Interactions within Ecosystems - Study Notes

Key Concepts

Food Webs and Energy Flow

Energy enters ecosystems through sunlight, which is captured by producers (green plants) during photosynthesis
Producers (also called autotrophs) convert light energy into chemical energy stored in glucose
Consumers (also called heterotrophs) obtain energy by eating other organisms
Energy flows in one direction through an ecosystem: from sun → producers → consumers
Energy is lost at each level as:
- Heat energy during respiration
- Movement and other life processes
- Undigested material in faeces
Only about 10% of energy is transferred from one trophic level to the next
Food chains show a single pathway of energy transfer (e.g., grass → grasshopper → frog → snake)
Food webs show multiple interconnected food chains in an ecosystem, giving a more realistic picture
Food webs show:
- What organisms eat
- Feeding relationships between species
- How energy flows through the community
- How organisms depend on each other

Trophic Levels

Trophic level = the position an organism occupies in a food chain
First trophic level: Producers (plants, algae)
- Make their own food through photosynthesis
- Convert light energy to chemical energy
Second trophic level: Primary consumers (herbivores)
- Feed directly on producers
- Examples: caterpillars, rabbits, cattle, deer
Third trophic level: Secondary consumers (carnivores)
- Feed on primary consumers
- Examples: frogs, small birds, small fish
Fourth trophic level: Tertiary consumers (carnivores)
- Feed on secondary consumers
- Often apex predators with no natural predators
- Examples: eagles, sharks, lions
Pyramid of numbers: shows the number of organisms at each trophic level
Pyramid of energy: shows the amount of energy at each trophic level (always pyramid-shaped)
Pyramid of biomass: shows the total mass of living organisms at each trophic level

Decomposers and Nutrient Cycling

Decomposers break down dead organisms and waste materials
Main decomposers include:
- Bacteria (microscopic organisms)
- Fungi (including moulds and mushrooms)
Functions of decomposers:
- Break down complex organic compounds into simpler substances
- Release nutrients back into the soil
- Recycle minerals like nitrogen, phosphorus, and carbon
- Prevent build-up of dead matter
Nutrient cycling is the continuous movement of nutrients through ecosystems:
- Nutrients are absorbed by producers from soil/water
- Nutrients pass to consumers through feeding
- Nutrients return to environment through death and waste
- Decomposers break down materials and release nutrients
- Nutrients become available again for producers
Without decomposers:
- Nutrients would be locked in dead organisms
- Soil would become depleted
- Producers could not grow
- Ecosystems would collapse

Symbiosis — Mutualism, Commensalism, Parasitism

Symbiosis = a close relationship between two different species living together
Three main types of symbiotic relationships:

1. Mutualism (+/+)

Both species benefit from the relationship
Examples:
- Nitrogen-fixing bacteria in root nodules: Bacteria get food and shelter from legume plants; plants get nitrogen compounds for protein synthesis
- Bees and flowers: Bees get nectar for food; flowers get pollinated for reproduction
- Oxpecker birds and cattle: Birds get food (ticks and parasites); cattle get cleaned and pest control
- Clownfish and sea anemones: Fish get protection from predators; anemones get cleaned and food scraps

2. Commensalism (+/0)

One species benefits, the other is neither helped nor harmed
Examples:
- Remora fish and sharks: Remoras attach to sharks for transportation and food scraps; sharks are unaffected
- Orchids growing on tree branches: Orchids get support and sunlight; trees are unaffected
- Birds nesting in trees: Birds get shelter; trees are unaffected

3. Parasitism (+/−)

One species (parasite) benefits, the other species (host) is harmed
Parasite gets nutrients/shelter from the host
Host loses nutrients, may become weak or sick
Parasites usually don’t kill the host immediately (host keeps parasite alive)
Examples:
- Tapeworms in human intestines: Tapeworm gets digested food; human loses nutrients and may suffer malnutrition
- Head lice on humans: Lice feed on blood; humans experience itching and discomfort
- Mistletoe on trees: Mistletoe gets water and minerals; tree loses nutrients
- Plasmodium (malaria parasite) in mosquitoes and humans: Parasite reproduces; humans suffer fever and illness

Human Impact on Ecosystems

Negative Impacts

1. Deforestation

Clearing forests for agriculture, development, or logging
Effects:
- Loss of habitats → species extinction
- Disruption of food webs
- Soil erosion (no tree roots to hold soil)
- Reduced oxygen production
- Increased carbon dioxide (fewer trees to absorb CO₂)
- Climate change contribution

2. Pollution

Water pollution: Industrial waste, sewage, oil spills, pesticides
- Kills aquatic organisms
- Disrupts food chains
- Eutrophication (excess nutrients cause algae blooms, oxygen depletion)
Air pollution: Burning fossil fuels, vehicle emissions, factory smoke
- Acid rain damages plants and aquatic ecosystems
- Haze affects photosynthesis
- Respiratory problems in animals
Plastic pollution: Non-biodegradable waste
- Animals ingest or get entangled in plastic
- Microplastics enter food chains

3. Overfishing and Hunting

Excessive removal of species faster than they can reproduce
Effects:
- Population decline or extinction
- Disrupted food webs
- Loss of biodiversity

4. Introduction of Invasive Species

Non-native species introduced to new ecosystems
Effects:
- Compete with native species for resources
- Prey on native species
- Introduce diseases
- Disrupt existing food webs

Positive Conservation Efforts

1. Reforestation

Planting trees to replace those cut down
Restores habitats and prevents soil erosion

2. Establishing Protected Areas

Nature reserves, national parks, marine protected areas
Preserves biodiversity and habitats

3. Sustainable Practices

Sustainable fishing (size limits, quotas, seasonal restrictions)
Sustainable agriculture (crop rotation, organic farming)
Reduces environmental damage while meeting human needs

4. Recycling and Waste Management

Reduces pollution and conserves resources
Proper disposal of hazardous waste

5. Legislation and International Agreements

Laws protecting endangered species
Regulations controlling pollution
International cooperation on climate change

Important Definitions

Producer: An organism that makes its own food through photosynthesis, converting light energy into chemical energy (e.g., green plants, algae)

Consumer: An organism that obtains energy by feeding on other organisms

Herbivore: A primary consumer that feeds only on plants

Carnivore: A consumer that feeds on other animals (can be secondary, tertiary, or higher level consumers)

Omnivore: A consumer that feeds on both plants and animals

Decomposer: An organism that breaks down dead organisms and waste materials, recycling nutrients back into the ecosystem (e.g., bacteria, fungi)

Food Chain: A diagram showing the transfer of energy from one organism to another in a single pathway

Food Web: A diagram showing multiple interconnected food chains in an ecosystem, illustrating all feeding relationships

Trophic Level: The position an organism occupies in a food chain, representing its feeding level

Pyramid of Numbers: A diagram showing the number of organisms at each trophic level in an ecosystem

Pyramid of Energy: A diagram showing the amount of energy available at each trophic level (always decreases upwards)

Pyramid of Biomass: A diagram showing the total mass of living organisms at each trophic level

Symbiosis: A close, long-term relationship between two different species living together

Mutualism: A symbiotic relationship where both species benefit (+/+)

Commensalism: A symbiotic relationship where one species benefits and the other is unaffected (+/0)

Parasitism: A symbiotic relationship where one species (parasite) benefits at the expense of the other (host) (+/−)

Parasite: An organism that lives in or on another organism (host), obtaining nutrients from it and causing harm

Host: An organism that harbours a parasite, providing it with nutrients or shelter

Biodiversity: The variety of different species living in an ecosystem

Deforestation: The large-scale removal of trees from forested areas

Conservation: The protection and careful management of natural resources and ecosystems

Diagrams and Structures

Diagram 1: Simple Food Chain

How to draw it:

Draw 4 boxes in a horizontal row from left to right
Connect them with arrows pointing right (arrows show direction of energy flow)
Label the boxes:
- Box 1: “Grass (Producer)”
- Box 2: “Grasshopper (Primary Consumer/Herbivore)”
- Box 3: “Frog (Secondary Consumer/Carnivore)”
- Box 4: “Snake (Tertiary Consumer/Carnivore)”
Below the diagram, write: “Arrow means ‘is eaten by’ or ‘provides energy for’”

Important note: Arrows always point from food source to consumer (from what is eaten to what eats it)

Diagram 2: Food Web

How to draw it:

Draw a sun at the top
Draw multiple producers (plants) below the sun
Draw several herbivores in the next level
Draw carnivores in the level above
Draw top predators at the highest level
Connect organisms with arrows showing feeding relationships
Multiple arrows should come from and go to various organisms

Example structure:

Sun → Grass, Bush, Tree
Grass → Rabbit, Grasshopper
Bush → Caterpillar
Tree → Caterpillar, Beetle
Rabbit → Fox, Eagle
Grasshopper → Frog, Bird
Caterpillar → Bird
Beetle → Bird
Frog → Snake
Bird → Snake, Eagle
Snake → Eagle

Key features to show:

Multiple food chains interconnected
Some organisms appear in multiple chains
Some organisms eat more than one type of food
Some organisms are eaten by more than one predator

Diagram 3: Pyramid of Energy

How to draw it:

Draw a pyramid divided into 4 horizontal sections
Make each section narrower as you go up (represents decreasing energy)
Label from bottom to top:
- Bottom (widest): “Producers (10,000 kJ)”
- Second level: “Primary Consumers (1,000 kJ)”
- Third level: “Secondary Consumers (100 kJ)”
- Top (narrowest): “Tertiary Consumers (10 kJ)”
Add a label: “Energy decreases by about 90% at each level”
Add arrows on the side showing “Heat lost to surroundings” at each level

Important: Always pyramid-shaped because energy is lost at each level

Diagram 4: Nutrient Cycle

How to draw it:

Draw a circle showing the cyclical nature
Include these components:
- Producers (plants) at bottom
- Consumers (animals) to the right
- Dead organisms and waste in the middle
- Decomposers below the dead organisms
- Nutrients in soil at the bottom
Connect with arrows showing:
- Nutrients from soil → absorbed by producers
- Producers → eaten by consumers
- Consumers → produce waste and eventually die
- Dead organisms/waste → broken down by decomposers
- Decomposers → release nutrients back to soil
- Nutrients in soil → back to producers (completing the cycle)

Diagram 5: Symbiotic Relationships (Summary Table)

How to draw it: Create a table with 4 columns and 4 rows:

Type	Species A	Species B	Example
Mutualism	Benefits (+)	Benefits (+)	Bee and flower
Commensalism	Benefits (+)	Unaffected (0)	Orchid on tree
Parasitism	Benefits (+)	Harmed (−)	Tapeworm in human

Worked Examples

Example 1: Analyzing Energy Transfer in a Food Chain

Question: In a food chain, the producer contains 50,000 kJ of energy. Calculate how much energy is available at each subsequent trophic level if 10% of energy is transferred at each stage.

Step-by-step solution:

Identify the starting energy:
- Producers: 50,000 kJ
Calculate energy at primary consumer level:
- Only 10% is transferred
- Energy = 50,000 kJ × 10% = 50,000 kJ × 0.1
- Primary consumers: 5,000 kJ
Calculate energy at secondary consumer level:
- 10% of primary consumer energy
- Energy = 5,000 kJ × 0.1
- Secondary consumers: 500 kJ
Calculate energy at tertiary consumer level:
- 10% of secondary consumer energy
- Energy = 500 kJ × 0.1
- Tertiary consumers: 50 kJ

Summary:

Producers: 50,000 kJ
Primary consumers: 5,000 kJ
Secondary consumers: 500 kJ
Tertiary consumers: 50 kJ

Explanation of energy loss: 90% of energy is lost at each level through:

Heat from respiration
Movement and life processes
Undigested material
Not all organisms are consumed

Example 2: Predicting Effects of Removing an Organism from a Food Web

Scenario: Consider this food web:

Grass → Rabbit → Fox
Grass → Grasshopper → Frog → Snake
Grasshopper → Bird → Snake

Question: Predict what would happen if all the frogs were removed from this ecosystem.

Step-by-step analysis:

Identify what frogs eat:
- Frogs eat grasshoppers
- Effect: Grasshopper population will increase (fewer predators eating them)
Identify what eats frogs:
- Snakes eat frogs
- Effect: Snake population will decrease (less food available)
- Note: Snakes also eat birds, so they won’t disappear completely, but will decline
Follow the knock-on effects:

Effect on grasshoppers:
- More grasshoppers survive
- They will eat more grass
- Grass population may decrease
Effect on birds:
- More grasshoppers available as food
- Bird population may increase
- But more snakes may eat birds (compensating for lost frogs)
- Overall effect is uncertain
Long-term effects:
- If grasshopper population increases too much, grass may be overconsumed
- This could then lead to grasshopper population decline (not enough food)
- Ecosystem becomes unbalanced

Conclusion: Removing one organism creates a chain reaction throughout the food web, affecting multiple species and potentially destabilizing the entire ecosystem.

Example 3: Identifying Types of Symbiotic Relationships

Question: Classify each relationship as mutualism, commensalism, or parasitism, and explain why.

a) Cleaner fish and large fish:

Cleaner fish eat parasites off large fish
Answer: Mutualism (+/+)
Explanation: Cleaner fish gets food (parasites); Large fish gets cleaned and healthier

b) Barnacles on whales:

Barnacles attach to whale skin for transportation and access to plankton-rich waters
Whales are unaffected
Answer: Commensalism (+/0)
Explanation: Barnacles benefit from transportation and feeding opportunities; Whales are neither helped nor harmed

c) Ticks on dogs:

Ticks feed on dog’s blood
Dogs may develop anemia, skin irritation, diseases
Answer: Parasitism (+/−)
Explanation: Ticks benefit by getting blood for nutrition; Dogs are harmed by blood loss and potential disease

d) Fungi and algae in lichen:

Fungi provide structure and water retention
Algae perform photosynthesis to produce food
Both share nutrients
Answer: Mutualism (+/+)
Explanation: Fungi gets food from algae; Algae gets water, minerals, and protection from fungi

Common Mistakes to Avoid

Drawing arrows in the wrong direction in food chains
- ❌ Wrong: Grass ← Rabbit ← Fox
- ✓ Correct: Grass → Rabbit → Fox
- Remember: Arrow shows direction of energy flow (points to the organism receiving the energy)
Confusing the terms ‘eaten by’ and ‘eats’
- The arrow means “is eaten by” or “provides energy for”
- Grass → Rabbit means “Grass is eaten by rabbit”
Thinking all pyramids of numbers are pyramid-shaped
- Pyramids of energy are ALWAYS pyramid-shaped
- Pyramids of numbers can sometimes be inverted (e.g., one tree supporting many insects)
- Don’t confuse the two!
Forgetting that decomposers are essential to food webs
- Decomposers should be included when drawing complete ecosystem diagrams
- They operate at ALL trophic levels, breaking down dead organisms and waste
Misidentifying trophic levels
- Producers are ALWAYS first trophic level (never call them consumers)
- Herbivores are ALWAYS primary consumers (even if they’re large animals)
- Carnivores can be secondary, tertiary, or higher (depends on what they eat)
- Omnivores can occupy multiple trophic levels
Confusing the three types of symbiosis
- Remember: Mutualism = both benefit (MUtual benefit)
- Commensalism = one benefits, one unaffected (think “common” = neutral)
- Parasitism = one benefits, one harmed (Parasite harms host)
Stating that energy is recycled in ecosystems
- ❌ Wrong: “Energy is recycled”
- ✓ Correct: “Energy flows in ONE direction and is lost as heat”
- Only NUTRIENTS/MATERIALS are recycled, not energy
Forgetting to explain WHY energy decreases at each trophic level
- Don’t just state “energy is lost”
- Explain: lost as heat during respiration, movement, undigested material, not all organisms consumed
Thinking parasites kill their hosts immediately
- Parasites usually keep hosts alive (dead host = dead parasite)
- They weaken hosts over time but rarely cause immediate death
Providing vague answers about human impact
- ❌ Weak answer: “Pollution is bad for animals”
- ✓ Strong answer: “Water pollution kills aquatic organisms and disrupts food chains by removing key species, causing populations of their prey to increase and their predators to decrease”

Exam Tips

Keywords to Use for Food Chains/Webs and Energy Flow:

“Energy flows from… to…”
“Energy is transferred when…”
“Energy is lost as heat during respiration”
“Energy is lost through movement and life processes”
“Approximately 10% of energy is transferred to the next trophic level”
“90% of energy is lost at each level”
“Not all parts of organisms are consumed”
“Energy cannot be recycled” (but nutrients can)

Keywords for Trophic Levels:

“Producer/First trophic level”
“Primary consumer/Herbivore/Second trophic level”
“Secondary consumer/Carnivore/Third trophic level”
“Tertiary consumer/Top predator/Fourth trophic level”
Always specify the trophic level number when asked

Keywords for Decomposers:

“Break down dead organisms and waste materials”
“Release nutrients back into the soil/environment”
“Recycle nutrients such as nitrogen, phosphorus, carbon”
“Essential for nutrient cycling”
“Include bacteria and fungi”

Keywords for Symbiosis:

Always state BOTH organisms and how EACH is affected
Mutualism: “Both species benefit because…”
Commensalism: “Species A benefits while Species B is unaffected because…”
Parasitism: “The parasite benefits while the host is harmed because…”
Provide specific examples of the benefit/harm

Keywords for Human Impact:

“Deforestation leads to habitat loss and species extinction”
“Disruption of food webs when species are removed”
“Loss of biodiversity reduces ecosystem stability”
“Pollution affects organisms by…” (be specific)
“Conservation efforts such as…” (name specific methods)
“Sustainable practices ensure…” (explain long-term benefits)

For Data Analysis Questions:

Identify trends clearly: “increases,” “decreases,” “remains constant”
Compare specific numbers from tables/graphs
Link changes to ecological concepts (population effects, energy transfer)
Explain cause-and-effect relationships

For “Predict” Questions:

State the immediate effect on the population mentioned
Follow the knock-on effects through the food web
Consider BOTH what the organism eats AND what eats it
Mention at least 2-3 different populations affected
Use terms like “increase,” “decrease,” or “remains stable”

Answer Structure for Long Questions:

State the concept/definition
Explain the mechanism/process
Provide a specific example
Link to the ecosystem/broader context

Drawing Diagrams:

Use a ruler for arrows
Label ALL organisms clearly
Ensure arrows point in correct direction
Include a key if necessary
For food webs, show at least 3 interconnected chains

Quick Summary

Essential points to remember for exams:

✓ Energy flow: Energy flows one direction (sun → producers → consumers), cannot be recycled, ~10% transferred between levels, 90% lost as heat/movement/waste

✓ Food chains vs food webs: Chains show single pathways; webs show interconnected multiple chains and are more realistic

✓ Trophic levels: Producers (1st) → Primary consumers/herbivores (2nd) → Secondary consumers/carnivores (3rd) → Tertiary consumers (4th)

✓ Pyramids: Energy pyramids always pyramid-shaped; number pyramids can be inverted; show decreasing energy/biomass upwards

✓ Decomposers: Bacteria and fungi break down dead material, recycle nutrients (N, P, C) back to soil, essential for ecosystem functioning

✓ Nutrient cycling: Nutrients recycled continuously through absorption → feeding → death/waste → decomposition → back to soil (circular process)

✓ Mutualism: Both species benefit (+/+) – e.g., bees and flowers, nitrogen-fixing bacteria and legumes

✓ Commensalism: One benefits, one unaffected (+/0) – e.g., orchids on trees, barnacles on whales

✓ Parasitism: Parasite benefits, host harmed (+/−) – e.g., tapeworms in humans, ticks on dogs, parasites don’t immediately kill hosts

✓ Human negative impacts: Deforestation (habitat loss), pollution (kills organisms), overfishing/hunting (population decline), invasive species (competition/predation)

✓ Conservation efforts: Reforestation, protected areas, sustainable practices, recycling, legislation to protect species and reduce pollution

✓ Predicting ecosystem changes: Removing an organism affects its predators (decrease), prey (increase), and cascades through the entire food web creating chain reactions

Remember: Ecosystems are interconnected systems where every organism plays a role. Changes to one part affect the whole system. Always consider multiple effects when analyzing ecosystem interactions!