The Living World: Biodiversity
Fast-track review of all 7 topics — biodiversity types, ecosystem services, island biogeography, tolerance, succession, and key exam strategies.
Introduction to Biodiversity
Biodiversity encompasses three distinct types — each tested separately on the AP exam.
| Type | Definition | Key Example | Why It Matters |
|---|---|---|---|
| Species Diversity | Variety of species — includes richness (count) AND evenness (relative abundance) | Forest with 50 equally abundant species | Drives ecosystem stability and function |
| Genetic Diversity | Variation in genes within a species/population | Multiple wheat varieties; wolf pack genetics | Enables adaptation; resistance to disease and environmental change |
| Ecosystem Diversity | Variety of different ecosystems/habitats in a region | Landscape with forests, wetlands, grasslands | Supports different species communities; regional resilience |
Richness = raw count of species (e.g., 12 species). Diversity = richness PLUS evenness (relative abundance of each species). Two communities with 10 species each can have very different diversity if one is dominated by a single species.
Example: Forest A has 10 species at equal abundance. Forest B has 10 species but one makes up 90% of individuals. Forest A has higher diversity despite identical richness.
① FRQ classic: "Why is a monoculture ecologically vulnerable?" → Low genetic diversity → single pathogen or pest can exploit uniform susceptibility across all plants. Classic example: Irish Potato Famine (1845) — one late-blight pathogen destroyed genetically uniform potato variety.
② Biodiversity hotspots: regions with exceptional species richness AND high threat from human activity. Examples: California Floristic Province, Tropical Andes, Madagascar. Used in AP FRQs about conservation prioritization.
③ Species richness generally increases toward the tropics (Latitudinal Diversity Gradient) due to higher solar energy, more stable climate, longer evolutionary time.
❌ Richness ≠ Diversity. Never use them interchangeably. Richness = count only; diversity includes evenness.
❌ FRQs about "biodiversity" may specifically require all THREE types. Don't just discuss species richness.
❌ High dominance by one species reduces diversity even if richness doesn't change — it lowers evenness.
A farmer plants only one genetically identical variety of wheat across 10,000 acres. Which type of biodiversity is most severely reduced and what is the primary ecological risk?
- (A) Species diversity; competition from weeds will intensify
- (B) Ecosystem diversity; habitat fragmentation will occur
- (C) Genetic diversity; a single pathogen could destroy the entire crop
- (D) Species richness; native plants will be excluded
Ecosystem Services
Ecosystem services are benefits humans receive from functioning ecosystems. They are grouped into four TEEB/MEA categories — all four appear on the AP exam.
| Category | Definition | Examples |
|---|---|---|
| Provisioning | Direct products harvested from the ecosystem | Food (crops, fish, game), fresh water, timber, fiber, medicinal plants, genetic resources |
| Regulating | Ecosystem processes that benefit us — no direct product harvested | Climate regulation, flood control, water purification, pollination, pest control, disease regulation, erosion prevention |
| Cultural | Non-material benefits — spiritual, aesthetic, recreational | Ecotourism, recreation, spiritual value, aesthetic appreciation, scientific knowledge, cultural identity |
| Supporting | Foundational processes that enable all other services | Nutrient cycling, soil formation, photosynthesis, water cycling, primary production |
① Supporting services are the most fundamental category — they enable all others. Without soil formation and nutrient cycling, there is no agriculture (provisioning). They are NOT directly used by humans.
② Monocultures maximize provisioning services (crop yield) but eliminate most regulating services — no pollinator habitat, no natural pest control, high erosion risk, minimal carbon storage.
③ Global value of ecosystem services estimated at $33–145 trillion/year — exceeding global GDP — yet most are unpriced in markets. This market failure drives overexploitation.
④ FRQ format: "Identify and explain TWO ecosystem services of [forest/wetland/reef]." Must name the service AND explain the mechanism — "flood control" alone earns no credit.
Name service category → Identify specific service → Explain mechanism → State benefit. Example: "Forests provide a regulating service through carbon sequestration: trees absorb CO₂ from the atmosphere via photosynthesis and store carbon in wood, roots, and soil organic matter, reducing atmospheric GHG concentrations and mitigating climate change."
❌ Provisioning ≠ Regulating. Provisioning = products you physically remove (timber, fish). Regulating = processes working in background (pollination, flood control). Pollination is regulating, NOT provisioning.
❌ Supporting services are not "used" by people but are more fundamental than all others — they make every other service possible.
❌ Cultural services are economically significant — ecotourism revenues often exceed extractive industries in biodiversity hotspots. Don't dismiss them as trivial.
Island Biogeography
MacArthur & Wilson's theory (1967): species richness on an island reaches a dynamic equilibrium between immigration rate and extinction rate.
| Variable | Immigration Rate | Extinction Rate | Equilibrium Species # |
|---|---|---|---|
| Larger island | Slightly higher (bigger target) | Lower (more habitats; more space per species) | Higher |
| Smaller island | Slightly lower | Higher (less space; stochastic extinctions) | Lower |
| Closer to mainland | Higher (shorter dispersal) | Slightly lower (rescue effect) | Higher |
| Farther from mainland | Lower (fewer colonizers reach) | Higher | Lower |
① At equilibrium, species number is stable but composition keeps changing — species turnover. Old species go extinct, new ones arrive; the count stays approximately constant.
② Most critical AP application: Island biogeography applies to habitat fragments on the mainland. A forest patch surrounded by development behaves like an island for wildlife. Larger patches → lower extinction rate; patches closer together → higher immigration rate → more species.
③ Wildlife corridors increase immigration between habitat islands → raise equilibrium species number → function like bridges connecting islands.
④ SLOSS (Single Large Or Several Small) debate: one large reserve generally better than several small of equal total area because large area dramatically reduces extinction rate and minimizes edge effect proportion.
❌ Equilibrium ≠ no turnover. Species composition constantly changes even at equilibrium — don't confuse stable species NUMBER with stable species IDENTITY.
❌ Size → extinction rate. Distance → immigration rate. These are independent effects. A large distant island could have the same richness as a small nearby island if the effects cancel out.
❌ The most AP-relevant application is mainland habitat fragmentation, NOT literal oceanic islands.
A highway cuts through a forest, creating two separate patches. According to island biogeography theory, which action would most effectively maintain species richness in both patches?
- (A) Introduce additional species to each patch
- (B) Reduce hunting pressure in both patches
- (C) Build a wildlife corridor connecting the two patches
- (D) Plant additional trees inside each patch
Ecological Tolerance
Each species has a range of tolerance for abiotic conditions. Performance (growth, reproduction, survival) follows a bell curve peaking at the optimum and declining toward both extremes.
| Zone on Tolerance Curve | Conditions | Organism Response |
|---|---|---|
| Optimal Range | Central portion of curve — near the peak | Best performance: highest growth, reproduction, survival rates |
| Zone of Physiological Stress | Between optimal range and lethal limits | Survival possible but reduced performance; energy diverted to stress response |
| Zone of Intolerance | Beyond lethal limits on either side | Cannot survive; physiological systems fail |
| Limiting Factor | The abiotic variable closest to tolerance limit | Determines where species can live; restricts geographic range |
| Feature | Generalist | Specialist |
|---|---|---|
| Tolerance range | Broad — tolerates wide range of conditions | Narrow — requires specific conditions |
| Niche breadth | Wide; flexible diet and habitat | Narrow; specific food source or habitat |
| Examples | Raccoon, coyote, crow, cockroach, humans | Giant panda (bamboo only), koala (eucalyptus only), spotted owl (old-growth) |
| Disturbance response | Thrives in disturbed/altered habitats | First to decline; highly extinction-prone when conditions change |
| Indicator species value | Poor — doesn't respond sensitively | Excellent — sensitive decline signals environmental degradation |
Fundamental niche: The full range of conditions where an organism could potentially live — based on physiology alone, without competition or predation.
Realized niche: Where it actually lives after biological interactions (competition, predation, parasitism) restrict it further.
Rule: Realized niche ≤ Fundamental niche. Biological interactions can ONLY restrict the niche — never expand it. The realized niche is always a subset of (or equal to) the fundamental niche.
❌ Optimal range ≠ entire tolerance range. Organisms at the edge of their tolerance range are alive but physiologically stressed. Optimal range is only the central peak region.
❌ Realized niche CAN NEVER exceed the fundamental niche — this violates the definition.
❌ Specialists outcompete generalists WITHIN their specialized niche — they are more efficient in their specific conditions. Generalists only outperform specialists when conditions are highly variable.
Natural Disruptions to Ecosystems
Natural disturbances — fire, flood, hurricane, drought, volcanic eruption — are integral to most ecosystems. Many ecosystems evolved with and require periodic disturbance for long-term health.
| Concept | Definition | Example |
|---|---|---|
| Resistance | Ability to resist change during disturbance | Old-growth forest resists moderate wind; resistant crops resist pests without changing |
| Resilience | Ability to recover after disturbance | Grasslands recover rapidly after fire; coral reefs recover from mild bleaching |
Biodiversity is highest at intermediate levels of disturbance (intermediate frequency AND intensity).
• Low disturbance: Competitive exclusion allows dominant species to take over → diversity decreases
• High disturbance: Only highly tolerant species survive → diversity decreases
• Intermediate disturbance: No single species dominates; many species can coexist → highest diversity
AP graph: bell curve with "Species Diversity" on Y-axis, "Disturbance Level" on X-axis — peak in the middle.
🔴 In fire-adapted biomes (savanna, chaparral, longleaf pine forest, tallgrass prairie), fire is necessary for ecosystem health:
• Recycles nutrients locked in dead biomass • Opens forest canopy allowing sun-dependent species • Prevents competitive exclusion by dominant plants • Maintains habitat for fire-adapted species
🔴 Fire suppression problem: Suppressing fire in fire-adapted ecosystems allows fuel accumulation → when fire finally occurs, it is catastrophically intense → destroys more biodiversity than regular low-intensity fires would have.
❌ Resistance ≠ Resilience. Resistance = doesn't change DURING disturbance. Resilience = recovers AFTER disturbance. A species can have low resistance (bleaches easily) but high resilience (recovers quickly).
❌ Zero disturbance ≠ maximum biodiversity. IDH specifically predicts that NO disturbance leads to competitive exclusion and REDUCED diversity.
Adaptations
Adaptations are heritable traits that increase an organism's fitness (reproductive success) in its specific environment. They arise through natural selection acting on pre-existing genetic variation — not through individual effort or choice.
| Type | Definition | Examples |
|---|---|---|
| Structural (Morphological) | Physical body feature aiding survival/reproduction | CAM photosynthesis in cacti; polar bear hollow fur; thick bark on fire-adapted trees; succulent water storage; drip-tip leaves in rainforests |
| Behavioral | Action or behavior pattern improving survival/reproduction | Migration; hibernation; nocturnal activity (deserts); cooperative hunting; territorial behavior; estivation (summer dormancy) |
| Physiological | Internal process or biochemical mechanism | Camel concentrates urine; arctic fish antifreeze proteins; heat-shock proteins; ability to enter torpor; efficient fat metabolism |
Name the adaptation → Classify it (structural/behavioral/physiological) → Describe the mechanism → State the survival advantage. Example: "The thick, waxy cuticle on desert succulent leaves is a structural adaptation. The wax layer physically blocks water vapor from diffusing out through the leaf surface, dramatically reducing transpiration water loss. This allows the plant to survive prolonged drought without desiccating, enabling reproduction in arid environments."
① Natural selection acts on existing variation — does NOT create mutations. Antibiotics don't cause resistance; they select for pre-existing resistant variants in the population.
② Coevolution: reciprocal adaptation between two interacting species over evolutionary time. Predator-prey arms races; plant-pollinator matching (flower shape adapted to specific pollinator anatomy).
③ FRQs often pair this with biome content: "An organism in the tundra faces extreme cold and short growing season — describe TWO specific adaptations." Know adaptations for desert, tundra, tropical rainforest, aquatic environments.
❌ Adaptations do NOT occur within an individual's lifetime. An individual cannot "adapt" to pollution — populations evolve resistance over generations via natural selection.
❌ Naming an adaptation without explaining the mechanism = partial credit at best. Always explain HOW the trait works.
❌ Camel's hump stores fat (energy reserve), NOT water. The water adaptation is their ability to tolerate dehydration and produce highly concentrated urine.
Ecological Succession
Ecological succession = predictable, directional change in species composition over time following disturbance. Early-successional (pioneer) species modify the environment in ways that make it suitable for later species but unsuitable for themselves.
| Feature | Primary Succession | Secondary Succession |
|---|---|---|
| Starting condition | Bare substrate with no soil present | Disturbed area where soil remains intact |
| Pioneer species | Lichens (attach to bare rock; weather it; release organic acids) | Grasses, weeds, fireweed — fast-germinating r-selected species from seed bank |
| Speed | Centuries to millennia (must build soil from scratch) | Decades to centuries (soil already present) |
| Examples | New volcanic island, lava flow, retreating glacier | Abandoned farmfield, post-fire forest, cleared land with soil intact |
| Climax community | Relatively stable, self-sustaining community characteristic of the climate region. Not permanent — major disturbance can reset succession. | |
Bare rock → Lichens (weather rock; add organic matter) → Mosses (thicker soil layer) → Grasses & herbs → Shrubs → Pioneer trees (fast-growing, light-demanding; shade out themselves) → Climax forest (shade-tolerant species dominate)
Key mechanism: each seral stage modifies soil, light, and microclimate — creating conditions that favor the next stage but disadvantage the current pioneers.
① The ONLY criterion for primary vs. secondary: does soil exist? Soil present = secondary. No soil = primary. A completely clearcut forest with intact soil = secondary succession.
② Pioneer species in primary succession are r-selected (hardy, fast-growing, tolerant of extreme bare conditions). Later successional species tend toward K-selection.
③ Climax community ≠ "most complex." It is the stable end-point community for a given climate — can be a fire-maintained grassland or savanna in regions with regular fire regimes.
❌ First colonizers in primary succession are lichens, not mosses. Lichens go first because they can attach to bare rock without any soil. Mosses come after lichens have begun building thin soil.
❌ Secondary succession does NOT restart from bare rock. It begins in a more advanced state because soil, seed banks, and root systems are already present.
❌ Climax communities are not permanent or eternal. A major disturbance (hurricane, clearcut, eruption) can reset succession to any earlier stage.
A forest is clear-cut but the soil is left completely undisturbed. Which type of succession will occur, and what is the primary reason?
- (A) Primary succession; all vegetation was removed
- (B) Secondary succession; soil and seed banks are still present
- (C) Primary succession; pioneer lichens must colonize first
- (D) Secondary succession; recovery will take only a few years
Top Common Mistakes — Full Unit 2
- 🧬Forgetting genetic diversity is a separate type from species diversityAP FRQs about "biodiversity" often specifically require genetic diversity. Always address all three types: species, genetic, and ecosystem diversity.
- 📊Species richness ≠ species diversityRichness = count. Diversity = count + evenness (relative abundance). Two communities with identical species counts can have very different diversity if one is dominated by a single species.
- 🌿Confusing provisioning and regulating ecosystem servicesProvisioning = products you physically remove (food, timber, water). Regulating = background processes benefiting you without removal (pollination, flood control, carbon sequestration). Pollination is regulating, NOT provisioning.
- 🏝Island biogeography equilibrium = no change in speciesEquilibrium means the NUMBER of species is stable. Species COMPOSITION continuously changes through turnover — individuals of new species arrive, old ones go locally extinct, the count stays approximately constant.
- 🔬Realized niche can exceed fundamental nicheImpossible. The realized niche is always ≤ fundamental niche. Biological interactions (competition, predation) can only restrict the niche — never expand it beyond physiological capacity.
- 📈Maximum biodiversity occurs at zero disturbanceWrong — the Intermediate Disturbance Hypothesis predicts maximum biodiversity at INTERMEDIATE disturbance. Zero disturbance → competitive exclusion → reduced diversity.
- 🔥Fire suppression always protects ecosystemsIn fire-adapted biomes (chaparral, savanna, longleaf pine), fire suppression is harmful — fuel accumulation leads to catastrophically intense fires that destroy more biodiversity than periodic low-intensity fires would.
- 🪨Confusing primary and secondary successionSingle criterion: does soil exist? Soil intact = secondary (even if all trees removed). No soil/bare rock = primary. Don't let "complete vegetation removal" fool you into saying primary.
- 🌱Mosses are first primary succession pioneersLichens are first — they colonize bare rock directly. Mosses come second, after lichens have produced thin soil. Lichens first → mosses second.
- 🦝Island biogeography only applies to actual oceanic islandsThe most important AP application is to mainland habitat fragments — forest patches, wetlands, and nature reserves surrounded by developed land. These behave like islands for wildlife populations.
Unit 2 Exam Strategy & High-Yield Topics
| Topic | MCQ Angle | FRQ Angle |
|---|---|---|
| Biodiversity (2.1) | Identify type; richness vs. diversity; species count vs. evenness | Explain monoculture vulnerability in terms of genetic diversity |
| Ecosystem Services (2.2) | Classify service as provisioning/regulating/cultural/supporting | Identify + explain 2 services of a specific habitat with mechanisms |
| Island Biogeography (2.3) | Predict species richness from size/distance; equilibrium concept | Apply to habitat fragmentation; explain value of wildlife corridors |
| Ecological Tolerance (2.4) | Interpret tolerance curve; identify limiting factor; generalist vs. specialist | Fundamental vs. realized niche; explain indicator species usefulness |
| Natural Disruptions (2.5) | IDH graph; resistance vs. resilience distinction | Role of fire in fire-adapted biomes; fire suppression consequences |
| Adaptations (2.6) | Classify type; identify survival mechanism | Describe 2 adaptations + mechanisms for given environment/challenge |
| Succession (2.7) | Primary vs. secondary; identify seral stage from description | Sequence of succession stages after specified disturbance type |
Unit 2 connects directly to Unit 5 (human impacts on biodiversity), Unit 8 (pollution effects on ecosystems), and Unit 9 (climate change impacts on species ranges and biomes). Island biogeography principles appear in conservation reserve design FRQs in Unit 9. Ecosystem services are cited throughout Units 5, 8, and 9.
For FRQs: always explain mechanisms. "Biodiversity decreases" earns no credit — explain WHY: competitive exclusion, habitat loss reducing carrying capacity, reduced genetic diversity increasing disease susceptibility, etc.