Comprehensive Study Notes: Energy Flow in Ecosystems (IGCSE Biology 0610)
Hello Biologists! This chapter is all about how life on Earth gets its power. Think of energy flow as the fundamental currency of life—it determines who eats whom, why food chains exist, and ultimately, how ecosystems survive. Don't worry, we'll break down the concepts, starting from the biggest power source of all!
1. The Principal Source of Energy (19.1)
The Sun: The Ultimate Power Station
In almost every biological system on Earth, the energy journey begins with the Sun.
- Principal Source: The Sun provides the initial light energy input to biological systems (19.1.1).
- Energy Conversion: Producers (plants) are responsible for capturing this light energy and converting it into stored chemical energy (sugars) through photosynthesis.
The Flow of Energy (19.1.2)
Energy moves through the ecosystem in a simple, one-way street:
- Start: Light energy (from the Sun) is captured by producers.
- Transfer: This energy, now chemical energy stored in organic molecules (food), is transferred when one organism eats another (feeding).
- Loss: At every stage, most energy is lost, mainly as heat, due to metabolic processes like respiration and movement.
- End: All energy is eventually transferred back to the environment as heat, meaning the ecosystem constantly needs a new energy input from the Sun.
Key Takeaway: Energy flows, it does not cycle! Once chemical energy becomes heat, it cannot be recycled back into organic molecules by living organisms.
2. Food Chains and Food Webs (19.2 Core)
What is a Food Chain?
A food chain shows the transfer of energy from one organism to the next, starting with a producer (19.2.1).
- It is a simple diagram using arrows (19.2.2).
- The arrows point in the direction of energy flow (i.e., from the organism being eaten to the organism that eats it).
Example: Grass (eaten by) $\rightarrow$ Rabbit (eaten by) $\rightarrow$ Fox
Quick Tip: Think of the arrow as saying "is eaten by."
Roles within the Chain: Producers and Consumers
The position an organism holds in the food chain is called its trophic level (19.2.13).
1. Producers (Trophic Level 1) (19.2.4)
- Organisms that make their own organic nutrients (food).
- Usually plants, using light energy (photosynthesis).
- Example: grass, trees, algae.
2. Consumers (19.2.5)
Organisms that get their energy by feeding on other organisms (19.2.5). Consumers are classified based on what they eat:
- Primary Consumers (Trophic Level 2): Animals that eat producers (plants). They are herbivores (19.2.7). Example: Rabbits, cows.
- Secondary Consumers (Trophic Level 3): Animals that eat primary consumers. They are often carnivores (19.2.8). Example: Foxes, small snakes.
- Tertiary Consumers (Trophic Level 4): Animals that eat secondary consumers. Example: Eagles, large snakes.
- Quaternary Consumers (Trophic Level 5): Animals that eat tertiary consumers (19.2.6).
Decomposers (19.2.9)
These are organisms (like bacteria and fungi) that get their energy from dead or waste organic material. They are crucial because they break down waste and recycle essential nutrients back into the soil.
Food Webs (19.2.3)
In reality, ecosystems are not simple single chains. A food web is a network of interconnected food chains. Most organisms eat, and are eaten by, multiple different species.
Why are food webs important? They show complex feeding relationships and how stable an ecosystem is. If one population crashes (e.g., rabbits die), the foxes can switch to eating mice, maintaining the stability of the system.
Human Impact on Food Webs (19.2.10)
Humans often disrupt ecosystems, causing negative impacts:
- Overharvesting: Catching or harvesting too many individuals of a food species (e.g., overfishing) causes the population to crash, severely affecting the consumers that rely on that species.
- Introducing Foreign Species: Bringing in non-native species (like invasive predators) can destroy native populations that have no defense against them, radically altering the local food web.
3. Ecological Pyramids (19.2.11 - 19.2.16)
Ecological pyramids graphically represent the organisms at each trophic level. The producer level always forms the wide base.
A. Pyramids of Numbers (19.2.11)
- Shows the number of individual organisms present at each trophic level.
- Interpretation: Draw bars representing the total count of organisms in that level.
- Disadvantage: These pyramids can sometimes look "odd" or be inverted (upside down). Example: One massive oak tree (low number) supports thousands of caterpillars (high number). This is why they are often less useful.
B. Pyramids of Biomass (19.2.11, 19.2.12)
- Shows the total dry mass (or biological material) of the organisms at each trophic level.
- Advantage: Biomass gives a much better representation of the quantity of living matter transferred than numbers, and they are usually pyramid-shaped.
- Why use biomass over numbers? Numbers can be misleading due to huge differences in organism size (a tiny plankton vs. a massive whale). Biomass is more representative of the stored energy.
Quick Review Box:
Pyramid of Numbers = Counts individuals.
Pyramid of Biomass = Measures total dry weight.
4. Energy Transfer and Efficiency (19.2 Supplement)
Don't worry if this seems tricky at first—it’s just applying maths to biology. Remember the 10% rule!
C. Pyramids of Energy (19.2.15, 19.2.16)
- Shows the total energy content (e.g., measured in kJ/m²/year) available at each trophic level.
- Advantage: This is the most accurate representation of the flow of energy in an ecosystem.
- Crucial Point: Pyramids of energy are ALWAYS pyramid-shaped, because energy must decrease at each successive trophic level.
Inefficient Energy Transfer (19.2.17)
The transfer of energy from one trophic level to the next is often not efficient. Only a small percentage of energy taken in (around 10%) is actually stored and passed on to the next level.
Where does the energy go?
When a cow eats grass, 90% of the energy in the grass is "lost" to the environment because the cow uses it for:
- Respiration: Used for metabolic processes and released as heat (the largest loss).
- Movement: Energy required to find food or escape predators.
- Waste: Undigested food material lost as faeces (not all of the food consumed is absorbed).
Why are Food Chains Short? (19.2.18)
Food chains rarely have more than five trophic levels (Producer $\rightarrow$ Primary $\rightarrow$ Secondary $\rightarrow$ Tertiary $\rightarrow$ Quaternary). This is explained by the energy loss:
The significant loss of energy (90% at each step) means that after just a few steps, there is simply not enough energy left to support another viable trophic level.
Did you know? If the producer has 10,000 kJ of energy, the quaternary consumer only receives about 1 kJ!
Energy Efficiency and Human Diet (19.2.19)
Understanding energy transfer helps us see why certain diets are more efficient.
It is more energy efficient for humans to eat crop plants (producers) than to eat livestock that have been fed on crop plants (primary consumers).
-
Eating Plants Directly (Vegetarian): Energy pathway is short (2 trophic levels):
Producer (Crop) $\rightarrow$ Human (You only lose energy once). -
Eating Meat (Carnivorous): Energy pathway is longer (3 trophic levels):
Producer (Crop) $\rightarrow$ Primary Consumer (Livestock) $\rightarrow$ Human (You lose energy twice).
Because so much energy is lost between the crop and the livestock (cow, chicken, etc.), eating the crop directly provides much more usable food energy for the human population from the same amount of land.
Key Takeaway: Energy Flow Summary
- The Sun drives all energy input.
- Energy flows linearly (one way) and is constantly lost as heat (respiration).
- Food chains are short due to massive energy loss (about 90%) at each step.
- Pyramids of energy are the most reliable tool because they account for the actual energy content, which always decreases up the trophic levels.