The Living World: Biodiversity
APES Unit 2 Study Notes
Explore how living systems respond to change.
Understand biodiversity, ecosystem services and ecological change with detailed notes, worked examples and clear diagrams.
2.1 Introduction to Biodiversity
What is Biodiversity?
Biodiversity is variation within genes, among species and across ecosystems. These levels describe different features of living systems. Biodiversity can support ecosystem functions and the capacity to respond to change, but a high species count alone does not guarantee that every function will be maintained. Species identities, interactions and environmental conditions also matter.
For the AP Environmental Science exam, you must understand biodiversity operates at three interconnected levels: genetic, species, and ecosystem diversity.
The Three Levels of Biodiversity
1. Genetic Diversity
Definition: The variety of genes and alleles within a single species or population. It represents the genetic differences among individuals of the same species.
Importance: Genetic diversity is critical for a population's ability to adapt to environmental changes, resist diseases, and avoid inbreeding depression. Higher genetic diversity provides more "raw material" for natural selection and evolution. Populations with low genetic diversity are more vulnerable to diseases, environmental stresses, and extinction.
Examples:
- Inherited differences in coat traits or disease resistance among individuals of a dog population; visible differences can also reflect environment, so appearance alone is not a complete genetic measure.
- Variation in drought tolerance among individual corn plants in a field
- Blood type variations (A, B, AB, O) among humans
Threats: Genetic bottlenecks (drastic population reduction), founder effects (new population from small group), inbreeding, habitat fragmentation that isolates populations.
2. Species Diversity
Definition: The variety and abundance of different species in a particular area or ecosystem. It includes two components: species richness (number of different species) and species evenness (relative abundance of each species).
Species Richness: Simply the count of how many different species are present. An ecosystem with 50 species has higher richness than one with 10 species.
Species Evenness: Describes how evenly individuals are distributed among the different species. An ecosystem where all species have similar population sizes has high evenness; one dominated by a few species has low evenness.
Why it matters: Species with different functions or responses can sustain production and buffer some disturbances. Functional redundancy means some roles overlap, not that species are interchangeable. Resistance (how little a system changes) and resilience (its recovery) are distinct; the outcome depends on which species and functions are affected.
Examples:
- Tropical rainforests have extremely high species diversity (thousands of plant and animal species)
- Many deserts have lower local richness than humid tropical forests for some groups, but drylands support distinctive and sometimes highly endemic biota. Compare the same taxonomic group, area and sampling effort rather than assume every desert is species-poor.
- A crop monoculture has low diversity of planted crop species, although weeds, insects, soil microbes and other organisms may still occur. One crop species does not mean the entire field contains only one species.
Calculation example: Community A contains five bird species with counts 100, 90, 85, 80 and 75 (total 430). Community B has the same five species with counts 250, 50, 25, 15 and 10 (total 350). Richness is 5 in both; evenness is higher in A. The most abundant species makes up 100/430 × 100 ≈ 23.3% of A, versus 250/350 × 100 ≈ 71.4% of B. Compare relative abundances when totals differ; actual field comparisons also need consistent sampling.
3. Ecosystem Diversity (Habitat Diversity)
Definition: The variety of ecosystems, habitats, and ecological communities in a given area. It represents the diversity of physical environments and the interactions between living organisms and their habitats.
Importance: Greater ecosystem diversity supports more total species because different ecosystems provide different niches, resources, and conditions. It increases overall regional biodiversity and provides a buffer against large-scale disturbances.
Examples:
- A landscape with forests, wetlands, grasslands, and streams has high ecosystem diversity
- A region containing only one uniform habitat type has lower ecosystem diversity than a comparable landscape containing several distinct habitat types. A desert region can itself contain dunes, rocky slopes, washes and oases, so a single biome label does not prove uniform habitat.
- Coastal areas with estuaries, mangroves, coral reefs, and open ocean have high ecosystem diversity
Conservation Implications: Protecting ecosystem diversity requires maintaining habitat heterogeneity. Habitat loss and fragmentation (breaking continuous habitat into isolated patches) are the primary threats to ecosystem diversity.
Important Species Classifications
Keystone Species
Definition: A species that has a disproportionately large effect on its ecosystem relative to its abundance. Removal of a keystone species causes dramatic changes to ecosystem structure and function.
Examples:
- Sea otters: Eat sea urchins; without otters, urchins overgraze kelp forests
- Wolves: Predation can change herbivore abundance or behaviour and indirectly affect vegetation. The size and direction of a cascade depend on other predators, hunting, climate and habitat; wolf presence does not automatically prevent all overgrazing.
- Beavers: Create wetland habitats through dam-building that support many species
Indicator Species
Definition: Species whose presence, absence, or abundance reflects specific environmental conditions. They serve as biological "warning systems" for ecosystem health.
Examples:
- Lichens: The abundance and composition of pollution-sensitive lichen species can indicate air-quality conditions. Absence alone is not proof of pollution: moisture, suitable substrate and dispersal must also be considered.
- Trout: Many trout require cool, oxygenated water. Their presence can inform assessment of those conditions, but one species alone cannot certify that every aspect of stream water quality is healthy.
- Amphibians: Permeable skin makes them sensitive to pollutants and environmental changes
Specialist vs. Generalist Species
Specialist species: Use a relatively narrow range of resources or conditions in a specified niche dimension. They may be vulnerable when that resource or habitat disappears. Specialization can confer advantages in suitable conditions, but does not make every specialist the first species to decline.
Examples: Giant pandas have a diet dominated by bamboo; koalas feed mainly on selected eucalyptus leaves; northern spotted owls depend strongly on suitable mature and old forest structure. These are different dimensions of specialization, not literal claims that each species uses one resource exclusively.
Generalist species: Use a broader range of resources or habitats in the dimension being discussed. This flexibility can help under some disturbances, although limits, competition and life history still matter. Breadth of diet is not the same as tolerance of every abiotic condition.
Examples: Raccoons, rats, coyotes, cockroaches, humans
Exam tip: Richness counts species; evenness compares their relative abundances. A keystone species has a disproportionately large ecological effect relative to abundance; an indicator provides evidence about conditions. Explain why a particular disturbance threatens a specialist resource or habitat rather than assume all specialists decline first.
Measuring diversity and interpreting evidence
For a fair comparison, specify the area, season, taxonomic group, sampling method and effort. A larger sample may find more species without proving that the underlying habitat is more diverse. Genetic diversity is variation within populations; adding a new species increases richness but does not automatically restore variation in a bottlenecked population.
Matched-sample example: Two plots each contain 100 trees and four species. Plot X has counts 25, 25, 25, 25; Plot Y has 85, 5, 5, 5. Both have richness 4. X has higher evenness because each species accounts for 25%, whereas one species accounts for 85% in Y. State both richness and evenness when explaining species diversity.
2.2 Ecosystem Services
Overview
Ecosystem services are the benefits that humans obtain from healthy, functioning ecosystems. They represent the many ways nature supports human well-being, economic prosperity, and survival. Understanding ecosystem services helps connect biodiversity conservation to human needs and demonstrates the economic value of protecting natural systems.
The Millennium Ecosystem Assessment (2005) categorized ecosystem services into four types: provisioning, regulating, cultural, and supporting services. For the AP exam, you must know definitions, examples, and how human activities impact each category.
The Four Categories of Ecosystem Services
1. Provisioning Services
Definition: Tangible products or resources that humans directly obtain from ecosystems. These are the "goods" we harvest or extract from nature.
Examples:
- Food: Fish, game animals, crops, fruits, nuts, mushrooms, honey
- Fresh Water: Drinking water from rivers, lakes, aquifers
- Timber and Fiber: Wood for construction, paper, cotton, wool, silk
- Fuel: Firewood, peat, biofuels
- Biochemicals and medicines: Natural compounds can inspire pharmaceuticals: willow salicylates are related to the development of aspirin, and paclitaxel was originally obtained from Pacific yew. A natural source is not the same as saying the manufactured drug is simply harvested unchanged.
- Genetic Resources: Genes for crop improvement, pharmaceutical development
Human Impacts: Overharvesting (overfishing, deforestation), habitat destruction, pollution degrading water supplies, monoculture agriculture reducing genetic resources.
2. Regulating Services
Definition: Benefits obtained from the regulation of ecosystem processes. These are the "behind-the-scenes" services that maintain environmental conditions and protect against natural hazards.
Examples:
- Climate regulation: Vegetation and soils can store carbon; net sequestration occurs when uptake exceeds losses over the period assessed. Forests also respire and can release carbon after fire, decay or clearing. Oceans store heat and carbon, influencing climate.
- Water purification: Wetlands can trap sediment and transform or retain some nutrients and contaminants. Soil microbes can break down certain compounds. Capacity is finite and depends on hydrology and chemistry; neither wetlands nor microbes remove every pollutant.
- Water regulation: Vegetation, soils and wetlands influence infiltration, evapotranspiration, storage and runoff. They can moderate some flood peaks and maintain flow pathways, but the effect depends on the watershed, storm and management; forests do not guarantee protection from floods or drought.
- Erosion Control: Plant roots hold soil in place; vegetation slows water runoff
- Natural-hazard protection: Mangroves, reefs and coastal wetlands can reduce wave energy, erosion and some storm impacts. Protection varies with habitat condition, width and event magnitude; they are not a guarantee against storm surge or tsunamis.
- Pollination: Bees, butterflies, birds, and bats pollinate crops and wild plants
- Pest Control: Predators and parasites control agricultural pests naturally
- Disease regulation: Host diversity and community composition can change transmission pathways. A dilution effect occurs in some systems, but adding species does not universally reduce disease: host competence, vectors and contact rates matter.
Human Impacts: Deforestation reducing carbon storage, wetland drainage eliminating water purification, pesticide use killing pollinators, coastal development removing natural storm barriers.
3. Cultural Services
Definition: Non-material benefits people obtain from ecosystems through spiritual enrichment, cognitive development, reflection, recreation, and aesthetic experiences. These services affect psychological well-being and cultural identity.
Examples:
- Recreation and Ecotourism: Hiking, birdwatching, fishing, wildlife viewing, scuba diving
- Aesthetic Values: Scenic landscapes, beautiful flowers, inspiring natural vistas
- Spiritual and Religious Values: Sacred groves, religious significance of certain species or places
- Educational Values: Nature-based learning, field studies, scientific research opportunities
- Cultural Heritage: Traditional ecological knowledge, cultural practices tied to nature
- Inspiration: Art, music, literature, architecture inspired by nature
Human Impacts: Habitat destruction reducing recreational opportunities, pollution diminishing aesthetic values, urbanization limiting access to nature, loss of traditional lands affecting cultural practices.
4. Supporting Services
Definition: Fundamental ecosystem processes that are necessary for the production of all other ecosystem services. These services underlie and enable provisioning, regulating, and cultural services to function.
Examples:
- Photosynthesis and Primary Production: Plants convert solar energy to chemical energy, forming base of food chains
- Nutrient Cycling: Decomposition returns nutrients to soil; biogeochemical cycles (carbon, nitrogen, phosphorus) maintain nutrient availability
- Soil Formation: Weathering, organic matter decomposition create and maintain fertile soils
- Water Cycling: Hydrologic cycle distributes water through ecosystems
- Habitat Provision: Ecosystems provide living space and nursery areas for species
- Oxygen Production: Photosynthesis produces atmospheric oxygen
Human Impacts: Soil degradation from intensive agriculture, disrupted nutrient cycles from excess fertilizer use, altered water cycles from urbanization, habitat fragmentation preventing species movement.
Important note: Supporting services enable other benefits, such as primary production underpinning food supply. They often affect people indirectly and across long timescales, but nutrient cycling and photosynthesis also occur rapidly. Distinguish an ecological process from its final human benefit to avoid counting the same benefit twice.
Common pitfall: In the four-category framework used here, provisioning means products, regulating means moderation of processes, cultural means non-material benefits, and supporting means processes underpinning the others. Crop pollination is normally treated as regulating; nutrient cycling as supporting. The same habitat supplies multiple services, and different assessment frameworks may organize them differently. Identify the named benefit in the question.
Apply ecosystem-service categories
| Observed benefit | Category | Mechanism |
|---|---|---|
| Fish harvested for food | Provisioning | People obtain a material product. |
| Lower downstream sediment load | Regulating | Vegetation slows water and traps some particles. |
| Birdwatching and outdoor learning | Cultural | People obtain recreation and knowledge. |
| Decomposition recycles nutrients | Supporting | Nutrients become available for further production. |
Explain a loss: Draining the wetland can remove storage and vegetation, increasing rapid runoff and sediment transport under suitable conditions. Name the service, the change in the process and the consequence for people. A monetary estimate measures one aspect of value and does not capture every cultural, ecological or distributional concern.
2.3 Island Biogeography
Theory of Island Biogeography
The Theory of Island Biogeography, developed by ecologists Robert MacArthur and E.O. Wilson in the 1960s, explains how species richness on islands reaches an equilibrium based on the rates of immigration (colonization) and extinction. This theory applies not only to literal oceanic islands but also to "habitat islands" - isolated patches of habitat surrounded by unsuitable areas (forest fragments, mountain tops, lakes, protected reserves).
For the AP Environmental Science exam, you must understand how island size and distance from mainland affect species richness, and how this theory applies to conservation and habitat fragmentation.
Key Principles
Species Equilibrium
The number of species on an island reaches a dynamic equilibrium when the rate of new species immigration equals the rate of species extinction. At equilibrium, the total number of species remains relatively stable, but species composition may change over time (species turnover).
Important: This is a dynamic, not static, equilibrium. Individual species may go extinct and be replaced by new immigrants, keeping the total species count stable.
Immigration Rates
The rate at which new species colonize an island decreases as more species are already present because:
- Fewer new species remain in the "source pool" that haven't already colonized
- More niches are already occupied by resident species
- Competition makes successful establishment harder
In the simple model, the arrival rate of species not already present declines as richness approaches the regional source-pool size. This is not the same as a population carrying capacity K. An immigrant from a species already present can increase its population without adding a new species.
Extinction Rates
The rate at which species go extinct from an island increases as more species are present because:
- More species means more opportunities for extinction to occur
- Increased competition for limited resources
- Smaller population sizes per species (less space/resources per species)
- Interactions such as competition, predation and disease can affect local persistence. The classical upward extinction curve is a simplified model, not a rule that each additional species must make every other species less secure.
Extinction rate is lowest when few species are present and increases as species richness increases.
Effect of Island Size
Holding other conditions comparable, larger islands generally support more species than smaller islands because:
- Larger Target: Bigger islands are easier for dispersing organisms to find and reach
- More Habitat Diversity: Larger areas contain more varied habitats (microclimates, vegetation types), supporting more species with different needs
- Larger Populations: Each species can maintain larger populations, reducing extinction risk from random events (demographic stochasticity)
- More Resources: Greater total food, water, and space availability
- Lower Extinction Rate: The extinction curve shifts downward for larger islands
Species-Area Relationship:
S = cAz
Where: S is predicted richness, A is area, c depends on the taxa, region and area units, and z is the exponent (the slope on log–log axes). Values around 0.2–0.4 are illustrative, not universal. Area alone does not specify habitat quality or isolation.
This is a power-law relationship, not exponential growth with area. Taking logs gives log S = log c + z log A. If 0 < z < 1, richness rises with area less than proportionally; doubling area does not double predicted richness.
Effect of Distance from Mainland
Holding size, habitat and other conditions comparable, islands nearer a source population generally receive more immigrants and support higher equilibrium richness because:
- Higher Immigration Rate: Shorter distances are easier for organisms to disperse across (wind, water, swimming, flying)
- More Frequent Arrivals: Species reach the island more often
- Rescue Effect: Nearby populations can send immigrants to "rescue" declining populations from extinction
- Higher Immigration Curve: The immigration curve shifts upward for closer islands
Distant Islands: Have lower immigration rates, reach equilibrium at fewer species, and are more isolated. Species that do reach distant islands tend to be good dispersers (birds, plants with wind-dispersed seeds).
Applications to Conservation
Island biogeography theory has profound implications for conservation biology and reserve design:
Habitat Fragmentation
When continuous habitat is broken into isolated patches (by roads, agriculture, development), the fragments act as islands:
- Small fragments support fewer species (lower species richness)
- Isolation can reduce immigration and rescue; small or degraded patches may also have higher local extinction. The surrounding matrix can be partly permeable rather than uniformly unsuitable, and species differ in dispersal ability.
- Interior specialists and species needing large territories may be especially affected, while some edge-tolerant generalists persist or increase. Responses depend on the organism and type of fragmentation; the order of decline is not universal.
- Increased edge effects alter habitat quality
Reserve Design Implications
- SLOSS debate: Single Large Or Several Small reserves of equal total area? A large patch can support wide-ranging species and reduce edge exposure. Several patches can protect different habitats or species and spread localized risk. There is no universal winner: compare connectivity, habitat complementarity, threats and the conservation goal.
- Habitat corridors: Suitable connections can increase movement, gene flow and recolonization for target species. They require appropriate width and habitat quality and may also spread fire, disease or invasive species. A corridor is not equally useful to every organism.
- Buffer Zones: Surrounding reserves with partial protection reduces edge effects.
- Proximity: Reduced effective isolation can improve movement, but the best placement also depends on protecting distinct habitats, climate pathways and avoiding shared threats. Straight-line distance alone does not describe connectivity.
Edge Effects
The boundary between fragmented habitat and surrounding matrix creates altered conditions:
- Increased light, temperature, wind exposure at edges
- Changed species composition (edge specialists replace interior specialists)
- Increased predation and parasitism from matrix species
- For similarly shaped patches, smaller fragments have higher perimeter-to-area ratios and less core habitat. Shape matters too: a narrow, elongated reserve can have much more edge than a compact reserve with the same area.
Exam tip: In the simple model, large + near predicts higher richness than small + far when other conditions are comparable. A small-near versus large-far comparison cannot be ranked without more information. Immigration and extinction curves intersect at equilibrium richness. For a species–area calculation, use the stated z value and distinguish a long-term model estimate from immediate observed extinctions.
Worked species–area estimate and equilibrium reasoning
Area loss: Suppose a model predicts 100 species in a habitat, area falls to 50% of its original value and z = 0.25. Then Snew/Sold = (Anew/Aold)z = 0.50.25 ≈ 0.841. Predicted richness is about 84 species, a reduction of about 16%, not 50%. Different exponents give different predictions. Local losses can lag habitat change; this equation is not a timetable or a guaranteed outcome for each species.
Read a graph: Put richness on the horizontal axis and rates of new-species arrival and local extinction on the vertical axis. Immigration slopes downward and extinction upward in the simple model. Below the intersection, arrivals exceed local losses and richness tends to increase; above it, losses exceed arrivals. At the intersection, turnover can continue even with no net change in richness. Local extinction means disappearance from the island, not necessarily global extinction.
2.4 Ecological Tolerance
Overview
Ecological tolerance (also called range of tolerance) refers to the range of environmental conditions within which an organism or species can survive, grow, and reproduce. Every species has specific tolerance ranges for various abiotic factors such as temperature, moisture, pH, salinity, and sunlight. Understanding tolerance ranges is crucial for predicting species distributions, responses to environmental change, and vulnerability to climate change.
For the AP Environmental Science exam, you must understand the tolerance curve, be able to identify optimal ranges and zones of stress, and recognize how limiting factors constrain species distributions.
Interpreting a tolerance curve
A tolerance curve is a simplified graph of a measured response, such as abundance, growth or survival, against one environmental gradient. A bell-shaped curve is useful for learning the zones, but real responses can be asymmetric, plateau-shaped or affected by other factors. Identify both axes before interpreting a graph.
Zone of Optimal Range (Peak of Curve)
Definition: The favourable portion of the measured gradient, where the specified response is relatively high. The optimum for growth need not exactly match the optimum for reproduction or observed abundance. Field abundance also reflects food, competitors, predators and dispersal.
Characteristics:
- Highest survival rates and reproductive success
- Peak population abundance
- Optimal physiological functioning
- Most efficient energy use and resource allocation
Zones of Physiological Stress (Slopes of Curve)
Definition: Environmental conditions on either side of the optimal range where the organism can survive but experiences stress. These zones extend from the optimal range to the tolerance limits.
Characteristics:
- Reduced population abundance and individual fitness
- Lower growth and reproduction rates
- Increased energy expenditure to maintain homeostasis
- Higher vulnerability to disease, predation, and competition
- Organism can survive but not thrive
Example: A fish species may survive in water slightly warmer or cooler than optimal, but growth slows and reproduction decreases.
Zones of Intolerance (Beyond Curve Edges)
Definition: Environmental conditions beyond the organism's tolerance limits where survival is impossible. These represent the extreme boundaries outside which the organism cannot persist.
Characteristics:
- Organism cannot survive - death occurs
- Physiological processes fail or become lethally disrupted
- No self-sustaining population is expected under prolonged exposure beyond the relevant limits; duration and life stage matter.
- A self-sustaining population cannot persist beyond the relevant limits under the conditions assessed; a transient visitor or dispersing individual is not evidence of successful reproduction there.
Example: Use the limits supplied for a particular species and exposure period. There is no universal fish tolerance boundary at 0°C or 40°C; some cold-water fishes live in subzero seawater, while species and life stages differ greatly in heat tolerance.
Limiting Factors
A limiting factor restricts growth, reproduction, distribution or abundance. A scarce resource can limit production, while excessive temperature, salinity or contaminants can also impose limits. Abiotic conditions and biotic interactions may act together; the limiting factor can change through space or time.
Liebig's Law of the Minimum
Principle: Growth can be constrained by the essential resource in shortest supply relative to biological requirements, even when other resources are adequate. This resource-shortage model is different from the idea that the smallest numerical measurement is always limiting.
Example: If nitrogen is the only limiting resource for a plant, adding nitrogen can increase growth until another factor becomes limiting. Adding extra light or water alone would not remove nitrogen limitation. Real systems can be co-limited, so use experimental evidence rather than assume one nutrient always limits.
Analogy: Think of a barrel with staves of different heights - water can only fill to the height of the shortest stave (the limiting factor).
Shelford's Law of Tolerance
Principle: For each environmental factor, an organism has a range of tolerance with an optimal range in the middle. Both too much and too little of a factor can be limiting.
Example: Plants need water, but both drought (too little) and flooding (too much) are stressful or lethal. Temperature is similar - too hot or too cold both limit survival.
Key insight: Both deficiency and excess can limit performance for many factors. This supports the idea of a tolerance range, but does not require every response to be symmetric or bell-shaped. Survival, growth and reproduction may have different limits.
Common Limiting Factors
Terrestrial Ecosystems:
- Water availability (precipitation, soil moisture)
- Temperature (growing season length, freeze/heat stress)
- Nutrients (especially nitrogen and phosphorus in soil)
- Sunlight (especially in dense forests)
- Soil pH and mineral content
Aquatic Ecosystems:
- Dissolved oxygen levels
- Sunlight/light penetration (depth)
- Nutrients (nitrogen, phosphorus for algae/phytoplankton)
- Temperature
- Salinity (especially in estuaries)
- pH and water chemistry
Tolerance Ranges and Species Types
Eurytolerant Species (Wide Tolerance)
Definition: Species with broad tolerance ranges that can survive across a wide range of environmental conditions. The prefix "eury-" means "wide."
Characteristics: Broad tolerance of the specified factor can allow use of more environments and buffer some changes. It does not imply broad tolerance of every factor, a globally widespread range or invasive status. Invasiveness also requires introduction and successful spread with impacts.
Examples: Some carp tolerate a relatively broad range of oxygen and temperature conditions; many estuarine fishes tolerate substantial salinity variation. Raccoons and rats illustrate broad habitat or diet use, but that is a niche comparison rather than a measured physiological tolerance curve.
Stenotolerant Species (Narrow Tolerance)
Definition: Species with narrow tolerance ranges that require specific environmental conditions. The prefix "steno-" means "narrow."
Characteristics: Narrow tolerance of a specified abiotic factor can increase vulnerability when that factor moves outside the suitable range. This does not automatically mean a species is endangered, has a small geographic range or is specialized in every resource.
Examples: Many shallow tropical reef-building corals have relatively narrow thermal tolerances; deep and cold-water corals also exist. Many trout are sensitive to warming and low dissolved oxygen. Panda diet and polar-bear sea-ice dependence illustrate specialization, not proof of the same narrow tolerance for every abiotic variable.
Specific Tolerance Terms
For specific environmental factors, we use special terminology:
- Temperature: Eurythermal (wide range) vs. Stenothermal (narrow range)
- Salinity: Euryhaline (wide range) vs. Stenohaline (narrow range)
- Oxygen: Euryoxic (wide range) vs. Stenooxic (narrow range)
Exam tip: Label the measured response, gradient, optimum, stress zones and limits. Explain a change using the relevant species, life stage and duration of exposure. Wide tolerance of one factor can help, but vulnerability also depends on exposure, dispersal, habitat availability and interactions. Liebig emphasizes resource shortage relative to need; Shelford includes limits from both deficiency and excess.
Read data before assigning a tolerance limit
Hypothetical experiment: At 10, 15, 20, 25 and 30°C, mean surviving juveniles per tank are 4, 14, 20, 12 and 0 under otherwise matched conditions. The highest measured survival is at 20°C. Zero survivors at 30°C in this experiment indicates an unsuitable tested treatment, but does not locate the exact lethal threshold between 25 and 30°C. More temperatures, replication and exposure details are needed.
Individual versus species: A species-level range combines variation among individuals and populations; it need not equal the range of one individual. Eggs or larvae may be more sensitive than adults, and reproduction may fail before adults die. Acclimation can change an individual’s physiological response within a lifetime, whereas evolutionary adaptation changes heritable trait frequencies across generations.
Interacting factors: Warm water generally holds less dissolved oxygen at saturation, while organism oxygen demand can increase within a physiological range. A thermal discharge may therefore impose both temperature and oxygen stress. Do not attribute every field absence to one abiotic factor without considering habitat, competition and access.
2.5 Natural Disruptions to Ecosystems
Overview
Natural disruptions (also called natural disturbances) are events that cause rapid changes to ecosystem structure, composition, or function. While often perceived as destructive, natural disruptions are essential ecological processes that create habitat diversity, initiate succession, prevent competitive exclusion, and maintain biodiversity. Ecosystems have evolved with these disturbances and many species depend on them.
For the AP Environmental Science exam, you must understand the types of natural disruptions, their ecological roles, and how disturbance frequency and intensity affect ecosystem resilience and diversity.
Types of Natural Disruptions
Wildfires
Description: Wildfires burn vegetation or organic material outside a prescribed operation. Lightning is a major natural ignition source; people also ignite many fires. Hot, dry weather promotes ignition and spread but does not normally make living vegetation burst into flame without an ignition source. Rare self-heating can ignite accumulated organic material under particular conditions.
Ecological Effects:
- Release nutrients from organic matter back to soil (ash fertilization)
- Clear underbrush and promote new growth
- Open up canopy, allowing light to reach forest floor
- Heat can open serotinous cones in some lodgepole and jack pine populations, releasing seeds. Other species resprout or respond to smoke cues. Do not generalize one fire response to every population or call every fire beneficial.
- Create habitat mosaic of different successional stages
- Prevent accumulation of excessive fuel loads
Fire-Adapted Ecosystems: Grasslands, savannas, chaparral, some coniferous forests (especially jack pine, longleaf pine)
Human impact: In some historically frequent-fire forests, prolonged exclusion can increase fuels and later fire severity. Other systems respond differently; frequent burning can damage fire-sensitive ecosystems. Warming and drying can increase fire-conducive conditions in many regions, while ignition, fuels, wind and management shape individual fires.
Floods
Description: Water covers normally dry ground after heavy precipitation, snowmelt, storm surge or altered flow. A dam failure can also cause flooding, but it is a failure of human infrastructure rather than a purely natural disturbance.
Ecological Effects:
- Deposit nutrient-rich sediments on floodplains
- Create wetland habitats
- Redistribute seeds and organisms
- Scour channels and create pools in rivers
- Maintain connectivity between aquatic habitats
- Flooding can change survival and recruitment, favouring flood-tolerant traits where those traits are heritable. An individual does not acquire an inherited adaptation merely by being flooded.
Flood-Adapted Species: Many riparian plants can survive temporary inundation; fish use floodplains for spawning
Human Impact: Dams and levees prevent natural flooding, degrading floodplain ecosystems. Urbanization increases flood risk by reducing infiltration.
Hurricanes, Tornadoes, and Severe Storms
Description: Intense weather events with high winds, heavy precipitation, and in the case of hurricanes, storm surges.
Ecological Effects:
- Uproot trees, creating canopy gaps and allowing succession
- Fallen trees become habitat (nurse logs) for decomposers and seedlings
- Increase structural complexity of forests
- Mix ocean and freshwater in coastal areas
- Transport seeds and organisms to new areas
- Reset succession in some areas
Climate connection: Warmer oceans can supply more energy and moisture to tropical cyclones. Assessments project stronger intensity and heavier rainfall in a warmer climate, while changes in total storm frequency and regional tracks are more complex. Do not infer that warming makes every storm more frequent or attribute one event to a single cause.
Droughts
Description: Extended periods of abnormally low precipitation leading to water shortage.
Ecological Effects:
- Select for drought-tolerant species
- Reduce populations of water-dependent species
- Lower lake and river levels, concentrating pollutants
- Increase wildfire risk
- Cause die-offs of trees and other vegetation
- Can shift ecosystem types (e.g., forest to grassland)
Climate connection: Changes in precipitation, snow storage, soil moisture and atmospheric evaporative demand can alter drought risk. Trends vary by region and drought definition. Water withdrawals and land management can intensify ecological impacts even when rainfall is unchanged.
Volcanic Eruptions
Description: Release of magma, ash, gases, and debris from Earth's interior.
Ecological Effects:
- Severe lava, heat or burial can kill much existing vegetation and create new substrate. Other patches retain soil, roots, seeds or organisms. A dramatic-looking volcanic landscape is not necessarily sterile everywhere.
- Volcanic ash adds minerals to soil
- Create new land (islands like Hawaii, Surtsey)
- Trigger primary succession on barren lava flows
- Large explosive eruptions can temporarily cool climate when sulfur gases reach the stratosphere and form reflective sulfate aerosols. Not every eruption has this effect; size, gas emissions and injection height matter.
- Create unique habitats around geothermal features
Example: The 1980 Mount St. Helens eruption produced a mosaic: some new deposits supported primary succession, while areas retaining soil and biological legacies followed other recovery pathways, including secondary succession. Classify the starting substrate and surviving organisms, not the entire volcano under one label.
Other Natural Disruptions
- Landslides and Avalanches: Create bare patches for colonization; mix soil layers
- Tsunamis: Reshape coastlines; deposit marine sediments inland
- Earthquakes: Alter topography; create dams or release water; trigger other disturbances
- Insect outbreaks: Native bark beetles can cause major mortality during outbreaks; introduced spongy moths can also defoliate forests. Outbreaks may have natural drivers, but species introductions and human-altered climate or habitat can change their extent.
- Disease epidemics: Pathogens can reshape communities. Chestnut blight and Dutch elm disease are examples strongly associated with introduced pathogens in affected regions, so they are not examples of disturbances wholly independent of human influence.
Ecological Role of Disturbances
Intermediate disturbance hypothesis: This hypothesis proposes that, in some communities and at a specified scale, intermediate disturbance can prevent dominance while allowing more species to persist. Results vary with productivity, disturbance type, taxa and measurement scale; a peak at intermediate disturbance is not a universal ecological law or a reason to prescribe disturbance indiscriminately.
Positive Effects of Natural Disturbances
- Create habitat heterogeneity and diversity
- Can create early-stage patches and reduce dominance by some established species; the effect depends on disturbance size, severity and frequency.
- Release nutrients bound in biomass
- Provide resources for specialist species adapted to disturbance
- Can maintain habitat mosaics and regeneration processes within a characteristic disturbance regime; disturbances outside that regime can instead reduce recovery capacity.
- Drive evolutionary adaptation
Common pitfall: Evaluate the disturbance regime: type, frequency, intensity, severity, spatial extent and duration. A low-severity fire may help maintain one ecosystem while repeated severe fires degrade another. Natural and human influences can interact. Distinguish immediate mortality from later regeneration, and avoid calling every disturbance either beneficial or harmful.
Resistance, resilience and disturbance regimes
Resistance describes how little an ecosystem property changes during disturbance. Resilience describes its capacity or rate of recovery afterward. A grassland can lose much aboveground biomass during fire (low resistance for that measure) yet regrow rapidly from surviving roots (high resilience for biomass). Species composition can recover more slowly than vegetation cover.
Compare timescales: Seasonal floods, episodic fires and long-term climatic shifts operate over different durations and areas. Species ranges can expand, contract or shift through time; past environmental conditions provide context for present distributions. Fossils, sediment records, tree rings and repeated surveys provide different forms of evidence about change.
Worked comparison: Plot A loses 10% of its biomass and recovers in five years. Plot B loses 60% and recovers in two years. For this measure, A shows greater resistance and B faster recovery. State the baseline and response variable; these observations alone do not establish that B has recovered its original species composition or every ecosystem service.
2.6 Adaptations
Overview
Adaptations are inherited traits or characteristics that increase an organism's ability to survive and reproduce in its specific environment. Adaptations result from natural selection acting over many generations - individuals with advantageous traits are more likely to survive, reproduce, and pass those traits to offspring. Over time, these beneficial traits become more common in the population.
For the AP Environmental Science exam, you must understand the three types of adaptations (structural, physiological, behavioral) and be able to identify examples and explain how they enhance survival or reproduction.
Types of Adaptations
1. Structural (Morphological) Adaptations
Definition: Inherited anatomical or morphological features that can improve survival or reproduction in a particular environment. Some are externally visible, while others require examining internal structure; visibility is not the defining distinction.
Examples:
- Cacti: Thick, waxy cuticle reduces water loss; shallow, widespread roots absorb water quickly; spines (modified leaves) defend against herbivores and provide shade; expandable stem stores water
- Arctic foxes: Thick fur for insulation; small ears to reduce heat loss; white winter coat for camouflage
- Birds: Beak shapes can suit particular feeding methods, such as nectar feeding, seed cracking or tearing meat. Many birds have lightweight, internally braced and partly air-filled bones. Traits are evaluated by their function and evolutionary context, not by assuming every bird has an identical skeleton.
- Giraffes: Long necks reach high leaves; long tongues grasp branches
- Fish: Streamlined bodies can reduce drag and gill structures support gas exchange. Many bony fishes have swim bladders used in buoyancy control, but not all fishes do; sharks lack a swim bladder.
- Camels: Humps store fat (not water) for energy; wide feet prevent sinking in sand; long eyelashes and sealable nostrils keep out sand
- Venus flytraps: Modified leaves trap insects to obtain nitrogen in nutrient-poor soils
- Deciduous trees: Broad, flat leaves maximize photosynthesis in summer; ability to shed leaves prevents water loss and damage in winter
Camouflage and Mimicry: Special structural adaptations where organisms resemble their environment or other species for protection. Cryptic coloration (blending in), Batesian mimicry (harmless species resembles harmful one), Müllerian mimicry (multiple harmful species resemble each other).
2. Physiological (Biochemical) Adaptations
Definition: Inherited biochemical or physiological mechanisms affecting function, such as water balance, enzymes or metabolic regulation. Processes can have visible effects; classify the mechanism rather than decide only by whether it can be seen.
Examples:
- CAM Photosynthesis: Desert plants (cacti, succulents) open stomata at night to absorb CO₂, minimizing water loss during hot days
- Antifreeze proteins: Some polar marine fishes produce proteins or glycoproteins that inhibit ice-crystal growth and help prevent freezing in subzero seawater. These adaptations occur in both Arctic and Antarctic groups and are more specific than a universal cold-water response.
- Venom production: Snakes, spiders, scorpions produce toxins for hunting and defense
- Hibernation metabolic changes: Bears and ground squirrels dramatically slow metabolism, heart rate, and breathing to conserve energy in winter
- Countercurrent heat exchange: Closely associated vessels carrying blood in opposite directions transfer heat from outgoing arterial blood to returning venous blood. Vessel arrangement is structural; the heat-transfer process is physiological. Both contribute to conserving core heat in some cold-adapted animals.
- Drought tolerance: Some plants can survive with very low cellular water content; others close stomata tightly to prevent water loss
- Chemosynthesis: Deep-sea bacteria near hydrothermal vents use chemical energy instead of sunlight to produce food
- Lactase persistence: In some human populations, inherited regulatory variants maintain production of the lactase enzyme into adulthood. This increases capacity to digest lactose; drinking milk does not by itself create an inherited evolutionary adaptation.
- Plant toxins: Many plants produce alkaloids, tannins, or other chemicals that deter herbivores
Important: Physiological adaptations often work in conjunction with structural adaptations to maximize survival.
3. Behavioral Adaptations
Definition: Actions or response patterns can improve survival and reproduction. Natural selection can shape inherited behaviours and learning capacities. A learned behaviour itself is acquired through experience rather than copied directly into DNA; behavioural flexibility and evolutionary adaptation should be distinguished.
Examples:
- Migration: Some birds move between breeding and nonbreeding areas, caribou track seasonal resources, and salmon migrate to spawning habitat. Directions vary with species and hemisphere; not every bird moves south or migrates at all.
- Hibernation: Bears, bats, and ground squirrels enter dormant state to survive winter when food is scarce. (Note: this has both physiological and behavioral components)
- Estivation: Some animals (lungfish, snails) become dormant during hot, dry periods
- Nocturnal activity: Desert animals are active at night to avoid daytime heat
- Courtship displays: Male birds perform elaborate dances, displays, or songs to attract mates
- Territorial behavior: Animals defend areas to secure resources and mating opportunities
- Herding/Schooling: Group living provides protection from predators (safety in numbers, dilution effect)
- Tool use: Chimpanzees use sticks to extract termites; sea otters use rocks to crack open shellfish
- Playing dead: Opossums feign death when threatened by predators
- Food caching: Squirrels bury nuts; crows hide food for later consumption
- Cooperative hunting: Wolves hunt in packs to take down larger prey
Innate versus learned: Innate behaviours develop with a strong inherited basis and need not be performed immediately at birth; web construction is an example. Learned behaviours depend on experience, such as tool-use techniques acquired socially. The categories can interact through inherited learning abilities and environmental cues.
Natural Selection and Evolution
How Adaptations Develop: Adaptations arise through natural selection, the process by which individuals with traits better suited to their environment survive and reproduce at higher rates than others.
The Process of Natural Selection
- Variation: Mutation creates new alleles; recombination and sexual reproduction reshuffle existing variation. Gene flow can introduce alleles from other populations. Mutations do not arise because an organism needs a particular adaptive solution.
- Inheritance: Some variation affecting the trait is heritable. Environmental effects can also change phenotype, so not every visible difference will respond to selection in the same way.
- Differential Survival: Some variations provide advantages in survival or reproduction (fitness)
- Reproduction: Individuals with favourable heritable traits leave, on average, more surviving reproductive offspring under the stated conditions. Survival alone is not sufficient if it does not contribute to reproduction.
- Frequency change: Heritable variants associated with higher reproductive success can increase across generations. Drift, gene flow and changing environments can also affect allele frequencies; evolution is not purposeful progress toward perfection.
Important Concepts
- Fitness: Reproductive success - the ability to survive and produce offspring that also survive to reproduce
- Selection Pressure: Environmental factors (predation, climate, disease, competition) that influence which traits are favored
- Trade-offs: Adaptations often involve compromises - a trait advantageous in one context may be disadvantageous in another
- Time Required: Significant adaptations typically take many generations to evolve (though some can occur rapidly in fast-reproducing organisms)
💡 Exam Tip: Be able to classify adaptations into the three types (structural, physiological, behavioral) and explain HOW each adaptation increases survival or reproduction. Remember: adaptations develop through natural selection over many generations - organisms don't "choose" to adapt during their lifetime (that's not evolution!). Camouflage is structural; venom production is physiological; migration is behavioral.
Adaptation, acclimation and natural selection
Separate the timescales: A fish adjusting its physiology after a temperature change may be acclimating. A population showing a heritable shift across generations is evolving. Individuals can develop, learn and acclimate; populations evolve. Plasticity itself can have a heritable basis, but one lifetime response is not evidence that a new allele evolved.
Selection example: A hypothetical insect population already contains a heritable resistance variant. After pesticide application, resistant insects leave more offspring on average. Across generations, the variant can become more common. The pesticide selects among variation; it does not deliberately teach insects resistance. A resistance cost can reduce the variant’s advantage when pesticide pressure is removed.
Connect disturbance to persistence: Genetic variation, generation time, reproductive output, dispersal and accessible habitat affect a population’s response to environmental change. Rapid change may outpace adaptation, and a useful trait in one environment may impose a trade-off in another. Explain the mechanism before classifying a feature as structural, physiological or behavioural; several mechanisms may operate together.
2.7 Ecological Succession
Overview
Ecological succession is change in community composition and structure following newly available substrate or a disturbance. Some broad patterns can be predicted, but arrival order, surviving organisms, resources, climate and further disturbances alter the pathway. A relatively persistent later community is sometimes called a climax community in simplified models; it is not a permanently fixed or inevitable endpoint.
For the AP Environmental Science exam, you must distinguish between primary and secondary succession, understand the stages of succession, know the characteristics of pioneer and climax species, and recognize factors that influence succession rates.
Primary Succession
Definition: Primary succession begins on newly formed or newly exposed substrate lacking developed soil and substantial biological legacies, such as fresh lava or recently exposed rock. The location may have supported an ecosystem in the distant past; “no developed soil at the start” is more useful than claiming life has never existed there.
Where Primary Succession Occurs
- Newly formed volcanic islands (Hawaii, Surtsey)
- Newly exposed bedrock or relatively undeveloped glacial deposits after glacier retreat. Glacial till is deposited sediment, not simply another name for bare rock.
- Cooling lava flows
- New sand deposits or dunes lacking developed soil can undergo primary succession. A disturbed dune retaining soil, roots or a seed bank requires a more site-specific classification.
- New deposits or scoured substrates after volcanic eruptions where developed soil and biological legacies are absent. Other eruption-affected patches can retain life and follow secondary recovery.
Stages of Primary Succession
Stage 1: Pioneer Species
- First colonizers: Depending on the substrate, microbes, lichens, mosses or stress-tolerant vascular plants may establish. Lichens combine a fungus with a photosynthetic partner, often algae or cyanobacteria. There is no mandatory lichen-first sequence at every site.
- Characteristics: Pioneer colonizers tolerate the particular exposed conditions and may have low soil requirements. They still need essential nutrients and appropriate water and energy supplies. Different pioneers obtain resources in different ways; they do not live on sunlight, air and water alone.
- Role: Begin soil formation by secreting acids that weather rock and accumulating organic matter from dead tissues
- Time: May take decades to hundreds of years depending on climate
Stage 2: Early Successional Species (Grasses and Small Plants)
- Species: Annual grasses, herbs, small flowering plants
- Requirements: Thin soil layer created by pioneers
- Characteristics: Fast-growing, short-lived, produce many seeds, shade-intolerant
- Role: Add more organic matter to soil; deeper root systems further break up rock; hold soil in place
- Result: Soil deepens and becomes richer in nutrients
Stage 3: Intermediate Successional Species (Shrubs and Small Trees)
- Species: Perennials, shrubs and fast-growing trees may establish where conditions permit. Alders can form associations with nitrogen-fixing Frankia bacteria; not every intermediate-stage species fixes nitrogen.
- Characteristics: Larger, live longer than early species, create shade
- Role: Shade out grasses; further enrich soil; increase structural complexity
- Result: Environment becomes less suitable for sun-loving pioneers, more suitable for shade-tolerant species
Stage 4: Later community (illustrative forest pathway)
- Species: In a suitable temperate forest example, later stages may contain long-lived trees such as maples, beeches, oaks or hemlocks. Light tolerance and regeneration requirements differ among these groups; not every later species is slow-growing or strongly shade-tolerant.
- Characteristics: Shade-tolerant, long-lived, slow-growing, produce few large seeds
- Result: Later communities may develop greater biomass and structural complexity than initial stages, but neither biodiversity nor biomass must increase continuously to a universal maximum. Resource limits, competition and disturbance shape the outcome.
- Time: Soil and later forest development can require centuries or longer at some sites, while initial colonization and some functions can develop sooner. Rates depend on substrate, climate and dispersal; these examples do not define a fixed climax timetable.
Key Point about Primary Succession:
Key point about primary succession: Soil development can make early establishment slow. Colonizers can facilitate later species by changing nutrients, shade or substrate, but they can also inhibit them or simply tolerate their arrival. These mechanisms may occur together; no single facilitation sequence explains every site.
Secondary Succession
Definition: Ecological succession that occurs in an area where an existing community has been disturbed or removed, but soil remains. Because soil and often seeds, roots, and organisms persist, secondary succession proceeds much faster than primary succession.
Where Secondary Succession Occurs
- Abandoned agricultural fields (old-field succession)
- After fires where developed soil persists. Severity and surviving roots, microbes or seeds affect recovery; visible vegetation loss alone does not establish primary succession.
- After hurricanes, tornadoes, or windstorms that blow down trees
- After flooding that doesn't remove soil
- Logging areas where trees are cut but soil remains
- Areas cleared for development but later abandoned
Stages of Secondary Succession (Example: Abandoned Farm Field)
Stage 1: Early herbs and grasses (illustrative first years)
- Species: Annual crabgrass and ragweed may occur alongside perennial dandelions or clovers in an abandoned field. The first plants are not all annuals, and surviving perennials can resprout immediately.
- Source: Seeds already in soil (seed bank) or wind-dispersed from nearby areas
- Characteristics: Many early colonizers grow rapidly and disperse abundant seeds. Annuals complete a life cycle in one growing season; perennial survivors also contribute to early recovery.
Stage 2: Perennial grasses and herbs (timing varies)
- Species: Goldenrod, asters, perennial grasses
- Characteristics: Live multiple years, deeper roots, shade out annuals
- Result: More stable community with higher plant density
Stage 3: Shrubs and young trees (timing varies)
- Species: Blackberry, sumac, pines, cedars, aspens
- Characteristics: Fast-growing trees, shade-intolerant, pioneer tree species
- Result: Create young forest; shade inhibits earlier grass/herb species
Stage 4: Later forest community (where climate and land use allow)
- Species: Oaks, maples, hickories, beeches (varies by region)
- Characteristics: Shade-tolerant, long-lived, slow-growing
- Result: A later forest may have substantial biomass and complex structure, with continuing gaps, recruitment, deaths and population fluctuations. Species diversity can peak earlier or differ among groups; maturity does not mean an unchanging system.
- Regeneration: Some later forest species tolerate shade and establish beneath a canopy. Others require gaps or disturbances. Mature forests remain dynamic, and not all long-lived trees regenerate equally well in deep shade.
Key Point about Secondary Succession:
Key point about secondary succession: Surviving soil, roots, organisms and seed banks often allow faster initial recovery than on new substrate. Decades or centuries may be needed for particular forest properties, while some functions recover sooner. The original 70–150-year temperate-forest range is only an illustration, not a universal deadline or proof of a fixed endpoint.
Pioneer and later-stage species: common tendencies
Pioneer Species
- Fast-growing
- Short-lived
- Small size
- Produce many seeds
- Wind or animal dispersal
- Shade-intolerant
- Hardy (tolerate extremes)
- Early communities may initially contain few species; this is a community property, not an individual-species trait.
- Early communities often have low standing biomass.
- Often associated with rapid growth and dispersal; the r-selected label is a simplified life-history model.
Later-stage species (simplified forest model)
- Often slower-growing in this forest comparison; exceptions occur.
- Long-lived
- Large size
- Reproductive output varies; some later trees produce many seeds.
- Large, heavy seeds
- Shade-tolerant
- Often persist under established vegetation; disturbances and gaps can still be important.
- Later communities can be diverse, but richness need not reach its maximum at the latest stage.
- Later communities often accumulate substantial standing biomass.
- Often associated with persistence and longevity; the K-selected label is a simplified life-history model.
Factors Affecting Succession Rate
- Climate: Warmer, wetter climates speed succession; cold or dry climates slow it
- Soil Quality: Nutrient-rich soil accelerates plant growth and succession
- Seed Availability: Proximity to seed sources speeds colonization
- Disturbance frequency: Repeated events can interrupt tree recruitment or maintain early-stage patches. Responses depend on severity, intervals and surviving organisms; a disturbance need not reset all ecosystem properties to zero.
- Competition: Strong competitors can slow or alter succession pathways
- Human Intervention: Can accelerate (planting trees) or prevent (mowing lawns) succession
Exam tip: Classify the starting conditions first: primary succession lacks developed soil; secondary succession retains it. Identify pioneers from the site rather than always naming lichens. Later vegetation depends on climate, resources, dispersal, biological legacies and disturbance. Biomass often rises during recovery, but biodiversity need not rise continuously and not every pathway ends in forest.
Use the pioneer–later comparison as a model
The trait lists above summarize common patterns in a temperate forest example. Early plants often disperse well and grow rapidly, while some later trees are long-lived and tolerate shade. These are tendencies, not definitions of every species. The labels r-selected and K-selected are simplified life-history endpoints rather than two rigid categories, and seed number alone does not identify succession stage.
Diversity versus biomass: Diversity is a property of a community, not a trait of an individual pioneer or later species. Standing biomass is an accumulated stock; productivity is a rate. Biomass can continue to rise while the mix and evenness of species change in either direction. A mature forest can have high total biomass without the highest current NPP.
Classify three sites: Fresh lava without developed soil suggests primary succession. A burned forest with soil and living roots suggests secondary succession. A grazed grassland with persistent soil and repeated disturbance may remain grassland rather than inevitably become forest. Name the evidence before selecting the process.
Track recovery: Compare soil depth, vegetation cover, biomass, species richness and composition using repeated measurements. Different properties recover at different rates. A site that looks green again may not yet provide its previous habitat or ecosystem services.
Unit 2 Key Takeaways for AP Exam Success
Must-Know Concepts
- 3 levels of biodiversity
- 4 categories of ecosystem services
- Island biogeography (size + distance)
- Tolerance curve zones
- Liebig's & Shelford's Laws
- Types of adaptations (3)
- Primary vs. secondary succession
- Pioneer vs. climax species
Common Mistakes to Avoid
- Confusing species richness & evenness
- Mixing up ecosystem service types
- Forgetting: LARGE + CLOSE = MORE species
- Tolerance zones mixed up
- Natural disturbances aren't always bad!
- Mixing structural/physiological/behavioral
- Primary succession has soil (FALSE!)
- Treating every later-stage species as shade-tolerant or succession as a fixed path to an unchanging climax
Study Strategies
Create comparison tables for primary vs. secondary succession and pioneer vs. climax species. Draw tolerance curves from memory. Practice classifying ecosystem services into the four categories. Make flashcards linking species to adaptation types. Draw island biogeography graphs showing how size and distance affect equilibrium species number.
APES Unit 2 study notes FAQs
What do APES Unit 2 study notes cover?
These notes cover all seven Unit 2 topics: biodiversity, ecosystem services, island biogeography, ecological tolerance, natural disruptions, adaptations and ecological succession. They include detailed examples, diagrams, comparisons and worked calculations.
What is the difference between species richness and evenness?
Richness is the number of species in a defined sample or area. Evenness describes how similar their relative abundances are. Two communities can have equal richness and different evenness; compare the same sampling effort when using field data.
Why do larger, nearer islands usually support more species?
In the simple island-biogeography model, larger areas generally reduce local extinction and nearer source populations increase immigration. Hold other conditions comparable. A small-near island cannot automatically be ranked against a large-far island without additional information.
How do primary and secondary succession differ?
Primary succession begins on substrate without developed soil, such as fresh lava. Secondary succession begins where soil persists after disturbance and often retains roots, seeds or other biological legacies. Neither requires an identical sequence of species or a permanently fixed endpoint.
Does an individual evolve when it adjusts to environmental change?
An individual can acclimate, develop or learn within its lifetime. Evolutionary adaptation concerns heritable differences and changes across generations in a population. Natural selection favours variants with greater reproductive success under particular conditions.
How much of the APES multiple-choice section is Unit 2?
College Board assigns The Living World: Biodiversity a 6–8% weighting in the multiple-choice section. Use the range to guide revision rather than predict an exact question count. Practise interpreting data and explaining mechanisms as well as recalling terms.