IB Biology SL: Theme B - Form & Function

B4.2 - Ecological Niches

Understanding Species Roles, Nutrition, and Resource Use

🎯 Introduction to Ecological Niches

In any ecosystem, every species plays a specific role—like actors in a complex theatrical production. This role encompasses not just where an organism lives (its habitat), but how it lives: what it eats, when it's active, how it interacts with other species, and how it responds to environmental conditions.

This multidimensional role is called an organism's ecological niche. Understanding niches helps us explain why certain species can coexist while others cannot, how competition shapes communities, and how organisms obtain energy to survive.

In this topic, we'll explore the concept of niches, the difference between fundamental and realized niches, the various modes of nutrition organisms use to obtain energy, adaptations for feeding, and the critical process of photosynthesis that powers most life on Earth.

🌿 The Ecological Niche: More Than Just a Home

What is an Ecological Niche?

An ecological niche is the complete role that a species plays in its ecosystem—encompassing all its interactions with both biotic (living) and abiotic (non-living) components of the environment.

Think of a habitat as an organism's address (where it lives), while its niche is its profession (what it does for a living and how it affects the environment).

📋 Components of an Ecological Niche

A species' niche includes multiple dimensions:

1. Resource Use (What it consumes)

Food sources: Type of prey, plants, or nutrients consumed
Water sources: Where and how water is obtained
Territory/space: Physical area utilized for feeding, breeding, shelter
Example: Hawks hunt small mammals in open fields during daytime

2. Temporal Activity (When it's active)

Diurnal: Active during day (most birds, butterflies)
Nocturnal: Active at night (owls, bats, many rodents)
Crepuscular: Active at dawn/dusk (deer, rabbits)
Seasonal patterns: Migration, hibernation, dormancy

3. Physical Location (Where in habitat)

Microhabitat: Specific location within broader habitat
Vertical stratification: Canopy, understory, forest floor, underground
Example: Different warbler species feed at different heights in same tree

4. Biotic Interactions

Predator-prey relationships: What it hunts; what hunts it
Competition: Other species competing for same resources
Symbiosis: Mutualism, commensalism, parasitism
Pollination/seed dispersal: Services provided to plants

5. Abiotic Tolerances

Temperature range: Optimal and tolerable temperatures
Humidity/moisture needs
pH tolerance (soil or water)
Light requirements (for plants)
Salinity tolerance

6. Reproduction and Life History

Breeding requirements: Nesting sites, mating behavior
Parental care strategies
Dispersal mechanisms

🔑 The Competitive Exclusion Principle

Gause's Principle: "No two species can occupy exactly the same niche in the same place at the same time."

If two species have identical niches and compete for exactly the same resources, one will always outcompete the other, eventually leading to the competitive exclusion (local extinction) of the inferior competitor.

Implication: Every species must have a unique niche to coexist. Even species that seem similar have subtle differences in their resource use, timing, or location that allow them to avoid direct competition.

⚖️ Fundamental vs. Realized Niche

Understanding the Difference

While we talk about "a species' niche," there are actually TWO concepts: the potential niche (fundamental) and the actual niche (realized). The difference between them reveals the powerful effect of competition and other biotic interactions.

📊 The Two Types of Niches

🌐 Fundamental Niche

Definition:

The complete potential range of environmental conditions and resources a species could occupy and use based solely on its physiological adaptations and tolerance limits—in the absence of competition, predation, or other biotic interactions.

Key Characteristics:

Theoretical/Ideal: What's possible without interference
Broader: Larger than realized niche
Based on: Physiology, adaptations, abiotic tolerances
Ignores: Competition, predation, disease

🔬 Example:

A species of barnacle can potentially live in both upper and lower intertidal zones (temperature, salinity tolerance).

📍 Realized Niche

Definition:

The actual range of environmental conditions and resources a species does occupy and use in nature—after accounting for competition, predation, and other biotic interactions.

Key Characteristics:

Real-world/Actual: What we observe in nature
Smaller: Subset of fundamental niche
Based on: Competition, predation, other species interactions
Includes: All limiting biotic factors

🔬 Example:

The same barnacle is actually found only in upper intertidal zone because a competing species dominates lower zone.

🔑 Key Relationship:

Realized Niche ⊆ Fundamental Niche

The realized niche is always a subset (part of) the fundamental niche. It's never larger, because biotic interactions restrict species distribution—they don't expand it beyond physiological limits.

🔬 Classic Experiment: Connell's Barnacles

Joseph Connell's 1961 experiment with two barnacle species beautifully demonstrates the fundamental vs. realized niche concept:

🔹 Species Involved:

Chthamalus stellatus: Found in upper intertidal zone (harsh conditions—more exposure, desiccation)
Balanus balanoides: Found in lower intertidal zone (less harsh—more submersion, less drying)

🔹 Experiment:

Connell removed Balanus from lower zone and transplanted Chthamalus to lower zone (removing competition).

🔹 Result:

Chthamalus successfully colonized and survived in lower zone! This showed that Chthamalus was physiologically capable of living there (fundamental niche includes both zones).

🔹 Conclusion:

Fundamental niche of Chthamalus: Both upper and lower zones (can tolerate both)
Realized niche of Chthamalus: Only upper zone (competitively excluded from lower zone by Balanus, which grows faster and smothers/crushes it)

💡 Take-Home Message:

Competition and other biotic interactions restrict species to smaller portions of their potential niche. Where we find species in nature (realized niche) doesn't necessarily reflect their full physiological capabilities (fundamental niche).

🔄 Niche Partitioning (Resource Partitioning)

Niche partitioning is a process by which competing species use the environment differently to reduce competition. Each species exploits a different part of the available resources.

Examples of Resource Partitioning:

Spatial partitioning: Different warbler species feed at different heights in the same tree (canopy, mid-level, lower branches)
Temporal partitioning: Owls hunt at night, hawks hunt during day—both eat rodents, but at different times
Food partitioning: Different finch species on Galápagos have different beak sizes → eat different seed sizes
Behavioral partitioning: Different hunting strategies (ambush vs. chase)

Result: Multiple species can coexist in the same habitat by occupying slightly different realized niches, even if their fundamental niches overlap significantly!

🍃 Types of Nutrition: How Organisms Obtain Energy

Nutrition: The Foundation of Life

Nutrition refers to the process by which organisms obtain and process energy and nutrients necessary for survival, growth, and reproduction. All life requires energy (usually stored in chemical bonds of organic molecules) to power cellular processes.

The MODE of nutrition is a fundamental aspect of an organism's niche—it determines what resources it needs, where it lives, and how it interacts with other species.

🌍 The Two Major Modes of Nutrition

🌱 AUTOTROPHIC Nutrition

Definition:

Auto = "self" | Troph = "feeding"
Organisms that synthesize their own organic molecules (food) from simple inorganic substances.

Role in Ecosystem:

PRODUCERS (Primary producers)
Foundation of food chains/webs

Two Types:

1. Photoautotrophs (Most common)

Energy source: Light (sunlight)
Process: Photosynthesis
Examples: Green plants, algae, cyanobacteria
Convert light energy → chemical energy (glucose)

2. Chemoautotrophs

Energy source: Chemical reactions (oxidation of inorganic compounds)
Process: Chemosynthesis
Examples: Some bacteria (deep-sea hydrothermal vents, sulfur bacteria)
Oxidize H₂S, NH₃, or Fe²⁺ for energy

✓ Key Feature:

SELF-SUFFICIENT
Don't depend on other organisms for organic molecules

🦁 HETEROTROPHIC Nutrition

Definition:

Hetero = "other" | Troph = "feeding"
Organisms that obtain organic molecules (food) by consuming other organisms or organic matter.

Role in Ecosystem:

CONSUMERS (Primary, secondary, tertiary)
Depend on producers for energy

Types of Heterotrophs:

1. Herbivores (Plant-eaters)

Feed on plants/algae. Examples: Cows, deer, grasshoppers, caterpillars

2. Carnivores (Meat-eaters)

Feed on other animals. Examples: Lions, wolves, hawks, snakes

3. Omnivores (Both plants & animals)

Feed on both. Examples: Bears, humans, pigs, raccoons

4. Detritivores (Feed on dead matter)

Feed on dead organic material. Examples: Earthworms, millipedes, dung beetles

5. Decomposers (Break down dead matter)

Secrete enzymes to digest externally. Examples: Fungi, bacteria

6. Parasites (Feed on living host)

Live on/in host, obtain nutrients. Examples: Tapeworms, ticks, mistletoe

✓ Key Feature:

DEPENDENT
Must consume pre-made organic molecules from other organisms

⚖️ Comparison Summary:

Feature	Autotrophs	Heterotrophs
Food Source	Self-synthesized from inorganic substances	Other organisms/organic matter
Energy Source	Light (photo-) or chemicals (chemo-)	Organic molecules (stored energy)
Carbon Source	CO₂ (inorganic carbon)	Organic compounds
Trophic Level	Producers (1st level)	Consumers (2nd, 3rd+ levels)
Examples	Plants, algae, cyanobacteria	Animals, fungi, most bacteria
Independence	Independent (self-sufficient)	Dependent on other organisms

🔄 The Energy Flow Connection

The relationship between autotrophs and heterotrophs forms the basis of energy flow through ecosystems:

Sun → Autotrophs (Producers) → Herbivores → Carnivores → Decomposers

Autotrophs capture energy from the environment (sun or chemicals) and convert it into organic molecules. Heterotrophs consume these molecules, transferring energy up the food chain. Ultimately, decomposers return nutrients to the environment, completing the cycle.

🦷 Adaptations for Feeding: Structure Reflects Function

Form Follows Function

An organism's feeding adaptations are specialized physical and behavioral features that enable it to obtain and process food efficiently. These adaptations are clear examples of how structure reflects function—the physical characteristics match the dietary needs.

Feeding adaptations are key components of an organism's ecological niche, determining what it eats and where/how it obtains food.

🌿 HERBIVORE Adaptations

Challenge: Plant material is tough (cellulose cell walls), low in nutrients, difficult to digest

1. Teeth Adaptations

Broad, flat molars: Large surface area for grinding tough plant material (leaves, grass)
Chisel-like incisors: Sharp front teeth for cutting/cropping vegetation
Large premolars: Additional grinding surface
No/reduced canines: Don't need sharp teeth for tearing meat
Continuous growth: Teeth grow throughout life (counteracts wear from grinding)
Example: Cow teeth—no upper incisors (uses tough pad), massive grinding molars

2. Digestive System Adaptations

Long digestive tract: Provides extended time for breaking down cellulose (plant fiber). Herbivore intestines can be 10-12× body length!
Multiple stomach chambers (ruminants): Cows, sheep, deer have 4-chambered stomach for extensive fermentation and breakdown
Symbiotic bacteria: Gut contains bacteria/protists that produce cellulase enzyme to digest cellulose (mammals cannot produce this enzyme!)
Regurgitation & rechewing (cud): Ruminants regurgitate partially digested food, chew again, re-swallow for further processing
Cecum & appendix: Enlarged fermentation chambers for bacterial breakdown of cellulose

3. Behavioral Adaptations

Grazing/browsing: Spend much of day feeding (plants = low energy density)
Selective feeding: Choose most nutritious plant parts (young leaves, buds)
Herd behavior: Safety in numbers (more eyes watching for predators)

4. Other Physical Adaptations

Strong jaw muscles: For continuous chewing
Long necks (giraffes): Reach high vegetation
Specialized mouthparts (insects): Caterpillars have mandibles for chewing leaves

🦁 CARNIVORE Adaptations

Challenge: Must catch, kill, and consume prey; meat is easy to digest but prey fights back/runs

1. Teeth Adaptations

Long, sharp canines: For stabbing, gripping, and killing prey
Sharp, pointed incisors: For tearing flesh from bones
Blade-like carnassial teeth: Specialized molars/premolars that act like scissors to shear meat and crush bones
No flat grinding surfaces: Don't need to grind—meat is pre-digested by prey!
Example: Lion teeth—4 cm canines, scissor-like carnassials

2. Digestive System Adaptations

Short digestive tract: Meat is easy to digest (proteins, fats readily broken down). Carnivore intestines only 3-6× body length
Simple stomach: Single chamber—no need for fermentation
Strong stomach acid: Low pH (~1-2) kills bacteria in decaying meat, breaks down proteins
Powerful digestive enzymes: Pepsin (protein digestion), lipase (fat digestion)
No cellulose digestion: Cannot digest plant material efficiently

3. Hunting/Predation Adaptations

Sharp claws/talons: For catching, gripping, and killing prey (cats retract claws to keep sharp)
Keen senses: Excellent vision (forward-facing eyes for depth perception), hearing, smell for detecting prey
Speed/agility: Fast running (cheetah), powerful flight (hawks)
Stealth/camouflage: Stalking ability, coat patterns for hiding
Powerful jaws: Strong bite force to crush bones, hold struggling prey

4. Behavioral Adaptations

Hunting strategies: Ambush (crocodiles), pursuit (cheetahs), pack hunting (wolves)
Territorial behavior: Defend hunting grounds
Intermittent feeding: Can go days between meals (unlike herbivores that must feed constantly)

📊 Comparison: Herbivore vs. Carnivore Adaptations

Feature	Herbivores	Carnivores
Teeth	Broad flat molars (grinding), chisel incisors	Sharp canines (killing), carnassials (shearing)
Digestive Tract	LONG (10-12× body length)	SHORT (3-6× body length)
Stomach	Multiple chambers (ruminants); simple but large	Simple, single chamber; very acidic
Cellulose Digestion	YES (via symbiotic bacteria)	NO (cannot digest plant fiber)
Claws	Blunt hooves or small nails	Sharp, retractable claws
Eyes Position	Sides of head (wide field of view for predator detection)	Front of head (binocular vision for depth perception)
Feeding Frequency	Almost constant (low energy density)	Intermittent (high energy density)
Energy Source Ease	Easy to find, hard to digest	Hard to catch, easy to digest

☀️ Photosynthesis: Powering Life on Earth

The Most Important Biological Process

Photosynthesis is the process by which photoautotrophs (green plants, algae, cyanobacteria) convert light energy into chemical energy stored in glucose, using carbon dioxide and water as raw materials.

Photosynthesis is arguably the most important process on Earth—it provides the organic molecules and oxygen that sustain nearly all life, removes CO₂ from the atmosphere, and forms the base of virtually all food chains.

⚗️ The Overall Equation

Word Equation:

Carbon Dioxide + Water + Light Energy →
Glucose + Oxygen

Balanced Chemical Equation:

6CO₂ + 6H₂O + Light Energy → C₆H₁₂O₆ + 6O₂

(in presence of chlorophyll)

📍 Key Points About the Equation:

Inputs (Reactants): 6 molecules of CO₂, 6 molecules of H₂O, light energy
Outputs (Products): 1 molecule of glucose (C₆H₁₂O₆), 6 molecules of O₂
Energy conversion: Light energy → Chemical energy (stored in glucose bonds)
Endergonic reaction: Requires energy input (light) to proceed
Opposite of respiration: Photosynthesis builds glucose; respiration breaks it down

🏭 Where Photosynthesis Occurs

Location: Chloroplasts

Photosynthesis occurs in specialized organelles called chloroplasts, found primarily in leaf mesophyll cells. Each chloroplast contains:

1. Chlorophyll (The Light-Absorbing Pigment)

Green pigment that absorbs light energy (mainly red and blue wavelengths, reflects green). Located in thylakoid membranes.

2. Thylakoids (Membrane Stacks)

Flattened membrane sacs stacked into grana. Site of light-dependent reactions (light reactions).

3. Stroma (Fluid Matrix)

Fluid-filled space surrounding thylakoids. Site of light-independent reactions (Calvin cycle).

⚡ The Two Stages of Photosynthesis

Photosynthesis occurs in TWO linked stages: Light-Dependent Reactions and Light-Independent Reactions

☀️ STAGE 1: Light-Dependent Reactions (Light Reactions)

Location:

Thylakoid membranes (inside chloroplasts)

Requirement:

LIGHT (only occurs in presence of light)

What Happens:

Step 1: Light Absorption

Chlorophyll molecules in thylakoid membrane absorb light energy → electrons in chlorophyll become excited (raised to higher energy level)

Step 2: Water Splitting (Photolysis)

Light energy splits water molecules:
2H₂O → 4H⁺ + 4e^- + O₂
Oxygen released as byproduct (the O₂ we breathe!)

Step 3: Electron Transport Chain

Excited electrons pass through series of carrier proteins in thylakoid membrane, releasing energy at each step

Step 4: ATP Production (Photophosphorylation)

Energy from electrons pumps H⁺ ions across membrane → creates proton gradient → H⁺ flows back through ATP synthase enzyme → produces ATP (energy currency)

Step 5: NADPH Production

At end of electron transport chain, electrons combine with H⁺ ions and NADP⁺ to form NADPH (electron carrier)

📌 Summary of Light Reactions:

INPUTS: Light, H₂O, ADP, NADP⁺
↓
OUTPUTS: O₂, ATP, NADPH

🌙 STAGE 2: Light-Independent Reactions (Calvin Cycle)

Location:

Stroma (fluid space in chloroplast)

Requirement:

NO LIGHT NEEDED (but requires products from light reactions: ATP and NADPH)

What Happens (Simplified):

Step 1: Carbon Fixation

CO₂ from atmosphere enters leaf through stomata → attaches to 5-carbon molecule (RuBP) → catalyzed by enzyme RuBisCO → forms unstable 6-carbon compound that immediately splits into two 3-carbon molecules (3-PGA)

Step 2: Reduction

ATP and NADPH (from light reactions) provide energy and electrons to convert 3-PGA into G3P (glyceraldehyde-3-phosphate, a 3-carbon sugar)

Step 3: Sugar Formation

Some G3P molecules exit cycle → two G3P molecules combine to form glucose (C₆H₁₂O₆)

Step 4: Regeneration

Remaining G3P molecules used (with more ATP) to regenerate RuBP → cycle continues

📌 Summary of Calvin Cycle:

INPUTS: CO₂, ATP, NADPH
↓
OUTPUTS: Glucose (C₆H₁₂O₆), ADP, NADP⁺

🌿 Adaptations of Plant Form for Harvesting Light

Plants have evolved numerous adaptations to maximize light capture for photosynthesis—a critical component of their ecological niche:

1. Broad, Flat Leaves

Large surface area maximizes light interception. Flat shape spreads photosynthetic tissue for optimal absorption.

2. Leaf Arrangement (Phyllotaxy)

Alternate, opposite, or spiral patterns minimize leaf overlap, reducing self-shading. Each leaf gets maximum light exposure.

3. Chloroplast-Rich Palisade Layer

Column-shaped palisade cells packed with chloroplasts positioned just below upper leaf surface—first to receive incoming light.

4. Thin Leaves

Minimal thickness allows light to penetrate to lower cell layers. Reduces diffusion distance for CO₂.

5. Transparent Epidermis

No chloroplasts in upper epidermis—allows light to pass through to photosynthetic mesophyll below without absorption by outer layer.

6. Multiple Pigments

Chlorophyll a & b, carotenoids—absorb different wavelengths of light, maximizing total light energy captured across spectrum.

7. Sun Tracking (Heliotropism)

Some plants orient leaves to face sun throughout day, maximizing perpendicular light angle for optimal absorption.

8. Shade Tolerance Adaptations

Shade plants: Larger, thinner leaves; higher chlorophyll content; lower light compensation point—adapted to low-light understory.

🌍 Global Importance of Photosynthesis

Produces oxygen: Nearly all atmospheric O₂ comes from photosynthesis (21% of atmosphere)
Foundation of food webs: All heterotrophs ultimately depend on glucose produced by autotrophs
Removes CO₂: Helps regulate atmospheric carbon dioxide levels, mitigating climate change
Energy storage: Fossil fuels (coal, oil, gas) are ancient photosynthetic products
Supports ~99% of life: Virtually all ecosystems (except deep-sea hydrothermal vents) depend on photosynthesis

🎯 Key Concepts Summary

✓ Ecological Niche

The complete role of a species in its ecosystem, including resources used, activity patterns, location, biotic interactions, and abiotic tolerances. No two species occupy identical niches (competitive exclusion principle).

✓ Fundamental vs. Realized Niche

Fundamental niche = potential range based on adaptations and tolerances (without competition). Realized niche = actual range occupied (smaller, restricted by competition, predation, other biotic factors). Realized ⊆ Fundamental.

✓ Types of Nutrition

Autotrophs (producers) synthesize their own food from inorganic substances using light (photoautotrophs) or chemicals (chemoautotrophs). Heterotrophs (consumers) obtain organic molecules by consuming other organisms—herbivores, carnivores, omnivores, detritivores, decomposers, parasites.

✓ Feeding Adaptations

Herbivores have flat grinding teeth, long digestive tracts, symbiotic cellulose-digesting bacteria. Carnivores have sharp canines and carnassials, short digestive tracts, strong stomach acid, hunting adaptations (claws, keen senses, speed). Structure reflects function.

✓ Photosynthesis

The process where autotrophs convert light energy into chemical energy (glucose). It has two stages: light-dependent reactions (in thylakoids) and light-independent reactions (in the stroma).

✓ Abiotic Factors and Adaptations

Abiotic factors (temperature, water, light) determine biome distribution. Organisms in extreme environments like deserts (xerophytes, nocturnal animals) and rainforests (drip tips, epiphytes, camouflage) show specialized adaptations to survive.

📚 About the Author

Adam

Co-Founder @RevisionTown

Adam is a dedicated mathematics and science educator with extensive experience in international curricula. As Co-Founder of RevisionTown, he is committed to creating comprehensive, high-quality educational resources that help students excel in their studies.

His expertise spans multiple examination systems including IB (International Baccalaureate), AP (Advanced Placement), GCSE, and IGCSE, making him uniquely qualified to provide clear, accurate, and exam-focused content for students worldwide.

📧 Contact Information:

Email: info@revisiontown.com

LinkedIn: linkedin.com/in/kumar-k-87346a153/

🎓 Educational Expertise

Specializing in IB, AP, GCSE, and IGCSE curricula across Mathematics and Sciences