Effects of Human Activities on Ecosystems and Environmental Conservation
Senior 3 · Biology
Effects of Human Activities on Ecosystems and Environmental Conservation
Lesson 2 of 5
25 min
Satellites have photographed Rwanda for decades. Two Landsat images of the Gishwati forest area, north-east of Lake Kivu, tell a clear story. A report prepared by Rwanda's national authorities for the FAO explains what happened. Until the mid-1970s Gishwati was a mountain rainforest of about 280 km². In the early 1980s, 100 km² was turned into pasture for cattle ranches and another 100 km² into pine plantations, and a growing population needed more grazing, farmland and firewood. After the 1994 Genocide against the Tutsi, returning refugees were resettled in the area and more land was cleared. By 2005 more than 90% of Gishwati was cultivated, and only 600 hectares of natural forest remained. The area then suffered floods, landslides, soil erosion, falling soil fertility and muddy, silted rivers. This lesson looks at the two biggest ways people damage ecosystems: destroying habitats and polluting them.
Definition
Habitat destruction: the damage or removal of a natural habitat so that it can no longer support the species that lived there. Example: clearing a forest for farms.
Deforestation: the permanent clearing of forest for another use.
Habitat fragmentation: the breaking up of a large habitat into small, separate patches.
Pollution: the release into the environment of substances or energy (such as heat or noise) that harm living things. The harmful substance is a pollutant.
Biodegradable: able to be broken down by decomposers. Most plastics are non-biodegradable.
| Cause | Example |
|---|---|
| Farming expands onto natural land | forests cleared for crops and cattle; marshes drained for farming |
| Settlements, towns and roads | houses and roads built on farmland, wetlands and hillsides |
| Cutting trees for firewood, charcoal and timber | 76% of households cooked with firewood in 2022 |
| Mining and quarrying | illegal mining pits in Gishwati, later restored |
| Wetland drainage | the Rugezi marsh, drained around 2000 (Lesson 5) |
Effects: species lose food, shelter and breeding sites and may disappear; small fragments hold small populations that are easily wiped out; bare slopes lose soil in heavy rain, causing landslides and floods downstream; springs dry up; and burned or rotting vegetation releases carbon dioxide (Unit 3).
Worked example 1: how much forest was lost?
Use the figures in the FAO country report: about 280 km² of forest until the mid-1970s, and 600 hectares of natural forest in 2005. (.) (a) Convert the original area to hectares. (b) What percentage of the forest remained in 2005? (c) What percentage was lost?
Note: other reports give different figures, because they measure from different years and boundaries. Always say where a figure comes from and what year it describes.
| Type and main causes | Consequences | Prevention |
|---|---|---|
| Air: smoke from firewood and charcoal stoves; vehicle exhaust; factories; burning rubbish | coughs, pneumonia and other lung diseases; soot and acid gases damage plants; carbon dioxide adds to global warming | improved cookstoves, gas or electricity for cooking, vehicle inspections, filters on chimneys, no open burning |
| Water: raw sewage; fertiliser and manure runoff; soil washed off bare land; waste from mines and factories | diseases such as cholera and typhoid; eutrophication; fish die; silted rivers and lakes | toilets and sewage treatment; careful fertiliser use; tree and grass strips along rivers; treating factory waste |
| Land: plastics and other solid waste; spilled oil and chemicals; mining spoil | blocked drains and floods; animals eat or are trapped by plastic; poisoned soil | reduce, reuse, recycle; waste collection and managed landfills; banning plastic bags |
| Noise: traffic, machines, loud music | stress, hearing damage, disturbed wildlife | limits on noise, quieter machines |
Worked example 2: a river below a sewage outlet
Students measured dissolved oxygen in a river at points above and below an outlet that releases untreated sewage. (Illustrative data.)
Figure 1: Dissolved oxygen above and below a sewage outlet (illustrative data).
(a) By how much does the oxygen fall from just above the outlet to 0.5 km below it? (b) Explain the fall. (c) Over what distance is the water unsuitable for most fish? (d) Explain the recovery.
Warning
Common mistakes. (1) "Sewage kills fish because it is poisonous": the main cause is the bacteria that decompose it, which use up the oxygen. (2) Calling every change "pollution": pollution means a substance or energy that harms living things. (3) Forgetting that habitat destruction and pollution work together: a cleared hillside also pollutes rivers with soil.
GCSE Biology: Deforestation and Land Use (Cognito). Watch for the reasons forests are cleared and the effects on biodiversity and carbon dioxide.
Tip
Exam technique: "cause → effect → solution". Pollution questions usually give marks for naming the pollutant and its source, explaining its effect on living things, and suggesting a realistic way to prevent it.
Practice: habitat destruction and pollution
Key points
Sources and credits
Written by Ganzaa: the tables, Figure 1 (illustrative data), the worked examples and the practice questions.
