Unit 2 Introduction to environmental biology
Key unit competence: Apply environmental concepts in describing feeding relationships and energy flow in an ecosystem.
Key ideas
- Ecology is the study of how organisms interact with each other and with their environment. The environment is everything around an organism: living (biotic) and non-living (abiotic) factors.
- Key terms: a habitat is the place where an organism lives; a population is all organisms of one species in an area; a community is all the populations living together in an area; an ecosystem is a community plus its physical environment, interacting; the biosphere is the part of the Earth where life exists.
- An ecosystem = biotope (the physical environment) + biocenosis (the community of living things). Examples: Lake Kivu, Nyungwe forest, Akagera savanna, a school garden.
- Abiotic factors: light, temperature, water, humidity, wind, soil (pH, minerals), altitude. Biotic factors: other organisms, such as predators, prey, competitors, parasites, pollinators and decomposers.
- Producers (green plants, algae) make food by photosynthesis using light energy. Consumers eat other organisms: primary consumers (herbivores) eat plants, secondary consumers eat herbivores, tertiary consumers eat secondary consumers. Decomposers (bacteria, fungi) break down dead remains and return minerals to the soil.
- A food chain shows who eats whom; arrows point in the direction energy flows, from the food to the feeder. Example: grass → grasshopper → frog → snake → eagle. Each step is a trophic level.
- A food web is many interconnected food chains in an ecosystem. It is more stable than one food chain, because consumers have other food if one species is lost.
- Ecological pyramids show trophic levels as bars: a pyramid of numbers (number of organisms), a pyramid of biomass (dry mass) and a pyramid of energy (energy at each level). The pyramid of energy is always upright.
- Energy flow: the Sun is the source of energy. Only a small part of the energy at one level (about 10%) passes to the next; the rest is lost as heat in respiration, in movement, in faeces and urine, and in parts not eaten. This is why food chains are short (usually 4 or 5 levels).
- Energy flows one way through an ecosystem and is lost as heat, but nutrients (minerals) are recycled by decomposers.
Common misconceptions
- "Arrows in a food chain show who eats whom, so they point from the eater to the food." No: arrows point from the food to the organism that eats it, showing the direction of energy flow.
- "Decomposers are consumers at the top of the food chain." No: decomposers feed on dead remains from every trophic level and recycle minerals.
- "Energy is recycled in an ecosystem like minerals." No: energy flows one way and is lost as heat; only nutrients are recycled.
- "A pyramid of numbers is always upright." No: one large tree can feed thousands of insects, giving an inverted pyramid of numbers.
Worked examples
Example 1. In the food chain maize → grasshopper → chicken → human, the maize stores 10 000 kJ of energy. About how much energy reaches the human? Explain.
Solution
About 10% passes at each step: grasshopper 1000 kJ, chicken 100 kJ, human about 10 kJ. The rest is lost as heat in respiration, movement, faeces and parts not eaten.
Example 2. Explain why a food web is more stable than a single food chain.
Solution
In a food web most consumers have several food sources. If one species decreases, the consumer can eat another, so populations do not collapse; in a single chain, losing one link affects every level above it.
Practice questions
Define the terms: (a) habitat; (b) population; (c) community; (d) ecosystem.
Show answer
(a) The place where an organism lives. (b) All the organisms of one species in an area. (c) All the populations of different species living together in an area. (d) A community and its physical environment interacting together.
Common mistake: Confusing population with community.
List three abiotic and three biotic factors that affect organisms in a pond.
Show answer
Abiotic: light, temperature, dissolved oxygen, pH, water depth, minerals. Biotic: predators (fish, birds), competitors, parasites, producers (algae, water plants), decomposers.
Common mistake: Listing living things as abiotic.
Construct a food chain with four trophic levels from a Rwandan grassland and label each level.
Show answer
Grass (producer) → grasshopper (primary consumer) → lizard/bird (secondary consumer) → hawk/snake (tertiary consumer).
Common mistake: Drawing arrows from the eater to the food.
Explain why energy decreases along a food chain.
Show answer
At each level energy is lost as heat in respiration, in movement, in faeces and urine, and in parts that are not eaten (bones, roots), so only about 10% passes on.
Common mistake: Saying the energy is destroyed.
Distinguish between a food chain and a food web.
Show answer
A food chain is a single feeding sequence; a food web is many interconnected food chains showing all the feeding relationships in an ecosystem.
Common mistake: Saying a food web has no producers.
State the role of decomposers in an ecosystem.
Show answer
They break down dead plants, animals and waste, releasing minerals (such as nitrates) back into the soil for producers to use; they also clean the environment.
Common mistake: Saying decomposers produce energy.
Give two reasons why a pyramid of numbers may not be upright.
Show answer
One large producer (a tree) may support many small consumers; parasites are many and small on one large host.
Common mistake: Thinking all pyramids must be upright.
Suggest two ways in which humans disturb food chains in Rwanda and one effect of each.
Show answer
Cutting forests removes producers and habitats, so herbivores and predators decrease; using pesticides kills insects that birds eat; overfishing removes fish that other animals depend on; draining wetlands destroys breeding sites.
Common mistake: Giving the activity without its effect.
Matching and drag-and-drop
Matching
Match each item in column A with the correct answer in column B. Write the letter next to each number.
Column A Column B 1. The part of the Earth where life exists is the: a. biosphere 2. Green plants are called producers because they: b. an ecosystem 3. Which pyramid is always upright? c. make food by photosynthesis 4. Nyungwe forest, with its plants, animals, soil and climate, is an example of: d. The pyramid of energy Show answer
1 → a (biosphere) · 2 → c (make food by photosynthesis) · 3 → d (The pyramid of energy) · 4 → b (an ecosystem)
- It includes land, water and the lower air.
- They trap light energy.
- Energy always decreases up the levels.
- It includes living and non-living parts.
Drag and drop
Choose the correct word or phrase from the box for each gap. One word in the box is not needed.
producer (green plant) · food web · heat from respiration · primary consumer (herbivore)- Most energy lost between trophic levels is lost as .
- Many interlinked food chains form a .
- An organism that eats only plants is a .
Show answer
1: heat from respiration · 2: food web · 3: primary consumer (herbivore)
- Respiration releases heat.
- Most animals eat more than one type of food.
- It is at the second trophic level.
Matching
Match each item in column A with the correct answer in column B. Write the letter next to each number.
Column A Column B 1. Each feeding level in a food chain is called a: a. trophic level 2. The study of how organisms interact with each other and their environment is: b. ecology 3. The place where an organism lives is its: c. too little energy is left at the top 4. Food chains rarely have more than five links because: d. habitat Show answer
1 → a (trophic level) · 2 → b (ecology) · 3 → d (habitat) · 4 → c (too little energy is left at the top)
- Producers are the first trophic level.
- Oikos means house or home.
- Example: a fish's habitat is a lake.
- About 90% is lost at each step.
Drag and drop
Choose the correct word or phrase from the box for each gap. One word in the box is not needed.
decomposers · secondary consumer · primary consumer · the direction of energy flow- In a food chain, the arrows show .
- Bacteria and fungi that break down dead organisms are .
- In grass → rabbit → fox, the fox is a .
Show answer
1: the direction of energy flow · 2: decomposers · 3: secondary consumer
- They point from the food to the eater.
- They return minerals to the soil.
- It eats a herbivore.