Unit 4 Effects of human activities on ecosystems and environmental conservation
Key unit competence: Evaluate the effects of human activities on ecosystems and propose ways of conserving the environment.
Key ideas
- Feeding a growing population requires more food supply. Methods include clearing land, monoculture (growing one crop over a large area), intensive livestock production, irrigation, fertilisers and pesticides. These raise yields but can harm ecosystems.
- Problems of large monocultures: loss of biodiversity, rapid spread of pests and diseases, soil nutrient depletion, heavy use of pesticides and fertilisers. Intensive livestock: large wastes, water pollution, antibiotic overuse, overgrazing.
- Habitat destruction by deforestation, draining wetlands, urban expansion, mining and overgrazing causes loss of species, soil erosion, floods, landslides, reduced rainfall and reduced carbon storage.
- Air pollution: smoke, carbon monoxide, sulphur dioxide, nitrogen oxides and dust from vehicles, factories and burning. Acid rain forms when SO₂ and nitrogen oxides dissolve in rain; it damages trees, lakes, fish, buildings and soils. Excess CO₂ and methane cause global warming.
- Water pollution: sewage, fertilisers (eutrophication), pesticides, industrial chemicals, oil and plastics. Effects: waterborne diseases (cholera, typhoid), death of fish, poisoning of food chains.
- Land pollution: plastic bags and bottles, household waste, e-waste and mining waste. Rwanda banned plastic bags in 2008 and holds monthly Umuganda community work to clean the environment.
- Biomagnification: some pesticides (e.g. DDT) and heavy metals are not broken down; they accumulate in organisms and become more concentrated at each trophic level, so top predators are most harmed.
- Sustainable development meets the needs of the present without harming the ability of future generations to meet theirs. Sustainable resources: renewable energy (solar, hydro, biogas), sustainable forestry (replanting), fishing quotas and closed seasons, recycling.
- Conservation protects species and habitats: national parks (Volcanoes, Nyungwe, Akagera, Gishwati-Mukura), protecting endangered species (mountain gorilla), banning poaching, captive breeding, seed banks, laws, environmental education, agroforestry, terracing, reforestation.
- Extinction causes: habitat loss, overhunting/poaching, pollution, climate change, invasive species (water hyacinth, which blocks lakes and reduces fish and oxygen).
Common misconceptions
- "Monoculture always increases food security." No: it gives high yields but makes crops vulnerable to pests and diseases and lowers biodiversity.
- "Acid rain only affects plants." No: it also acidifies lakes (killing fish), damages buildings and leaches minerals from soil.
- "Conservation means stopping all use of nature." No: sustainable use allows people to use resources without exhausting them.
- "Pesticides harm only the pests they are sprayed on." No: they can kill useful insects and accumulate in food chains (biomagnification).
Worked examples
Example 1. DDT in a lake was 0.02 ppm in water, 5 ppm in small fish and 2000 ppm in fish-eating birds. Explain these figures.
Solution
DDT is not broken down or excreted easily, so it is stored in body fat. Each small fish eats much plankton and each bird eats many fish, so DDT becomes more concentrated at each trophic level: biomagnification. Top predators like birds are poisoned (thin eggshells, death).
Example 2. Explain how draining a wetland in Kigali for building affects the environment.
Solution
Wetlands are habitats for many plants, birds and fish; draining destroys them and reduces biodiversity. Wetlands also store floodwater and filter pollutants, so floods and water pollution increase downstream. Peat and plants that store carbon are lost.
Practice questions
State three advantages and three disadvantages of monoculture.
Show answer
Advantages: high yields, easy mechanisation, simple management and marketing. Disadvantages: pests and diseases spread fast, loss of biodiversity, soil nutrients depleted, more pesticides and fertilisers needed.
Common mistake: Giving only advantages.
Explain how acid rain is formed and two of its effects.
Show answer
Burning fuels releases sulphur dioxide and nitrogen oxides, which dissolve in rainwater to form sulphuric and nitric acids. Effects: damages leaves and kills trees, acidifies lakes killing fish, corrodes buildings, leaches soil minerals.
Common mistake: Saying CO₂ alone causes acid rain.
State four causes of water pollution and one effect of each.
Show answer
Sewage (waterborne diseases); fertilisers (eutrophication); pesticides (poison aquatic life); industrial chemicals/heavy metals (poisoning, biomagnification); oil (coats birds and fish); plastics (choke animals).
Common mistake: Listing causes without effects.
Explain biomagnification using an example.
Show answer
A non-biodegradable chemical such as DDT is taken up by plankton; small fish eat much plankton and store it; birds eat many fish and store even more; the concentration increases at each trophic level, poisoning top predators.
Common mistake: Saying it decreases along the chain.
Define sustainable development and give three examples of sustainable practices.
Show answer
Development that meets present needs without compromising future generations' ability to meet theirs. Examples: renewable energy, replanting trees, fishing quotas, recycling, terracing and agroforestry.
Common mistake: Equating it with fast economic growth only.
Give four causes of extinction of species.
Show answer
Habitat destruction; overhunting/poaching; pollution; climate change; invasive species; disease.
Common mistake: Listing natural features of species.
State four ways in which Rwanda conserves its environment.
Show answer
National parks and protected areas; ban on plastic bags; Umuganda; tree planting/reforestation; terracing; protecting wetlands; laws against poaching; environmental education.
Common mistake: Giving examples from other countries only.
Explain three effects of deforestation.
Show answer
Soil erosion and landslides (no roots); loss of habitats and biodiversity; less rainfall (less transpiration); more CO₂ and global warming; floods.
Common mistake: Saying deforestation increases rainfall.
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 main gas causing global warming from burning fuels is: a. cholera and typhoid 2. Sewage in drinking water can cause: b. sustainable development 3. Rwanda banned plastic bags in: c. 2008 4. Using resources without harming future generations is: d. carbon dioxide Show answer
1 → d (carbon dioxide) · 2 → a (cholera and typhoid) · 3 → c (2008) · 4 → b (sustainable development)
- It traps heat in the atmosphere.
- They are waterborne diseases.
- This reduces land pollution.
- It balances present and future needs.
Drag and drop
Choose the correct word or phrase from the box for each gap. One word in the box is not needed.
an endangered species · Volcanoes National Park · deforestation · an invasive species- Cutting down forests is .
- A species that is introduced and spreads harmfully is .
- The mountain gorilla is protected mainly in .
Show answer
1: deforestation · 2: an invasive species · 3: Volcanoes National Park
- It destroys habitats.
- Example: water hyacinth.
- It is an endangered species.
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. Which is a renewable energy source? a. carbon monoxide 2. Biogas is produced from: b. rapid spread of pests and diseases 3. A disadvantage of monoculture is: c. Solar energy 4. A gas from car exhausts that reduces oxygen carried by blood is: d. animal dung and plant waste Show answer
1 → c (Solar energy) · 2 → d (animal dung and plant waste) · 3 → b (rapid spread of pests and diseases) · 4 → a (carbon monoxide)
- It will not run out.
- It is a renewable fuel.
- The same crop everywhere helps pests.
- It binds to haemoglobin.
Drag and drop
Choose the correct word or phrase from the box for each gap. One word in the box is not needed.
overgrazing · extinction · monoculture · agroforestry- Growing a single crop over a large area is .
- The permanent loss of all members of a species is .
- Growing trees together with crops is .
Show answer
1: monoculture · 2: extinction · 3: agroforestry
- It reduces biodiversity.
- It cannot be reversed.
- Trees protect soil and give fodder and fuel.