Outline the causes and describe the consequences of deforestation, including its effects on: biodiversity, extinction, loss of soil, flooding, and concentration of carbon dioxide in the atmosphere.
Describe the impacts humans have through:
(a) over-harvesting of plant and animal species
(b) introducing a non-native species to an ecosystem
Describe the harmful effects of:
(a) water pollution by untreated sewage and nitrogen-containing fertilisers leading to eutrophication
(b) air pollution by greenhouse gases (carbon dioxide and methane), contributing to global warming
(c) pollution due to insecticides and herbicides
(d) non-biodegradable plastics in aquatic and terrestrial ecosystems
Deforestation means cutting down or clearing large areas of forest — usually to use the land or the wood for human purposes.
Causes of deforestation
Agriculture: Forests are cleared to create farmland for growing crops or raising livestock (animals like cattle).
Logging: Trees are cut down for their timber (wood), which is used to make furniture, buildings, and paper.
Urban development: Forests are cleared to build roads, towns, cities, and factories.
Mining: Forests are removed to access minerals and fuels beneath the ground.
Consequences of deforestation
1. Reduction in Biodiversity
Biodiversity means the variety of different species (types of living organisms) living in an area.
Forests — especially tropical rainforests — are home to an enormous number of different species of plants, animals, fungi, and other organisms.
When forests are cleared, these species lose their habitat (the place where they live). They lose their food sources, shelter, and breeding grounds.
With nowhere to live, many species decline in number very rapidly.
The result is a dramatic fall in biodiversity — fewer different types of organisms remain in the area.
2. Extinction
Extinction means that every individual of a species has died — the species no longer exists anywhere on Earth.
When large areas of forest are destroyed, some species cannot survive anywhere else. If their numbers fall too low, they cannot reproduce fast enough to keep the population going.
Once a species is extinct, it is gone forever — it can never return.
Many species unique to certain forests — especially those found nowhere else on Earth — are at very high risk of extinction when those forests are cut down.
3. Loss of Soil (Soil Erosion)
Tree roots hold the soil together and anchor it in place. The leaves and branches of trees also act like an umbrella — they intercept (catch) rainfall and slow it down before it hits the ground.
When trees are removed, the soil is left bare and exposed.
Rainwater hits the bare soil with full force, washing it away. This process is called soil erosion.
Wind also blows away dry, unprotected soil.
The top layer of soil — called topsoil — is the most fertile layer, full of minerals and nutrients that plants need to grow. Once it is washed or blown away, the land becomes much less productive and may eventually turn into desert (this is called desertification).
4. Flooding
Trees absorb large amounts of water through their roots. They also intercept rainfall, slowing the movement of water into the ground and rivers.
When forests are removed, water is no longer absorbed by tree roots. Rainwater flows rapidly across the bare land surface (this is called surface run-off) and pours straight into rivers.
Rivers fill up much faster and can overflow, causing floods.
Deforestation therefore increases both the frequency (how often) and severity (how bad) of flooding.
5. Increased Carbon Dioxide in the Atmosphere
Trees absorb carbon dioxide (CO₂) from the atmosphere and use it in photosynthesis to make food. This means living forests act as a carbon store — they lock up carbon in their wood, roots, and leaves.
When trees are cut down and burned (a common way to clear land quickly), the carbon stored in the wood is released back into the atmosphere as CO₂.
When trees are cut down and left to rot (decompose), decomposers (bacteria and fungi) break down the wood and also release CO₂.
With fewer trees left standing, less CO₂ is absorbed from the atmosphere by photosynthesis.
The combined effect of releasing CO₂ from burning/rotting wood and having fewer trees to absorb CO₂ means the concentration of CO₂ in the atmosphere rises.
This contributes to the greenhouse effect and global warming (covered in more detail in Section 3b).