition (-/-), predation (+/-), mutualism (+/+), commensalism (+/0), parasitism (+/-).
Keystone Species
A species with a disproportionately large effect on its community relative to its abundance — removing it can reshape the whole ecosystem.
Biodiversity & Stability
Higher biodiversity generally increases ecosystem resilience by providing more functional redundancy against disturbance.
Ecosystem Disruptions
Wildfires, invasive species, and climate change can reshape community structure and reduce biodiversity.
Energy Flows; Matter Cycles
Energy moves one-directionally through an ecosystem (lost as heat). Nutrients (carbon, nitrogen) are recycled through biogeochemical cycles.
The key terms you must know
- Population density / dispersion — individuals per unit area / the spatial pattern of those individuals.
- Exponential growth — unlimited, J-shaped population growth with no resource constraints.
- Logistic growth — S-shaped growth that slows as the population approaches carrying capacity.
- Carrying capacity (K) — the maximum population size an environment can sustainably support.
- Density-dependent / density-independent factors — limiting factors whose effect scales with population size vs. factors that don't.
- Trophic level — a feeding position in a food chain (producer, primary consumer, etc.).
- Food chain / food web — a single linear energy path vs. the full interconnected network of feeding relationships.
- Competition / predation / mutualism / commensalism / parasitism — the five major types of species interactions.
- Keystone species — a species with outsized influence on its community relative to its abundance.
- Biodiversity — the variety of species (and genetic variation) within an ecosystem.
- Ecosystem disruption — a disturbance (natural or human-caused) that alters ecosystem structure and function.
Key themes to remember
- Energy flows; it doesn't cycle. Energy enters as sunlight and is ultimately lost as heat at every trophic transfer. Matter, by contrast, is recycled.
- The 10% rule explains almost everything about food chain length and biomass. Why there are far fewer top predators than producers, and why food chains rarely go past 4-5 levels.
- Real populations follow logistic, not exponential, growth. Exponential growth is the exception (e.g., invasive species in a new habitat with no limits yet).
- Density-dependent factors get WORSE as a population grows; density-independent factors don't care how big the population is.
- Biodiversity is a buffer. More species and genetic variation generally mean more redundancy and resilience when a disturbance hits.
- Disruptions aren't always "bad" in an evolutionary sense — they're selective pressures. They change which traits and species are favored going forward.
Common exam traps
- Exponential growth is the EXCEPTION, not the default. Most real populations follow logistic growth because resources are limited.
- Density-dependent ≠ "depends on the environment." It specifically means the factor's effect intensifies as the population gets MORE crowded (e.g., competition, disease spread).
- Mutualism and commensalism are NOT the same. Mutualism: both benefit. Commensalism: one benefits, the other is unaffected (not harmed).
- A keystone species isn't necessarily the most abundant species. Its importance comes from its disproportionate ECOLOGICAL EFFECT, not its population size.
- Energy pyramids get smaller going up because of energy LOSS, not because animals "eat less." Most energy is lost as metabolic heat at each level.
- Biodiversity loss doesn't just mean "fewer species" — it reduces the functional redundancy that makes ecosystems resilient to future disturbances.