What the exam asks
- Evaluate resilience: Explain why an ecosystem with greater genetic and species diversity is better able to withstand or recover from environmental disruptions.
- Identify keystone species: Use experimental removal data (bar graphs with error bars or species-richness timelines) to identify a keystone species and justify the claim.
- Predict trophic cascades: Predict the short-term and long-term consequences of adding or removing a producer, consumer, keystone species, or essential abiotic factor.
- Quantify diversity: Calculate or compare Simpson’s Diversity Index values using the formula provided on the AP Equations and Formulas sheet.
Core ideas
Biodiversity and ecosystem resilience
Resilience is an ecosystem’s ability to resist change and recover quickly after a disturbance (such as drought, fire, or a novel pathogen). Natural and artificial ecosystems with fewer component parts and little diversity among the parts are often less resilient to changes in the environment.
- Genetic diversity within a population: Ensures a greater probability that some individuals carry alleles conferring resistance to a new disease or abiotic stress.
- Species diversity within a community: Provides functional redundancy. If multiple species occupy similar ecological roles (e.g., several distinct grass species acting as primary producers), the decline of one species is compensated for by the others, stabilizing energy flow to higher trophic levels.
Quantitatively, AP Biology measures diversity using Simpson’s Diversity Index (given on your formula sheet):
where is the total number of organisms of a particular species and is the total number of organisms of all species. Values range from to ; values closer to indicate higher species richness and evenness, and therefore higher resilience.
Keystone species, producers, and essential factors
The structure and diversity of a community are maintained by critical biotic and abiotic components:
| Component | Ecological role | Consequence of removal |
|---|---|---|
| Keystone species | Exerts strong control on community structure not by numerical abundance, but by its pivotal ecological niche (e.g., top predator, mutualist, or ecosystem engineer). | Ecosystem collapse: Often triggers a trophic cascade where one prey species monopolizes a limiting resource through competitive exclusion, drastically reducing overall species diversity. |
| Primary producers | Capture light or chemical energy and convert inorganic carbon into organic biomass, supporting all higher trophic levels. | Bottom-up collapse: Immediate drop in net primary productivity (NPP) and carrying capacity for all heterotrophs. |
| Essential abiotic factors | Nonliving resources (water, sunlight, dissolved oxygen, limiting nutrients like nitrogen and phosphorus) required for metabolic processes. | Alters habitat viability and shifts competitive balances (e.g., nutrient runoff causing eutrophication and hypoxia). |