Unit 2 – The Living World: 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:

• Different fur colors, sizes, and disease resistances among dogs of the same breed
• 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: Higher species diversity typically correlates with greater ecosystem stability, productivity, and resilience to disturbances. Diverse ecosystems have functional redundancy - if one species declines, others can perform similar ecological roles.

Examples:

• Tropical rainforests have extremely high species diversity (thousands of plant and animal species)
• Deserts have lower species diversity due to harsh conditions
• Agricultural monocultures have very low species diversity (only one crop species)

Calculation Example: If Community A has 5 bird species (100 sparrows, 90 robins, 85 jays, 80 cardinals, 75 finches) and Community B has 5 bird species (250 sparrows, 50 robins, 25 jays, 15 cardinals, 10 finches), both have equal species richness (5), but Community A has higher species evenness because the populations are more evenly distributed.

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 with only desert has low ecosystem diversity
• 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.

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: Control deer populations, preventing 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: Sensitive to air pollution; absence indicates poor air quality
• Trout: Require clean, cold, well-oxygenated water; presence indicates healthy streams
• Amphibians: Permeable skin makes them sensitive to pollutants and environmental changes

Specialist vs. Generalist Species

Specialist Species: Have narrow ecological niches; require specific habitat conditions, food sources, or climate. More vulnerable to environmental changes and habitat loss but excel in stable environments.

Examples: Giant pandas (bamboo only), koalas (eucalyptus only), spotted owls (old-growth forests)

Generalist Species: Have broad ecological niches; can tolerate wide range of conditions and utilize many resources. More resilient to environmental changes but may be outcompeted by specialists in stable habitats.

Examples: Raccoons, rats, coyotes, cockroaches, humans

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: Pharmaceutical compounds from plants (aspirin from willows, cancer drugs from Pacific yew)
• 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: Forests absorb CO₂ (carbon sequestration); oceans moderate temperatures
• Water Purification: Wetlands filter pollutants; soil microorganisms break down contaminants
• Water Regulation: Forests regulate water flow, reducing flood and drought intensity
• Erosion Control: Plant roots hold soil in place; vegetation slows water runoff
• Natural Hazard Protection: Mangroves and coral reefs protect coasts from storms and tsunamis
• Pollination: Bees, butterflies, birds, and bats pollinate crops and wild plants
• Pest Control: Predators and parasites control agricultural pests naturally
• Disease Regulation: Biodiversity can dilute disease transmission (dilution effect)

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: Unlike the other three categories, supporting services act on longer timescales and provide benefits indirectly. Loss of supporting services cascades to affect all other service categories.

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

Immigration rate is highest when the island is empty (no species present) and decreases toward zero as the island approaches its maximum species capacity.

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)
• Greater predator-prey interactions and disease transmission

Extinction rate is lowest when few species are present and increases as species richness increases.

Species-Area Relationship:

S = cA^z

Where: S = number of species, A = area, c = constant, z = slope (typically 0.2-0.4)
This means species richness increases exponentially with island area.

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)
• Isolated fragments have lower immigration, higher extinction
• Specialist species decline first; generalists persist longer
• Increased edge effects alter habitat quality

Reserve Design Implications

• SLOSS Debate: Single Large Or Several Small reserves? Generally, one large reserve is better than several small ones of equal total area because larger reserves support more species and have lower edge-to-area ratios.
• Habitat Corridors: Connecting isolated reserves with corridors increases immigration rates and reduces extinction, effectively making reserves "less distant."
• Buffer Zones: Surrounding reserves with partial protection reduces edge effects.
• Proximity Matters: Reserves should be as close together as possible to facilitate species movement.

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
• Smaller fragments have higher edge-to-interior ratios, reducing core habitat

Zone of Optimal Range (Peak of Curve)

Definition: The ideal environmental conditions where the organism thrives best. In this range, the species exhibits maximum growth, reproduction, and population size.

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 population can be sustained
• Species is absent from these environmental conditions

Example: Most fish cannot survive if water temperature exceeds 40°C or drops below 0°C (freezing) - these are zones of intolerance.

Liebig's Law of the Minimum

Principle: Growth and reproduction are limited by the resource or condition that is in shortest supply (the scarcest or most limiting factor), even if all other factors are abundant.

Example: If a plant has abundant sunlight, water, and all nutrients except nitrogen, nitrogen becomes the limiting factor. Adding more sunlight or water won't increase growth - only adding nitrogen will help.

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: This law explains why tolerance curves are bell-shaped rather than linear - organisms are limited by extremes in both directions.

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

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: Generalists, widespread distributions, more resistant to environmental change, often invasive

Examples: Raccoons (various habitats and diets), cockroaches (temperature extremes), brown rats, dandelions, carp (various water conditions)

Stenotolerant Species (Narrow Tolerance)

Definition: Species with narrow tolerance ranges that require specific environmental conditions. The prefix "steno-" means "narrow."

Characteristics: Specialists, limited distributions, vulnerable to environmental change, often endangered

Examples: Coral (warm, clear water only), trout (cold, oxygen-rich water), pandas (bamboo forests), polar bears (arctic ice), tropical orchids (specific humidity/temperature)

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)

Wildfires

Description: Uncontrolled fires that burn vegetation and organic matter. Can be caused by lightning strikes, volcanic eruptions, or spontaneous combustion in hot, dry 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
• Some species require fire for reproduction (serotinous cones of lodgepole pine)
• 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: Fire suppression leads to fuel accumulation and more severe fires when they do occur. Climate change is increasing fire frequency and intensity.

Floods

Description: Overflow of water onto normally dry land. Caused by heavy rainfall, snowmelt, storm surges, or dam failures.

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
• Remove less flood-tolerant species, maintaining adaptation

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 Change Connection: Warmer ocean temperatures are likely increasing hurricane intensity (though not necessarily frequency).

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 Change Connection: Many regions experiencing more frequent and severe droughts due to changing precipitation patterns and increased evaporation.

Volcanic Eruptions

Description: Release of magma, ash, gases, and debris from Earth's interior.

Ecological Effects:

• Can completely destroy existing ecosystems (sterilize landscape)
• Volcanic ash adds minerals to soil
• Create new land (islands like Hawaii, Surtsey)
• Trigger primary succession on barren lava flows
• Large eruptions can temporarily cool global climate (volcanic winter)
• Create unique habitats around geothermal features

Example: Mount St. Helens (1980) provided opportunity to study primary succession from bare volcanic substrate.

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: Bark beetles, gypsy moths can kill large areas of forest
• Disease Epidemics: Chestnut blight, Dutch elm disease dramatically altered forests

Positive Effects of Natural Disturbances

• Create habitat heterogeneity and diversity
• Reset succession, preventing dominance by climax species
• Release nutrients bound in biomass
• Provide resources for specialist species adapted to disturbance
• Maintain ecosystem resilience and adaptive capacity
• Drive evolutionary adaptation

1. Structural (Morphological) Adaptations

Definition: Physical features of an organism's body structure, form, or anatomy that enhance survival. These are physical characteristics you can see.

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: Beaks shaped for specific diets (long thin beaks for nectar, heavy beaks for cracking seeds, hooked beaks for tearing meat); hollow bones reduce weight for flight
• Giraffes: Long necks reach high leaves; long tongues grasp branches
• Fish: Streamlined bodies reduce water resistance; gills extract oxygen from water; swim bladder controls buoyancy
• 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: Internal body processes, metabolic functions, or chemical mechanisms that enhance survival. These are internal processes you cannot see but affect how the organism functions.

Examples:

• CAM Photosynthesis: Desert plants (cacti, succulents) open stomata at night to absorb CO₂, minimizing water loss during hot days
• Antifreeze proteins: Arctic fish produce proteins that lower the freezing point of their blood, preventing ice crystal formation
• 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: Arctic animals have blood vessels arranged to conserve heat in extremities while maintaining core temperature
• 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
• Lactose tolerance: Some human populations evolved ability to digest milk sugar into adulthood
• 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 responses organisms perform to increase survival or reproductive success. These are things organisms do rather than physical traits they have. Can be instinctive (innate) or learned.

Examples:

• Migration: Birds fly south for winter; caribou move to seasonal grazing areas; salmon return to spawning grounds. Allows access to resources year-round.
• 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 vs. Learned: Some behavioral adaptations are innate (genetically programmed, present from birth - like spider web-building). Others are learned through experience (like chimpanzee tool use taught by parents).

The Process of Natural Selection

1. Variation: Genetic differences exist within populations (from mutations, sexual reproduction)
2. Inheritance: Traits are passed from parents to offspring through genes
3. Differential Survival: Some variations provide advantages in survival or reproduction (fitness)
4. Reproduction: Individuals with advantages reproduce more successfully
5. Frequency Change: Over generations, advantageous traits become more common in the population

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)

Where Primary Succession Occurs

• Newly formed volcanic islands (Hawaii, Surtsey)
• Bare rock exposed by retreating glaciers (glacial till)
• Cooling lava flows
• Sand dunes
• Areas after catastrophic volcanic eruptions (Mount St. Helens' blast zone)

Stages of Primary Succession

Stage 1: Pioneer Species

• First colonizers: Lichens (symbiosis of fungus and algae/cyanobacteria) and mosses
• Characteristics: Extremely hardy; can survive on bare rock with minimal nutrients; tolerate temperature extremes; require only sunlight, air, and water
• 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: Perennial plants, shrubs, fast-growing trees (often nitrogen-fixing species like alders)
• 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: Late Successional/Climax Community

• Species: Large, slow-growing trees (oaks, maples, beeches, hemlocks)
• Characteristics: Shade-tolerant, long-lived, slow-growing, produce few large seeds
• Result: Relatively stable, mature ecosystem with high biodiversity, complex structure, and maximum biomass
• Time to Reach: Can take hundreds to thousands of years for primary succession to reach climax

Key Point about Primary Succession:

Primary succession is SLOW because soil must be created from scratch. Each stage facilitates the next by improving environmental conditions (facilitation), but also makes conditions less favorable for itself, eventually being replaced.

Where Secondary Succession Occurs

• Abandoned agricultural fields (old-field succession)
• After forest fires that don't completely sterilize soil
• 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: Annual Weeds and Grasses (Years 1-2)

• Species: Crabgrass, ragweed, dandelions, clover
• Source: Seeds already in soil (seed bank) or wind-dispersed from nearby areas
• Characteristics: Fast-growing, complete life cycle in one year, produce abundant seeds

Stage 2: Perennial Grasses and Herbs (Years 3-10)

• 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 Softwood Trees (Years 10-40)

• 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: Hardwood Forest/Climax Community (Years 70+)

• Species: Oaks, maples, hickories, beeches (varies by region)
• Characteristics: Shade-tolerant, long-lived, slow-growing
• Result: Mature forest with high species diversity, complex structure, stable populations
• Self-Perpetuating: Climax species can reproduce in their own shade; their seedlings grow in forest understory

Key Point about Secondary Succession:

Secondary succession is FASTER than primary succession (decades instead of centuries) because soil already exists, along with seeds and roots. The area doesn't need to start from bare rock. Typical time to reach climax: 70-150 years in temperate regions.

✓ 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!)
• Climax species are shade-intolerant (FALSE!)