The Living World — AP Environmental Science
1. Ecosystems and Biodiversity ★★☆☆☆ ⏱ 4 min
An ecosystem is a discrete functional unit consisting of a community of living organisms interacting with their surrounding physical environment (e.g., a temperate forest, a coral reef, or a small pond). Biodiversity refers to the variety of life across all levels of ecological organization, with three core measurable dimensions.
- **Genetic diversity**: Variation in genes within a single species, which increases resilience to disease and environmental change (e.g., variation in drought tolerance among individual oak trees)
- **Species diversity**: Variety of distinct species in a habitat, measured via species richness (total number of distinct species) and species evenness (relative abundance of each species)
- **Ecosystem diversity**: Variety of distinct habitat types across a geographic region.
H' = -\sum (p_i \ln p_i)
Where $p_i$ is the proportion of total individuals in a sample belonging to species $i$.
2. Energy Flow and Biogeochemical Cycles ★★☆☆☆ ⏱ 4 min
Energy enters all ecosystems via primary producers (autotrophs like plants or algae) through photosynthesis, and transfers through the ecosystem across trophic levels (producer → primary consumer → secondary consumer → tertiary consumer).
6CO_2 + 6H_2O + \text{solar energy} \rightarrow C_6H_{12}O_6 + 6O_2
Energy movement across trophic levels follows the **10% rule**: only ~10% of energy from one trophic level transfers to the next. The remaining 90% is lost as heat via cellular respiration, excreted as waste, or contained in unconsumed biomass.
Biogeochemical cycles describe the movement of essential nutrients between biotic and abiotic pools. The most commonly tested cycles on the AP exam are:
- **Carbon cycle**: Largest pool is ocean sediments and fossil fuels; human activity (burning fossil fuels, deforestation) adds excess CO₂ to the atmosphere, driving climate change
- **Nitrogen cycle**: 78% of the atmosphere is N₂, but only nitrogen-fixing bacteria can convert it to usable ammonia (NH₃) for plants; synthetic fertilizer use adds excess nitrogen to aquatic systems, causing eutrophication
- **Phosphorus cycle**: No atmospheric pool, all phosphorus comes from weathering of rock; it is the most common limiting nutrient for freshwater ecosystems.
3. Population Dynamics and Ecological Succession ★★★☆☆ ⏱ 4 min
Population dynamics describes how the size of a species population changes over time in response to resource availability and environmental conditions. There are two core growth models used to describe population change:
N_t = N_0 e^{rt}
Where $N_0$ = initial population size, $r$ = intrinsic growth rate, $t$ = time.
N_t = \frac{K N_0 e^{rt}}{K + N_0 (e^{rt} - 1)}
Ecological succession describes the predictable sequence of species colonization of a habitat after a disturbance. There are two main types:
- **Primary succession**: Occurs on bare rock with no existing soil (e.g., after volcanic eruption, glacial retreat). Pioneer species (lichens, mosses) break down rock to form soil, followed by grasses, shrubs, and finally a climax community, taking 100+ years to complete.
- **Secondary succession**: Occurs after a disturbance that leaves soil intact (e.g., forest fire, abandoned farmland). It progresses 2-5x faster than primary succession, as soil and seed banks are already present.
4. Symbioses and Trophic Interactions ★★☆☆☆ ⏱ 4 min
Symbiosis is a long-term, close biological interaction between two species of different taxa, with three core types tested on the AP exam:
- **Mutualism (+/+)**: Both species benefit from the interaction (e.g., bees get nectar from flowering plants, plants get pollinated in return)
- **Commensalism (+/0)**: One species benefits, the other is completely unharmed and receives no benefit (e.g., barnacles attached to whales get access to food-rich moving water, whales experience no measurable impact)
- **Parasitism (+/-)**: One species (parasite) benefits by feeding off a host species, which is harmed but usually not immediately killed (e.g., ticks feeding on deer blood, which can transmit disease to the deer)
Trophic interactions describe feeding relationships between species across trophic levels. Two key terms that are frequently tested are:
- **Keystone species**: A species with a disproportionately large impact on its ecosystem relative to its biomass. For example, sea otters control sea urchin populations that would otherwise destroy kelp forests that support hundreds of other species.
- **Trophic cascade**: A change in the population size of a top trophic level causes cascading effects across all lower trophic levels. For example, reintroducing wolves to Yellowstone National Park reduced elk overgrazing, allowing tree populations to recover.
5. AP Style Concept Check ★★★☆☆ ⏱ 4 min
Common Pitfalls
Why: Students skim question prompts and skip labeling trophic levels before starting calculations.
Why: Students memorize cycle facts in isolation without active comparison.
Why: Students forget symbiosis requires a persistent, long-term association between species.
Why: Students forget primary succession requires complete removal of existing soil.
Why: Students rush calculations and skip the step of summing total individuals in the sample.