IB Biology SL: Theme B - Form & Function

B4.1 - Adaptation to Environment

How Organisms Survive and Thrive in Their Habitats

🌍 Introduction to Adaptation & Environment

Life on Earth exists in an astonishing variety of environments—from scorching deserts to frozen tundras, from deep ocean trenches to mountaintops. Each environment presents unique challenges: extreme temperatures, scarce water, intense sunlight, or complete darkness.

Adaptation is the key to survival. It refers to the inherited characteristics that increase an organism's ability to survive and reproduce in a particular environment. Through millions of years of natural selection, organisms have evolved remarkable adaptations—both structural and behavioral—that allow them to thrive where others cannot.

This topic explores how organisms are adapted to their habitats, how abiotic (non-living) factors shape species distribution, and how these principles apply across Earth's major biomes—from coral reefs to rainforests to deserts.

🏞️ Habitat: Where Life Happens

What is a Habitat?

A habitat is the natural place or environment in which a community, species, population, or organism lives. It provides everything needed for survival: food, water, shelter, and suitable conditions for reproduction.

Think of a habitat as an organism's "address"—it's the specific location where you would find that organism in nature. Every species has evolved to survive in a particular type of habitat.

🔍 Components of a Habitat

A habitat is defined by both physical features and biological factors:

Abiotic (Non-living) Features

Climate: Temperature range, precipitation patterns, humidity, wind
Geography: Altitude, latitude, landscape features (mountains, valleys, plains)
Soil/Substrate: Soil type, pH, mineral content, texture
Water: Availability, salinity, pH, clarity
Light: Intensity, duration (day length), wavelength
Air composition: Oxygen/CO₂ levels

Biotic (Living) Features

Food sources: Availability of prey, plants, nutrients
Predators: Organisms that hunt this species
Competitors: Other species competing for resources
Symbiotic partners: Species that live in close relationship
Vegetation: Plant communities present
Microorganisms: Bacteria, fungi, decomposers

🌱 Examples of Habitats

🌵 Desert

Example species: Cacti, camels, scorpions, fennec foxes
Key features: Very low rainfall (<30 cm/year), extreme temperatures, sandy/rocky soil

🌲 Tropical Rainforest

Example species: Monkeys, jaguars, tree frogs, orchids, bromeliads
Key features: High rainfall (200+ cm/year), consistently warm (25-27°C), high humidity, dense vegetation

🪸 Coral Reef

Example species: Coral polyps, clownfish, sea turtles, sharks
Key features: Warm shallow ocean (20-28°C), clear water, high light penetration, rich biodiversity

❄️ Tundra

Example species: Arctic fox, caribou, moss, lichens
Key features: Extremely cold (-40°C in winter), low precipitation, permafrost, short growing season

🌾 Grassland/Savanna

Example species: Zebras, lions, grasses, acacia trees
Key features: Moderate rainfall (25-75 cm/year), seasonal droughts, few trees, grazing animals

⚠️ Habitat Requirements are Species-Specific

Each species has specific habitat requirements that must be met for survival:

Polar bears need Arctic ice, cold temperatures, and access to seals
Cacti need hot, dry deserts with sandy soil and minimal water
Coral needs warm, clear, shallow ocean water with specific temperature and salinity
If habitat conditions change or are destroyed → species may migrate, adapt, or go extinct

🌍 Habitat Conservation is Critical:

Habitat loss (deforestation, urbanization, pollution) is the #1 threat to biodiversity. Protecting habitats means protecting species.

🌡️ Abiotic Factors: The Non-Living Environment

What are Abiotic Factors?

Abiotic factors are non-living physical and chemical components of the environment that affect living organisms. They shape the conditions of a habitat and determine which species can survive there.

Abiotic factors are contrasted with biotic factors (living components like predators, competitors, prey). Together, abiotic and biotic factors determine ecosystem structure and function.

📋 Major Abiotic Factors

1. Temperature

Effect: Influences enzyme activity, metabolic rate, growth, reproduction, and distribution

Examples: Tropical plants cannot survive freezing temperatures. Polar bears need cold climates. Coral reefs need 20-28°C water.

2. Water Availability

Effect: Essential for all life. Affects photosynthesis, cell turgor, biochemical reactions, and survival

Examples: Desert plants (xerophytes) have water-storage adaptations. Aquatic organisms require constant water immersion.

3. Light Intensity & Wavelength

Effect: Necessary for photosynthesis. Affects plant growth, flowering, and behavior (e.g., nocturnal vs. diurnal)

Examples: Rainforest floor plants adapted to low light. Deep-sea organisms live without sunlight. Coral reefs need shallow, clear water for light penetration.

4. Salinity (Salt Concentration)

Effect: Affects osmotic balance, water uptake, and survival in aquatic/marine environments

Examples: Freshwater fish cannot survive in seawater. Mangroves tolerate brackish (salty) water. Halophytes (salt-tolerant plants) thrive in saline soils.

5. pH (Acidity/Alkalinity)

Effect: Affects enzyme function, nutrient availability, and toxicity. Applies to soil and water

Examples: Blueberries prefer acidic soil (pH 4-5). Most fish thrive in pH 6-8. Ocean acidification threatens marine life.

6. Oxygen Concentration

Effect: Essential for aerobic respiration. Varies in aquatic environments (depends on temperature, turbulence, photosynthesis)

Examples: Cold, turbulent water has higher O₂. Stagnant, warm water has low O₂ (may cause fish kills). High-altitude organisms adapted to low O₂.

7. Humidity (Water Vapor in Air)

Effect: Affects transpiration rate in plants and water loss in animals

Examples: Rainforests have high humidity (reduces water stress). Deserts have low humidity (increases evaporation).

8. Wind Speed & Direction

Effect: Affects transpiration, seed dispersal, pollination, temperature regulation, erosion

Examples: Coastal plants adapted to strong winds. Mountain trees shaped by prevailing winds.

9. Turbidity (Water Clarity)

Effect: Cloudiness of water affects light penetration and photosynthesis in aquatic plants/algae

Examples: Coral reefs require clear water. Turbid water (sediment, algae) blocks light, reducing productivity.

10. Soil Composition & Nutrients

Effect: Soil texture (clay, sand, loam), nutrient content (N, P, K), and organic matter affect plant growth

Examples: Rainforest soil is nutrient-poor (nutrients in vegetation). Desert soil is sandy with low organic matter. Wetland soil is waterlogged.

📊 Abiotic Factors Affect Species Distribution

Range of Tolerance

Every species has a range of tolerance for each abiotic factor—a range of conditions within which it can survive and reproduce.

Key Components:

Optimum range: Conditions where species thrives best → highest population size, growth, reproduction
Zone of physiological stress: Conditions tolerable but not ideal → reduced growth, survival, reproduction
Zone of intolerance: Extreme conditions beyond tolerance → organism cannot survive
Critical minimum & maximum: Boundary points between survivable and lethal conditions

📈 Example: Temperature Tolerance in Fish

Optimum: 15-20°C (fish thrive, reproduce well)
Stress zones: 5-15°C and 20-30°C (survival but reduced activity)
Intolerance: <5°C or >30°C (fish die)

🗺️ Species Distribution Determined by Abiotic Factors:

Species are found only in areas where abiotic conditions fall within their range of tolerance for ALL limiting factors. If any single factor exceeds tolerance limits, the species cannot survive there—even if other conditions are ideal.

🔄 Limiting Factors

A limiting factor is an abiotic (or biotic) factor that restricts population size, distribution, or survival because it is in short supply or exceeds tolerance limits.

Examples:

Water is a limiting factor in deserts
Light is a limiting factor on rainforest floors
Temperature is a limiting factor in polar regions
Nutrients are limiting factors in open ocean

🪸 Coral Reef Formation: Living Architecture

What are Coral Reefs?

Coral reefs are massive underwater structures formed from the calcium carbonate skeletons of coral polyps accumulated over thousands of years. They're often called the "rainforests of the sea" due to incredible biodiversity.

Despite covering <1% of ocean floor, coral reefs support ~25% of all marine species! They provide food, shelter, breeding grounds, and protection for countless organisms.

🏗️ How Coral Reefs Form

The Coral Polyp: Reef Builder

At the heart of reef formation is the coral polyp—a tiny animal (typically 1-3 mm) related to jellyfish and sea anemones. Each polyp has a soft, cylindrical body with tentacles for catching plankton.

Formation Process:

Larval Settlement: Free-swimming coral larvae (planulae) settle on hard substrate (rock, dead coral)
Skeleton Secretion: Polyp secretes calcium carbonate (CaCO₃) skeleton around its base—forms protective cup
Asexual Reproduction: Polyp divides repeatedly, creating thousands of genetically identical clones (colony)
Colonial Growth: Each polyp adds to skeleton, colony grows larger
Accumulation: Over centuries/millennia, skeletons accumulate, creating reef structure
Reef Expansion: New corals grow on old skeletons, reef expands upward and outward

⚡ The Secret to Reef Growth: Zooxanthellae Symbiosis

The key to rapid reef formation is a mutualistic symbiotic relationship between coral polyps and microscopic algae called zooxanthellae (a type of dinoflagellate).

How the Symbiosis Works:

Coral Provides →

Protected environment
CO₂ (for photosynthesis)
Nutrients (N, P from waste)
Access to sunlight

Zooxanthellae Provides →

Organic compounds (glucose)
Up to 90% of coral's energy
Oxygen (byproduct)
Helps with calcification

🔑 Result:

This relationship provides corals with abundant energy, enabling rapid growth and calcium carbonate production. It's why reef-building corals grow 10-100× faster than non-symbiotic corals!

🌡️ Conditions Required for Coral Reef Formation

Coral reefs have very specific abiotic requirements. They only form where ALL these conditions are met:

1. Warm Water Temperature

Requirement: 20-28°C (68-82°F)

Why: Corals and zooxanthellae are adapted to warm temperatures. Too cold → slowed metabolism, reduced calcification. Found primarily in tropical/subtropical regions (between 30°N and 30°S latitude).

2. Clear, Clean Water (Low Turbidity)

Requirement: High water clarity, low sediment

Why: Zooxanthellae need sunlight for photosynthesis. Turbid (cloudy) water blocks light. Sediment can smother coral polyps. Reefs absent near river mouths with high sediment load.

3. Shallow Water Depth

Requirement: Usually <30-50 meters deep

Why: Light intensity decreases with depth. Below ~50m, insufficient light for zooxanthellae photosynthesis. Shallow water ensures adequate sunlight for symbiosis.

4. Stable Salinity

Requirement: Normal seawater salinity (32-42 parts per thousand)

Why: Corals are sensitive to salinity changes. Freshwater input (rivers, heavy rain) can kill corals. Reefs absent near river mouths and in areas with variable salinity.

5. High Oxygen Concentration

Requirement: Well-oxygenated water

Why: Corals need oxygen for respiration. Stagnant water with low O₂ is unsuitable. Wave action and currents help maintain high O₂ levels.

6. Hard Substrate for Attachment

Requirement: Solid surface (rock, dead coral, artificial structures)

Why: Coral larvae must attach to hard substrate. Cannot settle on soft sediment (sand, mud). Once established, reef itself provides substrate for new corals.

7. Low Nutrient Concentration

Requirement: Oligotrophic (nutrient-poor) waters

Why: Seems counterintuitive! High nutrients → algal blooms → block sunlight → smother corals. Coral-zooxanthellae symbiosis is efficient in nutrient-poor environments. Nutrient pollution (runoff) damages reefs.

⚠️ Coral Bleaching: When Symbiosis Breaks Down

Coral bleaching occurs when corals expel their zooxanthellae in response to stress (usually elevated temperature).

What Happens:

Stress triggers coral to expel zooxanthellae
Coral loses color (appears white/"bleached") → zooxanthellae give corals their color
Coral loses 90% of energy source
If stress continues → coral starves and dies
If conditions improve quickly → coral can recover, reacquire zooxanthellae

🔥 Main Cause:

Ocean warming due to climate change. Even 1-2°C above normal can trigger mass bleaching events. Other stressors: pollution, ocean acidification, disease, extreme low tides.

🌍 Biomes: Earth's Major Ecosystems

What is a Biome?

A biome is a large geographical area characterized by specific climate conditions (temperature and precipitation patterns) and distinctive communities of plants and animals adapted to those conditions.

Biomes consist of many ecosystems that share similar abiotic conditions and therefore contain similar types of organisms—even if they're located on different continents!

🌡️ Abiotic Factors as Determinants of Biome Distribution

Two PRIMARY abiotic factors determine which biome occurs in a given location:

1. Temperature

Determined by: Latitude (distance from equator), altitude, ocean currents, seasonal variation

Tropical → Temperate → Polar
(Hot → Moderate → Cold)

2. Precipitation

Determined by: Atmospheric circulation, proximity to water bodies, mountain ranges, prevailing winds

Rainforest → Grassland → Desert
(High → Moderate → Low rainfall)

🔑 Key Principle:

Biomes with similar temperature and precipitation patterns have similar vegetation and animal adaptations, even if they're on opposite sides of the planet! This is called convergent evolution—different species independently evolve similar adaptations to similar environmental challenges.

🗺️ Major Terrestrial Biomes

🌴 1. Tropical Rainforest

Climate:

Temperature: Warm year-round (25-27°C), no seasons
Precipitation: Very high (200-400 cm/year), evenly distributed
Location: Near equator (Central/South America, Africa, Southeast Asia)

Characteristics:

Highest biodiversity of any biome (>50% of world's species)
Dense, multi-layered vegetation (canopy, understory, forest floor)
Nutrient-poor soil (nutrients in vegetation, not soil)
High productivity due to constant warmth, moisture, sunlight

🌵 2. Hot Desert

Climate:

Temperature: Very hot days (40-50°C), cold nights (can drop below 0°C)
Precipitation: Very low (<25-30 cm/year), unpredictable
Location: 20-30° N and S of equator (Sahara, Arabian, Australian deserts)

Characteristics:

Sparse vegetation—widely spaced drought-adapted plants (xerophytes)
Sandy or rocky soil with low organic matter
Organisms adapted to water scarcity and temperature extremes
Low biodiversity but specialized species

🌾 3. Temperate Grassland

Climate:

Temperature: Hot summers, cold winters, distinct seasons
Precipitation: Moderate (25-75 cm/year), seasonal droughts
Location: Interior continents (North American prairies, Eurasian steppes, South American pampas)

Characteristics:

Dominated by grasses, few trees (insufficient water)
Fertile soil (rich in organic matter from decomposed grasses)
Large grazing animals (bison, antelope) and predators
Fire-adapted vegetation

🍂 4. Temperate Deciduous Forest

Climate:

Temperature: Four distinct seasons (warm summer, cold winter)
Precipitation: Moderate-high (75-150 cm/year), evenly distributed
Location: Eastern North America, Europe, East Asia

Characteristics:

Deciduous trees (lose leaves in fall)—oaks, maples, beeches
Layered vegetation (canopy, understory, shrubs, herbs)
Fertile soil with leaf litter
Animals show seasonal adaptations (migration, hibernation)

🌲 5. Taiga (Boreal Forest)

Climate:

Temperature: Long, cold winters (-40°C); short, mild summers
Precipitation: Low-moderate (40-100 cm/year), mostly snow
Location: Northern hemisphere (Canada, Russia, Scandinavia)

Characteristics:

Dominated by coniferous trees (pine, spruce, fir)—needle-like leaves
Acidic, nutrient-poor soil
Short growing season (~3-4 months)
Low diversity but high abundance

❄️ 6. Tundra

Climate:

Temperature: Extremely cold (-30 to -50°C winter); cool summer (3-12°C)
Precipitation: Very low (15-25 cm/year), mostly snow
Location: Arctic regions, high mountains

Characteristics:

No trees—low-growing plants (grasses, mosses, lichens, dwarf shrubs)
Permafrost (permanently frozen ground) beneath surface
Very short growing season (~50-60 days)
Lowest biodiversity of terrestrial biomes

🦎 Adaptations to Life in Deserts & Rainforests

Deserts and tropical rainforests represent opposite extremes of the environmental spectrum—one is hot and DRY, the other is hot and WET. Each presents unique challenges, and organisms have evolved remarkable adaptations to survive and thrive in these extreme conditions.

🌵 ADAPTATIONS TO HOT DESERTS

🌱 Desert Plant Adaptations (Xerophytes)

Xerophytes are plants adapted to survive in extremely dry conditions. Main challenges: minimize water loss, maximize water storage, obtain water efficiently.

1. Reduced Leaves or Spines

Adaptation: Leaves reduced to spines (cacti) or very small leaves

Benefit: Reduces surface area → minimizes transpiration water loss. Spines also deter herbivores and provide shade.

Example: Cacti have no leaves—photosynthesis occurs in green stems

2. Thick, Waxy Cuticle

Adaptation: Thick waxy coating on stems/leaves

Benefit: Waterproof barrier reduces evaporation from plant surface. Reflects sunlight (reduces heat absorption).

Example: Aloe vera has thick cuticle on succulent leaves

3. Water Storage in Succulent Tissues

Adaptation: Thick, fleshy stems or leaves store water

Benefit: Stores water during rare rainfall for use during drought. Can survive months without water.

Example: Cacti can store hundreds of liters of water; stems expand accordion-style

4. Extensive/Deep Root Systems

Two strategies:

Shallow, wide roots: Spread horizontally near surface to quickly absorb rare rainfall over large area (cacti)
Deep taproots: Penetrate deep underground (10-30+ meters) to reach groundwater (mesquite, acacia)

5. CAM Photosynthesis

Adaptation: Crassulacean Acid Metabolism—alternative photosynthesis pathway

How it works: Stomata open at NIGHT (when cooler/less evaporation) to take in CO₂. CO₂ stored as organic acids. During DAY, stomata close, CO₂ released internally for photosynthesis.

Benefit: Minimizes water loss—stomata closed during hot day. Can reduce water loss by 90% compared to normal plants!

6. Sunken Stomata & Leaf Rolling

Adaptation: Stomata in pits/grooves, leaves roll up

Benefit: Traps humid air near stomata, reduces water vapor gradient → reduces transpiration. Rolling creates protected microenvironment.

7. Ephemeral Life Cycle (Drought Avoidance)

Adaptation: Some desert plants complete entire life cycle in weeks after rainfall

Benefit: Seeds remain dormant for months/years. When rare rain comes, rapidly germinate, grow, flower, produce seeds, die—all in 6-8 weeks. Avoids drought by "sitting it out" as seeds.

🦎 Desert Animal Adaptations

1. Nocturnal Behavior

Active at night when temperatures cooler. Avoid hottest part of day. Examples: most desert rodents, snakes, scorpions, some insects.

2. Efficient Water Conservation

Concentrated urine, dry feces minimize water loss. Some animals (kangaroo rat) never drink—get all water from food metabolism!

3. Burrowing

Live underground during day. Burrows maintain cooler, more humid microclimate. Examples: desert tortoises, meerkats, rodents.

4. Large Ears (Heat Dissipation)

Large surface area with many blood vessels radiates excess heat. Examples: fennec fox (huge ears), jackrabbits.

5. Light-Colored Fur/Scales

Reflects sunlight, reduces heat absorption. Also provides camouflage against sandy background.

6. Behavioral Thermoregulation

Basking in sun when cold (morning), seeking shade when hot (midday). Lizards precisely control body temperature through behavior.

7. Water Storage (Camels)

Camel's hump stores fat (not water!), which can be metabolized to produce water. Can tolerate severe dehydration (25% body weight loss). Can drink 100+ liters at once to rehydrate.

🌴 ADAPTATIONS TO TROPICAL RAINFORESTS

🌿 Rainforest Plant Adaptations

Main challenges: intense competition for light, heavy rainfall, high humidity, nutrient-poor soil, climbing to canopy.

1. Drip Tips (Pointed Leaf Tips)

Adaptation: Leaves have elongated, pointed tips that channel water

Benefit: Quickly sheds heavy rainwater off leaf surface. Prevents water accumulation → reduces weight, prevents mold/bacterial growth, allows gas exchange. Essential in environment with daily rain.

2. Broad, Thin Leaves

Adaptation: Large surface area, thin blade

Benefit: Maximizes light capture in shaded understory. No need to conserve water (abundant!), so large leaves possible. Allows maximum photosynthesis despite low light levels.

3. Buttress Roots

Adaptation: Large, wing-like roots extending from trunk base

Benefit: Provides stability for tall trees in shallow soil. Spreads weight distribution. Increases surface area for nutrient absorption. Characteristic of emergent layer trees.

4. Epiphytes (Plants Growing on Other Plants)

Adaptation: Grow on branches/trunks of taller trees (not parasitic—don't harm host)

Benefit: Access to sunlight in canopy without growing from ground. Absorb water/nutrients from rain, air, and decaying organic matter. Examples: orchids, bromeliads, ferns, mosses.

5. Lianas (Woody Vines)

Adaptation: Climb using other trees for support, reach canopy without thick trunk

Benefit: Reaches sunlight efficiently without investing energy in structural support. Flexible stems can withstand tree swaying. Roots remain in soil for water/nutrients.

6. Smooth, Thin Bark

Adaptation: Thin bark, often smooth

Benefit: No need for thick protective bark (no cold winters, no water stress). Smooth surface sheds water easily, discourages epiphyte growth that would compete for light.

7. Shallow Root Systems

Adaptation: Roots spread horizontally near surface

Benefit: Most nutrients concentrated in thin topsoil layer (from decomposition). Deep roots unnecessary—water abundant at all depths. Quick nutrient uptake from leaf litter.

8. Cauliflory (Flowers/Fruit on Trunk)

Adaptation: Flowers and fruits grow directly from trunk or large branches

Benefit: Makes flowers/fruits accessible to ground-dwelling pollinators/seed dispersers in shaded understory. Example: cacao trees.

🦜 Rainforest Animal Adaptations

1. Camouflage

Coloration and patterns blend with environment (leaves, bark, forest floor). Helps both prey (avoid predators) and predators (ambush prey). Examples: leaf insects, stick insects, green tree pythons, jaguars (spotted for dappled light).

2. Arboreal Lifestyle & Climbing Adaptations

Many animals live in trees to access food/avoid predators. Adaptations: prehensile tails (monkeys), strong claws (sloths), opposable thumbs (primates), gripping pads (tree frogs), gliding membranes (flying squirrels).

3. Bright Warning Coloration (Aposematism)

Poisonous/toxic animals display bright colors (red, yellow, blue) to warn predators "don't eat me!" Examples: poison dart frogs (highly toxic skin), coral snakes.

4. Specialized Diets

Niche specialization reduces competition. Examples: hummingbirds (nectar), toucans (fruit), anteaters (termites/ants), howler monkeys (leaves—rare for primates).

5. Loud Vocalizations

Dense vegetation blocks sight—sound travels well. Animals use calls for communication (territory, mating, warning). Examples: howler monkeys (loudest land animal), parrots, toucans.

6. Nocturnality

Many animals active at night to avoid daytime heat, competition, and predators. Examples: jaguars, ocelots, many frogs and insects.