Introduction to Environmental Biology
Senior 2 · Biology
Introduction to Environmental Biology
Lesson 3 of 4
18 min
Let's begin
Pull all the grass out of Akagera and, within a season, the zebras, buffaloes and impalas would starve. Soon after, so would the lions and hyenas that eat them. Every animal in the park depends, directly or indirectly, on plants.
Food chains and food webs show who eats whom. They let ecologists predict what will happen when a species is removed, added or hunted.
A food chain is a single line of organisms through which food, and the energy it contains, passes as one organism eats another. The arrows point from the food to the eater, showing the direction energy flows.
grass → grasshopper → chameleon → snake → hawk
Each step in a food chain is a trophic level (feeding level):
| Trophic level | Name | Example above |
|---|---|---|
| 1st | producer: makes food by photosynthesis | grass |
| 2nd | primary consumer: a herbivore that eats producers | grasshopper |
| 3rd | secondary consumer: eats primary consumers | chameleon |
| 4th | tertiary consumer | snake |
| 5th | quaternary consumer, here the top (apex) consumer | hawk |
An aquatic example from Lake Kivu is: phytoplankton (microscopic algae) → zooplankton (tiny crustaceans) → isambaza → African fish eagle.
Decomposers, mainly bacteria and fungi, feed on the dead bodies and waste of every level. Detritivores, such as earthworms, millipedes and termites, eat dead material (detritus) and break it into smaller pieces. Together they recycle minerals back to the producers.
In real ecosystems, most animals eat more than one kind of food, and most are eaten by more than one predator. A food web is a set of interlinked food chains, showing all the feeding relationships in an ecosystem.
A simple Akagera savanna food web:
Notice that the impala is at the same trophic level wherever it appears, while the hyena can act as a predator and as a scavenger. An animal can occupy different trophic levels in different chains. A person who eats beans is a primary consumer, and the same person eating tilapia is a secondary or tertiary consumer.
Question: In the food web above, poachers kill most of the lions. Predict and explain two effects.
Solution: (1) The numbers of zebra, impala and buffalo will rise, because fewer are being eaten. (2) With so many grazers, the grass will be over-grazed and may be eaten faster than it regrows. The grazers then compete more for food and may starve in the dry season, and the bare soil may erode. Other predators, such as leopards and hyenas, may increase because more prey is available. One removal spreads through the whole web.
Tip
Exam technique: the arrow means "is eaten by" and points towards the eater, in the direction energy flows. Drawing it the other way is the most common way to lose marks.
When predicting changes in a web, follow every arrow into and out of the organism, and say why each population changes.
In the chain maize → rat → snake → hawk, name the producer, the primary consumer and the tertiary consumer.
Solution: producer: maize; primary consumer: rat; tertiary consumer: hawk.
What is the difference between a food chain and a food web?
Solution: A food chain is one line of feeding relationships. A food web is many food chains linked together, showing that most organisms eat, and are eaten by, more than one species.
Explain why a food web is more stable than a single food chain.
Solution: In a web, most consumers have several food sources. If one prey species declines, the predator can switch to another, so it does not starve. In a single chain, losing one link would starve every organism above it.
State the role of decomposers in an ecosystem.
Solution: They break down dead organisms and waste, releasing minerals (such as nitrates) back into the soil or water, where producers can absorb and reuse them.
Write a food chain of four organisms from Lake Kivu.
Solution: phytoplankton → zooplankton → isambaza → African fish eagle (or another sensible chain that starts with a producer).
Summary
Sources and credits
The Rwandan food chains and food web, the worked example and the practice questions are written by Ganzaa.
