Welcome to Ecological Relationships and Energy Flow!
Have you ever wondered why there are millions of blades of grass in a field, but only a handful of foxes or birds of prey? Or why life on Earth never runs out of carbon, even after billions of years? In this chapter, we will explore how living organisms interact with one another, how energy from the Sun flows through nature, and how vital nutrients are recycled. Don't worry if ecology terms feel new—we will break down every single idea step by step!
1. Feeding Relationships and Trophic Levels
Every living thing needs energy to survive. The main source of all energy for life on Earth is sunlight. Green plants trap this light energy to make their own food through photosynthesis.
Key Terms You Need to Know
Producer: An organism (like a green plant or alga) that makes its own food using sunlight via photosynthesis.
Consumer: An organism that gets its energy by eating (consuming) other organisms.
Herbivore (Primary Consumer): An animal that eats only plants (e.g., a caterpillar or a rabbit).
Carnivore (Secondary or Tertiary Consumer): An animal that eats other animals (e.g., a fox or a hawk).
Omnivore: An animal that eats both plants and animals (e.g., humans or badgers).
Decomposer: Microorganisms (mainly bacteria and fungi) that break down dead plant and animal matter, recycling nutrients back into the soil.
Trophic Levels
A trophic level simply means the feeding position an organism occupies in a food chain.
Trophic Level 1: Producers (e.g., Grass)
Trophic Level 2: Primary Consumers (e.g., Grasshopper)
Trophic Level 3: Secondary Consumers (e.g., Frog)
Trophic Level 4: Tertiary Consumers (e.g., Hawk)
Food Chains and Food Webs
A food chain shows a single path of energy transfer from one organism to the next.
Example: Grass \(\rightarrow\) Grasshopper \(\rightarrow\) Frog \(\rightarrow\) Hawk
Crucial Exam Tip: The arrows in a food chain always show the direction of energy flow (from the organism being eaten to the organism that eats it). Never draw the arrows pointing backwards!
In nature, feeding relationships are rarely a single straight line. Animals usually eat more than one type of food. A food web consists of many interconnected food chains showing the complex feeding relationships in an ecosystem.
Interpreting Changes in Food Webs
Exam questions often ask what happens if one species in a food web increases or decreases. Let's look at how to tackle these:
1. Identify the direct link: If frogs decrease, hawks have less food, so the hawk population may fall.
2. Look at the organism eaten by that species: With fewer frogs eating grasshoppers, the grasshopper population may rise.
3. Look at wider effects: More grasshoppers will eat more grass, causing grass numbers to decrease.
Key Takeaway: Energy flows from producers to consumers through trophic levels. Always remember that arrows represent the direction of energy transfer.
2. Energy Flow and Efficiency Calculations
Why do food chains rarely have more than four or five trophic levels? The answer lies in how energy is lost at each stage.
How Energy is Lost
Only about \(10\%\) of the energy at one trophic level is passed on to the next. The other \(90\%\) is lost in several ways:
1. Respiration: A large amount of energy is released during respiration for life processes like muscle contraction and maintaining body temperature (heat energy is lost to the surroundings).
2. Waste products: Energy is lost in excretion (urine) and egestion (undigested food/faeces).
3. Uneaten parts: Not all of an organism is eaten (e.g., roots, bones, feathers, fur, woody stems).
Calculating Percentage Efficiency of Energy Transfer
You can calculate how efficiently energy moves from one trophic level to the next using this simple formula:
\(\text{Percentage Efficiency} = \left( \frac{\text{Energy transferred to the next level}}{\text{Total energy available at previous level}} \right) \times 100\)
Worked Example:
A field of clover absorbs \(50\,000\text{ kJ}\) of light energy. The clover converts \(5\,000\text{ kJ}\) of this into plant tissue. Rabbits eat the clover and incorporate \(500\text{ kJ}\) of energy into their bodies.
What is the percentage efficiency of energy transfer from clover to rabbits?
\(\text{Efficiency} = \left( \frac{500\text{ kJ}}{5\,000\text{ kJ}} \right) \times 100 = 10\%\)
Key Takeaway: Energy is lost at each level through heat, respiration, movement, and waste. Because so much energy is lost at each step, there is not enough energy left to support higher trophic levels.
3. Ecological Pyramids
Ecological pyramids are graphical representations that show the structure of an ecosystem. There are two main types you need to know for your exam: Pyramids of Numbers and Pyramids of Biomass.
Rules for Drawing Ecological Pyramids
Rule 1: The producer always goes at the bottom (base bar).
Rule 2: Trophic levels must be stacked in order (Primary consumer on top of producer, etc.).
Rule 3: The height of each bar should be the same, and the width of each bar must represent the quantity accurately and symmetrically.
Rule 4: Every bar must be clearly labelled with the organism's name or trophic level.
1. Pyramids of Numbers
A pyramid of numbers represents the actual count of individual organisms at each trophic level.
Standard Pyramid: In a grassland ecosystem (e.g., \(10\,000\) Grass plants \(\rightarrow\) \(500\) Caterpillars \(\rightarrow\) \(20\) Blue tits \(\rightarrow\) \(1\) Hawk), the pyramid is broad at the bottom and narrows at the top.
Inverted/Non-standard Pyramid: When a single large producer supports many consumers (e.g., \(1\) Oak tree \(\rightarrow\) \(2\,000\) Caterpillars \(\rightarrow\) \(10\) Robins), the base bar is very narrow, creating an inverted shape.
2. Pyramids of Biomass
Biomass is the total mass of living material in an organism. To be scientifically accurate, it refers to dry mass (mass of the organism after all water has been removed).
Unlike pyramids of numbers, a pyramid of biomass is almost always pyramid-shaped (broad at the bottom, narrow at the top). Even if one oak tree supports thousands of caterpillars, the oak tree has a far greater total mass than all those caterpillars combined.
Quick Comparison Table
Pyramid of Numbers: Easy to collect data; shows count of organisms; can be irregular or upside down.
Pyramid of Biomass: Always pyramid-shaped for terrestrial ecosystems; shows actual biological mass; more difficult to measure because drying organisms kills them.
Key Takeaway: Pyramids of numbers can have irregular shapes due to organism size, but pyramids of biomass are always widest at the bottom because biomass decreases as you move up trophic levels.
4. Nutrient Recycling: The Carbon Cycle
While energy flows in one direction and is eventually lost as heat, nutrients and elements must be recycled so life can continue. Carbon is the fundamental building block of carbohydrates, fats, and proteins.
The Key Processes in the Carbon Cycle
1. Photosynthesis (Removes \( \text{CO}_2 \) from the atmosphere):
Green plants absorb carbon dioxide from the air and convert it into glucose: \(\text{Carbon dioxide} + \text{Water} \rightarrow \text{Glucose} + \text{Oxygen}\). This is the only natural process that removes carbon dioxide from the atmosphere!
2. Feeding (Passes carbon along):
Animals eat plants or other animals, incorporating carbon compounds into their own body tissues.
3. Respiration (Releases \( \text{CO}_2 \) into the atmosphere):
Plants, animals, and decomposers break down glucose to release energy, producing carbon dioxide as a waste product: \(\text{Glucose} + \text{Oxygen} \rightarrow \text{Carbon dioxide} + \text{Water}\).
4. Decomposition / Decay (Releases \( \text{CO}_2 \)):
When plants and animals die, decomposers (bacteria and fungi) feed on the dead matter. They respire and return carbon dioxide to the atmosphere.
5. Fossilisation:
In certain conditions where dead organisms do not decay completely (e.g., deep underground in acidic, low-oxygen bogs over millions of years), they turn into fossil fuels (coal, oil, and natural gas).
6. Combustion (Releases \( \text{CO}_2 \)):
Burning fossil fuels or biomass (wood) releases trapped carbon back into the atmosphere as carbon dioxide: \(\text{Fossil Fuel} + \text{Oxygen} \rightarrow \text{Carbon dioxide} + \text{Water}\).
Key Takeaway: Carbon enters living systems solely through photosynthesis and returns to the atmosphere through respiration, decomposition, and combustion.
5. Decomposition and the Rate of Decay
Decomposers are nature's cleanup crew. Without them, dead organisms would pile up, and essential nutrients like carbon and nitrogen would remain locked away.
Who are the Decomposers?
The two main groups of decomposers are fungi and bacteria. They secrete digestive enzymes onto dead material to break down complex organic molecules into simple soluble substances, which they absorb (a process called saprophytic nutrition).
Factors Affecting the Rate of Decay
Because decomposers are living microorganisms, their activity depends on environmental conditions:
1. Temperature: Warm temperatures increase enzyme activity, speeding up decay. However, if the temperature gets too hot (above \(\approx 45^\circ\text{C}\)), the enzymes denature and decay stops.
2. Moisture (Water): Microorganisms need water for cellular processes and to dissolve food substances. Decay is very slow in dry conditions.
3. Oxygen availability: Decomposers need oxygen for aerobic respiration to produce the energy required for growth and reproduction. In waterlogged soils or sealed containers where oxygen is scarce, decay slows down drastically.
Did You Know? Food preservation methods work by removing one of these conditions! For example: freezing lowers the temperature, drying/curing removes moisture, and vacuum packing removes oxygen.
Key Takeaway: Decomposition occurs fastest in warm, moist, and oxygen-rich environments because these conditions maximise the metabolic activity of bacteria and fungi.
Chapter Summary & Quick Revision Checklist
Make sure you can confidently answer the following before your exam:
- Can you explain why food chains have arrows pointing toward the consumer? (They show energy flow!)
- Can you list three ways energy is lost between trophic levels? (Respiration/heat, excretion/egestion, uneaten parts.)
- Can you calculate percentage energy efficiency using the formula?
- Can you explain why a pyramid of numbers can be inverted, but a pyramid of biomass is typically upright?
- Can you describe the six stages of the carbon cycle? (Photosynthesis, feeding, respiration, death/decomposition, fossilisation, combustion.)
- Can you state the three factors that affect decomposition? (Temperature, moisture, oxygen.)