Introduction to Environmental Biology
Senior 2 · Biology
Introduction to Environmental Biology
Lesson 4 of 4
22 min
Let's begin
Akagera supports thousands of impalas, zebras and buffaloes, but only a few dozen lions. Why so few? The answer is not about hunting skill. It is about energy.
At each step of a food chain, most of the energy is lost. This is why food chains are short, why top predators are rare, and why a hectare of land feeds more people growing beans than raising cattle.
Almost all energy in an ecosystem starts as sunlight. Producers capture a small part of it by photosynthesis and store it as chemical energy in their bodies. When a herbivore eats a plant, only part of that energy becomes new herbivore body tissue. The rest is lost:
On average only about 10% of the energy at one trophic level is passed to the next. Energy flows in one direction, from the sun through the chain and out as heat, and is never recycled. Minerals, however, cycle, returned by decomposers.
Question: In the chain grass → impala → lion, the grass stores 50 000 kJ of energy. Assuming 10% transfer at each step, how much reaches (a) the impalas and (b) the lions?
Solution:
(a) impalas = 10% of 50 000 kJ = 0.10 × 50 000 = 5 000 kJ
(b) lions = 10% of 5 000 kJ = 0.10 × 5 000 = 500 kJ
Only 1% of the energy in the grass reaches the lions. A fourth level would receive just 50 kJ, too little to support a population. This is why food chains rarely have more than four or five levels.
efficiency (%) = (energy at the higher level ÷ energy at the lower level) × 100
Example: producers in a pond hold 20 000 kJ/m² per year, and the primary consumers hold 1 600 kJ/m² per year. Efficiency = (1 600 ÷ 20 000) × 100 = 8%.
An ecological pyramid is a diagram of stacked bars, one for each trophic level, with the producers at the bottom. The width of each bar shows the size of that level.
| Pyramid of… | What each bar shows | Can it be upside-down? |
|---|---|---|
| Numbers | the number of individual organisms at each level | Yes. One avocado tree can feed thousands of caterpillars, which feed a few birds, so the bottom bar is narrower than the one above it. Parasites give the same effect: one cow can carry hundreds of ticks. |
| Biomass | the total dry mass of living material at each level (for example g/m²) | Occasionally. In a lake, the phytoplankton present at one moment can weigh less than the zooplankton, because the algae are eaten and reproduce very quickly. |
| Energy | the energy passing through each level over a period (for example kJ/m² per year) | Never, because each level always receives less energy than the level below it. |
Question: A study of one mango tree found: 1 tree → 5 000 aphids → 40 ladybirds → 2 birds. Sketch the pyramid of numbers and explain its shape.
Solution: The bottom bar (1 tree) is very narrow, the next bar (5 000 aphids) is very wide, and then the bars narrow again (40, then 2). The pyramid is inverted at the base because one large producer can feed thousands of small consumers. A pyramid of numbers ignores the size of organisms. A pyramid of biomass or of energy for the same chain would have a wide base, because the tree has far more mass and energy than all the aphids.
Tip
Memory hook: the 10% rule: divide by 10 at each step up.
Exam technique: when asked why food chains are short, give the full chain of reasoning: energy is lost as heat in respiration, in faeces and in uneaten parts, so too little energy is left to support another level.
Producers in a field fix 80 000 kJ. Using the 10% rule, how much energy reaches the secondary consumers?
Solution: primary consumers = 0.10 × 80 000 = 8 000 kJ; secondary consumers = 0.10 × 8 000 = 800 kJ.
Rabbits eat grass containing 3 000 kJ and build 270 kJ into their bodies. Calculate the efficiency of transfer.
Solution: (270 ÷ 3 000) × 100 = 9%.
Explain why a pyramid of energy can never be inverted.
Solution: Each trophic level gets its energy only from the level below, and some energy is always lost as heat, in faeces and in uneaten parts. Each level therefore always has less energy than the one below it.
Explain why more people can be fed from a hectare of beans than from a hectare used to raise cattle for beef.
Solution: Eating beans makes people primary consumers, so they receive the energy directly from the producer. When the land feeds cattle first, about 90% of the energy is lost in the cattle, so people eating beef receive only about a tenth as much.
Summary
Sources and credits
The explanation of the three ecological pyramids, the worked calculations, the Rwandan examples and the practice questions are written by Ganzaa.
