Biodiversity refers to the variety of life on Earth at all levels, from genes to ecosystems. It is measured across three interconnected levels: genetic diversity, species diversity, and habitat/ecosystem diversity. Higher biodiversity generally leads to greater ecosystem resilience and stability.
| Level | Definition | Examples | Why It Matters |
|---|---|---|---|
| Genetic Diversity | Variety of genes within a single species | Different coat colors in mice; disease-resistant vs. susceptible individuals | Enables adaptation to changing environments; reduces extinction risk |
| Species Diversity | Variety of different species in an ecosystem | Tropical rainforests: thousands of species; coral reefs: 25% of all marine species | More species = more functional redundancy = greater resilience |
| Habitat Diversity | Variety of ecosystems in a region | Forests, wetlands, grasslands, deserts within one continent | More habitats provide more niches for species |
Species diversity has two components: species richness (total number of species present) and species evenness (relative abundance of each species). An ecosystem with 100 species where each has equal numbers is more diverse than one with 100 species where one dominates.
A population bottleneck occurs when a population is dramatically reduced by a catastrophic event, leaving survivors with only a fraction of the original genetic diversity. All future generations are descended from this limited gene pool.
Hunted to ~20 individuals in 1890s. Recovered to ~200,000 but with extremely low genetic diversity.
Bottleneck ~10,000 years ago. All modern cheetahs are genetically near-identical, reducing disease resistance.
Isolated population showing inbreeding depression: genetic defects, kinked tails, heart problems.
Increased disease vulnerability, reduced fertility, harmful recessive traits expressed, higher extinction risk.
The AP exam frequently asks you to distinguish species richness (number) from species evenness (distribution). Also common: "Which level of biodiversity helps a population adapt?" → genetic diversity.
More species diversity → more functional redundancy (multiple species performing similar roles) → greater ecosystem resilience after disturbance. If one pollinator declines, others compensate.
A population of bighorn sheep was reduced to 12 individuals due to disease. Although the population has since recovered to 500, the sheep are all genetically very similar. This scenario best illustrates which concept?
❌ Confusing species richness with species evenness. Richness = count of species; evenness = how equally distributed they are.
❌ Thinking genetic diversity only matters for endangered species. ALL populations benefit from genetic diversity for adaptation.
Ecosystem services are the benefits that humans receive from properly functioning ecosystems. They are categorized into four types by the Millennium Ecosystem Assessment. The estimated global value of ecosystem services is $125-145 trillion/year, far exceeding global GDP.
| Category | Definition | Examples |
|---|---|---|
| Provisioning | Direct products obtained from ecosystems | Food, fresh water, timber, fiber, medicine, genetic resources |
| Regulating | Benefits from regulation of ecosystem processes | Climate regulation, flood control, water purification, pollination, disease regulation |
| Cultural | Non-material benefits from ecosystems | Recreation, ecotourism, spiritual values, education, aesthetic beauty |
| Supporting | Services necessary for all other services | Nutrient cycling, soil formation, primary production, water cycling, habitat provision |
75% of global food crops depend on animal pollinators. Bee colony collapse disorder threatens agriculture worldwide.
Wetlands filter pollutants, sediments, and excess nutrients. New York City saves $6-8 billion by protecting Catskill watershed instead of building filtration plant.
Forests absorb ~2.6 billion tonnes of CO₂/year. Oceans absorb ~25% of human CO₂ emissions through phytoplankton.
Mangroves reduce wave height by 66% and protect coastal communities. 1 acre of wetland can store 1-1.5 million gallons of floodwater.
Decomposers break down organic matter → humus. Takes 500-1,000 years to form 1 inch of topsoil naturally.
When asked to classify an ecosystem service, ask: "Is this a direct product (provisioning), a process benefit (regulating), a non-material benefit (cultural), or fundamental to other services (supporting)?" Supporting services are the foundation — without them, the others cannot exist.
A city decides to protect upstream wetlands instead of building a water treatment plant. This decision most directly takes advantage of which ecosystem service?
❌ Classifying "clean water" as provisioning. Fresh water as a resource = provisioning; water purification by ecosystems = regulating.
❌ Confusing supporting and regulating services. Supporting services (nutrient cycling, soil formation) underpin ALL other services. Regulating services control specific processes (flood control, pollination).
The Theory of Island Biogeography (MacArthur & Wilson, 1967) explains species diversity patterns on islands and isolated habitats. It applies to literal islands AND habitat "islands" like forest fragments, mountaintops, and nature reserves.
| Factor | Effect on Species Diversity | Why |
|---|---|---|
| Island Size | Larger islands → MORE species | More habitats, more resources, lower extinction rates, larger populations |
| Distance from Mainland | Closer islands → MORE species | Higher immigration/colonization rates; easier for organisms to reach |
Maximum species diversity. High immigration rate + low extinction rate. Example: Trinidad (close to South America, large).
Minimum species diversity. Low immigration rate + high extinction rate. Example: Remote Pacific atolls.
Species diversity reaches equilibrium when immigration rate = extinction rate. Species composition may change even if number stays constant.
Forest fragments act like islands. Smaller fragments lose species faster. Wildlife corridors mimic "bridges" to increase connectivity.
Island biogeography directly informs reserve design: one large reserve is generally better than several small ones (SLOSS debate). Corridors connecting fragments increase effective habitat size and immigration rates.
FRQ prompts often present two habitat fragments and ask you to predict which has higher diversity. Always consider BOTH size AND distance. A small fragment close to a source population may have more species than a large but extremely isolated one.
According to the theory of island biogeography, which forest fragment would be expected to have the greatest species diversity?
❌ Thinking only literal oceanic islands are covered. This theory applies to any isolated habitat: mountaintops, lakes, forest fragments, even urban parks.
❌ Forgetting that equilibrium means species NUMBER is stable, not species IDENTITY. Different species may continuously immigrate and go extinct.
Every species has a range of tolerance for each environmental factor (temperature, pH, salinity, dissolved oxygen, etc.). Outside this range, the organism cannot survive. Within the range, there is an optimum zone where the organism thrives best.
| Zone | Description | Organism Response |
|---|---|---|
| Optimum Range | Ideal conditions for growth and reproduction | Maximum population growth, highest fitness |
| Zone of Physiological Stress | Conditions are suboptimal but survivable | Reduced growth, fewer offspring, behavioral changes |
| Zone of Intolerance | Conditions exceed survival limits | Death — organism cannot survive |
| Lower/Upper Limit | Boundary between stress and intolerance | Critical threshold beyond which survival is impossible |
Narrow temperature tolerance. Example: coral reefs bleach at just 1-2°C above normal; trout need cold water (10-15°C).
Wide temperature tolerance. Example: raccoons live from Canada to Central America; coyotes thrive in deserts and forests.
Species with narrow tolerances serve as indicators of environmental quality. Trout indicate clean, cold, well-oxygenated water.
The environmental factor closest to an organism's tolerance limit. Often determines species distribution (Liebig's Law of the Minimum).
Specialist species have narrow tolerance ranges (stenothermal/stenohaline) — they are vulnerable to environmental change but highly competitive in their niche. Generalist species have wide tolerance ranges (eurythermal/euryhaline) — they are adaptable but may be outcompeted by specialists in stable environments.
Trout populations in a stream begin to decline as water temperature rises from 15°C to 22°C. This observation is best explained by which concept?
❌ Confusing stenothermal with eurythermal. Steno = narrow; eury = wide. Think: "steno" like stenography (specific, narrow shorthand).
❌ Assuming the optimum is always at the center of the tolerance range. For many species, it can be skewed toward one end.
Natural disruptions are events not caused by human activity that alter ecosystem structure and function. They can be catastrophic (sudden, large-scale) or gradual (slow, long-term). Ecosystems have evolved to recover from natural disruptions through ecological resilience.
| Disruption | Type | Short-Term Effects | Long-Term Effects |
|---|---|---|---|
| Wildfires | Catastrophic | Destroys vegetation, displaces animals, releases CO₂ | Returns nutrients to soil; promotes fire-adapted species (lodgepole pine, chaparral); triggers succession |
| Volcanic Eruptions | Catastrophic | Destroys all life in blast zone; ash blankets wide areas | Creates new land; enriches soil with minerals; primary succession begins |
| Earthquakes | Catastrophic | Habitat destruction, tsunamis, landslides | Alters drainage patterns; creates new habitats; redirects rivers |
| Hurricanes/Cyclones | Catastrophic | Flooding, wind damage, saltwater intrusion | Creates gaps in forest canopy allowing light-loving species; redistributes nutrients |
| Droughts | Gradual | Water stress, crop failure, animal migration | Shifts in species composition; desertification if prolonged |
| Disease Outbreaks | Variable | Population decline of affected species | May allow competitor species to expand; alters community structure |
Burned ~800,000 acres. Within 5 years, diverse plant regrowth. Lodgepole pine cones opened by heat, releasing seeds.
Eruption destroyed 230 sq miles of forest. 40+ years later, primary succession continues with pioneer species colonizing lava flows.
Warm Pacific waters disrupt weather globally. Coral bleaching, altered rainfall patterns, fisheries collapse in Peru.
Resistance: ability to withstand disturbance without change. Resilience: ability to recover after disturbance. High biodiversity increases both.
When asked about natural disruptions, emphasize that they are part of normal ecosystem dynamics. Many species are fire-adapted or disturbance-dependent. The key distinction from human disruptions is scale, frequency, and whether the ecosystem has evolutionary history with the disturbance.
After a wildfire destroys a pine forest, the first plants to grow are grasses and wildflowers. These are eventually replaced by shrubs, then shade-tolerant hardwoods. This process is an example of
❌ Calling all post-fire recovery "primary succession." If soil remains, it is SECONDARY succession. Primary succession only occurs on bare substrate with no prior soil.
❌ Viewing all wildfires as purely destructive. Many ecosystems (chaparral, grasslands, boreal forests) DEPEND on periodic fire for nutrient cycling and seed germination.
An adaptation is a heritable trait that increases an organism's fitness (ability to survive and reproduce) in a specific environment. Adaptations arise through natural selection over many generations — individuals with favorable traits are more likely to survive and pass those traits to offspring.
| Type | Definition | Examples |
|---|---|---|
| Structural/Physiological | Physical features or internal processes that improve survival | Cactus spines (reduce water loss), thick fur in arctic animals, waxy leaf cuticle, counter-current heat exchange |
| Behavioral | Actions or behaviors that improve survival | Migration, hibernation, nocturnal activity, courtship displays, pack hunting |
| Camouflage | Blending with environment to avoid predation | Arctic hare (white in winter, brown in summer), leaf insects, flounder |
| Mimicry | Resembling another organism for protection | Batesian: harmless viceroy butterfly mimics toxic monarch. Mullerian: two toxic species share warning colors |
Harmless species mimics a dangerous one. Viceroy butterfly → Monarch; king snake → coral snake. Predators avoid both.
Two harmful/toxic species evolve similar warning signals. Bees and wasps both have yellow-black stripes. Reinforces predator learning.
Two species evolve in response to each other. Predator-prey arms race: cheetah speed ↔ gazelle speed. Flower shape ↔ pollinator beak.
Species divide resources to reduce competition. Warblers feed at different heights in the same tree (MacArthur's warblers).
Adaptations are NOT intentional — organisms don't "choose" to adapt. Natural selection acts on existing genetic variation. Individuals with traits suited to the environment survive more, reproduce more, and pass those genes on. The population changes over generations, not individual organisms.
A non-venomous snake species has evolved coloring similar to a venomous coral snake. This adaptation is an example of
❌ Confusing Batesian and Mullerian mimicry. Batesian = harmless mimics dangerous; Mullerian = both species are dangerous/toxic.
❌ Saying organisms "adapt to survive." Adaptation is a population-level process through natural selection, not an individual choice.
Ecological succession is the gradual, predictable process of change in the species composition of a community over time. It occurs in two forms depending on whether soil is present: primary succession (no soil) and secondary succession (soil remains).
| Feature | Primary Succession | Secondary Succession |
|---|---|---|
| Starting Condition | Bare rock, lava, sand — NO soil | Disturbed area with soil intact |
| Initiating Events | Volcanic eruption, glacier retreat, new island | Wildfire, flood, farming abandonment, logging |
| Pioneer Species | Lichens, mosses (break down rock → soil) | Grasses, weeds, fast-growing herbs |
| Time to Climax | Hundreds to thousands of years | Decades to ~200 years |
| Soil Present? | No — must be created | Yes — seeds and nutrients remain |
First colonizers arrive. Primary: lichens + mosses on bare rock. Secondary: grasses + annual weeds. These species are r-selected, fast-growing, sun-loving.
Shrubs and small trees replace pioneers. Soil deepens, biodiversity increases. Shade-intolerant species gradually replaced by shade-tolerant ones.
Stable, mature ecosystem. Dominated by shade-tolerant, K-selected species. Remains until next major disturbance. Example: temperate deciduous forest (oak-hickory).
Lakes fill with sediment over time → marsh → meadow → forest. This natural aging process is called natural eutrophication (takes thousands of years).
The #1 tested concept: primary = no soil; secondary = soil present. If you see "abandoned farmland," "after a fire," or "after deforestation" → secondary. If you see "lava flow," "glacier retreat," or "new volcanic island" → primary.
When describing succession stages, always mention: (1) what pioneer species colonize first, (2) how soil develops and changes, (3) how biodiversity changes over time (increases), (4) what characterizes the climax community, and (5) the approximate timescale.
A glacier retreats, exposing bare rock. Lichens are the first organisms to colonize the surface. Over centuries, soil develops and eventually a coniferous forest becomes established. This process is an example of
Which of the following best explains why secondary succession typically occurs faster than primary succession?
❌ Thinking climax communities are permanent. They persist until the next disturbance (fire, storm, human activity) restarts succession.
❌ Confusing natural eutrophication (thousands of years, natural lake aging) with cultural eutrophication (decades, caused by nutrient pollution from human activity).
❌ Assuming secondary succession always follows fire. It follows ANY disturbance that leaves soil behind: farming abandonment, logging, floods, hurricanes.
A large tropical forest is fragmented by road construction and agricultural expansion, leaving four isolated forest patches of varying sizes. Scientists monitor biodiversity changes over 15 years.
(a) Using the theory of island biogeography, explain why the smallest and most isolated forest fragment is expected to lose species faster than the largest fragment closest to the remaining continuous forest. (3 points)
(b) Describe TWO specific edge effects that would be observed in the fragmented patches and explain how each reduces interior-species diversity. (4 points)
(c) A wildfire burns through one of the medium-sized fragments. Identify the type of ecological succession that would follow and describe TWO changes in species composition that would occur during recovery. (3 points)
Mixed MCQ and FRQ in AP APES exam style. Attempt each before revealing the answer.
A species of salamander is found only in cold, fast-flowing mountain streams with dissolved oxygen levels above 8 mg/L. Which of the following best describes this species?
After Hurricane Katrina destroyed large areas of coastal wetlands in Louisiana, residents experienced more severe flooding from subsequent storms. This outcome illustrates the loss of which ecosystem service?
A conservation organization is designing a nature reserve to protect a threatened bird species in a region where forests have been fragmented by agriculture.
(a) Using the theory of island biogeography, recommend whether the reserve should be one large area or several small areas, and justify your recommendation. (2 points)
(b) Explain how wildlife corridors connecting forest fragments could benefit the genetic diversity of the bird population. (2 points)
(c) Identify ONE adaptation the bird species might need to survive in edge habitat and explain why edge habitat differs from forest interior. (2 points)
A volcanic eruption creates a new island in the Pacific Ocean. Scientists begin monitoring colonization and species diversity over time.
(a) Identify the type of ecological succession that will occur and explain why. (2 points)
(b) Describe the role of pioneer species in this process and give TWO examples. (2 points)
(c) Predict how species diversity will change over time and explain the concept of equilibrium in island biogeography. (2 points)
Unit 2 is 6-8% of the exam. Focus on island biogeography (size + distance predictions), ecological succession (primary vs. secondary — the #1 tested concept), and ecosystem services (classifying the four categories). Know real-world examples for each concept and practice drawing tolerance curves. FRQs often combine multiple Unit 2 topics — e.g., habitat fragmentation + island biogeography + succession.