Introduction to Environmental Biology
Senior 2 · Biology
Introduction to Environmental Biology
Lesson 2 of 4
18 min
Let's begin
Plant the same bean seed in Musanze and in Bugesera and it will grow differently. Musanze is cooler, wetter and has rich volcanic soil; Bugesera is warmer and drier. The seed is the same, but its surroundings are not.
Everything around a living thing that affects it is a factor. Some factors are other living things, and some are physical conditions. Farmers, foresters and fish managers work by understanding and controlling these factors.
Biotic factors are the living organisms in an ecosystem and the ways they affect each other.
Organisms interact in several ways:
| Interaction | What happens | Example |
|---|---|---|
| Predation | a predator kills and eats its prey | a lion hunting an impala in Akagera |
| Competition | organisms struggle for the same limited resource | weeds and maize competing for light, water and minerals |
| Parasitism | a parasite lives on or in a host and harms it | ticks feeding on cattle; tapeworms in the intestine |
| Mutualism | both partners benefit | bees get nectar and pollinate coffee flowers; bacteria in bean root nodules get sugar and supply nitrogen |
| Commensalism | one benefits and the other is not affected | cattle egrets catching insects disturbed by grazing cattle |
Abiotic factors are the physical and chemical conditions of an ecosystem.
Question: In Nyungwe, the forest floor under the tall trees has few green plants, but a sunny gap where a tree has fallen quickly fills with young plants. Name the factor responsible and explain.
Solution: The factor is light, an abiotic factor. The tall trees' leaves absorb most of the sunlight, so plants on the dark forest floor cannot photosynthesise fast enough to grow well. When a tree falls, light reaches the ground, photosynthesis increases, and seedlings grow quickly. Competition for light between the seedlings, a biotic factor, then decides which ones reach the top.
Tip
Memory hook: bio means life, and a- means not. Biotic = living; abiotic = non-living.
Exam technique: to explain an ecological pattern, (1) name the factor, (2) say whether it is biotic or abiotic, and (3) link it to a life process such as photosynthesis, water loss or enzyme activity.
Classify each as biotic or abiotic: rainfall, fungi, soil pH, a tick, temperature, grass.
Solution: Biotic: fungi, a tick, grass. Abiotic: rainfall, soil pH, temperature.
Explain why a tea plantation is called an artificial ecosystem.
Solution: It is planned and maintained by people: they choose one crop, plant it in rows, add fertiliser, remove weeds and control pests. Without this care, it would slowly change back into a natural ecosystem.
Name the type of interaction: (a) weeds and beans in the same field; (b) nitrogen-fixing bacteria in bean root nodules; (c) a fish eagle catching a fish.
Solution: (a) competition; (b) mutualism; (c) predation.
Suggest why fewer fish can live in warm, still pond water than in a cool, fast-flowing stream.
Solution: Less oxygen dissolves in warm water than in cool water, and fast-flowing water mixes in oxygen from the air. The warm, still pond therefore has less dissolved oxygen, an abiotic factor, for fish to breathe.
Summary
Sources and credits
The lists of factors and interactions, the Rwandan examples, the worked example and the practice questions are written by Ganzaa.
