4d Human Influences on the Environment¶

Part of 4 Ecology and the Environment.

Human activity can alter ecosystems by changing gases, nutrients, water quality and land cover. The topic is mainly about tracing cause and effect from human action to biological consequence.

Learning Objectives¶

Air Pollution¶

Burning fossil fuels releases gases that can cause direct harm to organisms and indirect harm to ecosystems.

Sulfur dioxide: - Released when fossil fuels (particularly coal and oil) containing sulfur are burned. - In the atmosphere, it dissolves in water droplets to form dilute sulfuric acid, which falls as acid rain. - Acid rain makes rivers and lakes too acidic for many aquatic organisms, reducing biodiversity. - It leaches important mineral ions from the soil, preventing plant growth. - It also corrodes metals and erodes limestone structures.

Carbon monoxide: - Released during incomplete combustion of fossil fuels (i.e. when oxygen supply is limited). - Binds irreversibly to haemoglobin in red blood cells, forming a stable compound that cannot carry oxygen. - This reduces the oxygen-carrying capacity of the blood, causing tiredness, unconsciousness or, in severe exposure, death. - In pregnant women, reduced oxygen delivery to the foetus can impair growth and development.

Greenhouse Gases and Global Warming¶

Greenhouse gases absorb infrared radiation re-emitted from the Earth's surface and re-radiate some of it back downwards, trapping heat in the lower atmosphere. This is the greenhouse effect, which naturally maintains the planet's temperature. However, human activities have increased the concentrations of these gases, enhancing the effect and causing global warming.

Major greenhouse gases and their main sources:

Biological consequences of global warming: - Climate change alters the temperature and rainfall patterns of habitats. - Rising sea levels (as glaciers and polar ice melt) can flood coastal habitats. - Loss of habitats and range shifts force species to move or face extinction. - Changes in timing of seasonal events can break ecological relationships between species.

Water Pollution and Eutrophication¶

Sewage pollution: - Sewage discharged into rivers and lakes is rich in nutrients and organic matter. - Decomposers (bacteria) multiply rapidly and respire aerobically to break down the sewage, consuming dissolved oxygen. - Bacteria from the sewage also consume oxygen. - An algal bloom on the water surface blocks light reaching underwater plants, causing them to die. - Dead plant matter provides even more food for decomposers, further reducing oxygen levels. - As dissolved oxygen falls, aquatic animals (fish, invertebrates) die, drastically reducing biodiversity. - Sewage treatment works address this by providing oxygen through aeration (stirring or air injection).

Eutrophication from leached fertilisers: - Fertilisers applied to agricultural land can be washed by rain into nearby rivers and lakes (leaching). - The excess nitrate and phosphate ions stimulate explosive algal growth (an algal bloom) on the water surface. - The algae block light from reaching aquatic plants below, causing them to die. - Dead algae and plants are decomposed by bacteria, which consume dissolved oxygen (the same sequence as sewage pollution). - Low oxygen kills aquatic animals, reducing biodiversity — this is eutrophication.

Biology-Only Content: Deforestation¶

This content is required for Biology-only students and is not required for Combined Science students.

Deforestation is the large-scale removal of trees from an area, usually to create agricultural land or for timber.

Effects of deforestation: - Leaching: trees normally take up nutrients and minerals from the soil through their roots. Without trees, these minerals are washed by rain into nearby rivers and lakes, contributing to eutrophication. - Soil erosion: tree roots bind soil particles together and intercept rainfall. Without roots, soil is easily washed away or blown away, reducing land productivity. - Disruption of water cycle: trees release water vapour into the atmosphere through transpiration (evapotranspiration). Deforestation reduces this, altering local rainfall patterns and water cycles. - Imbalance in atmospheric gases: trees remove CO₂ from the atmosphere by photosynthesis and store carbon in their wood. Burning or rotting deforested trees releases this stored carbon as CO₂. Combined with reduced photosynthesis, deforestation increases atmospheric CO₂ levels and decreases oxygen production, contributing to the enhanced greenhouse effect.

Deforestation¶

• Removing trees increases leaching and soil erosion because roots no longer stabilise the soil and intercept rainwater.
• Deforestation reduces evapotranspiration, disrupts the carbon cycle and changes the balance of atmospheric gases by reducing photosynthesis and often increasing combustion.
• Its ecological impact is therefore both local and global.

Key Terms¶

• Greenhouse gas: a gas in the atmosphere that absorbs and re-emits infrared radiation, contributing to the warming of the Earth's surface.
• Greenhouse effect: the trapping of heat energy in the lower atmosphere by greenhouse gases.
• Global warming: a long-term increase in average global temperature caused by elevated greenhouse gas concentrations.
• Acid rain: rain made acidic by dissolved pollutants such as sulfur dioxide.
• Eutrophication: nutrient enrichment of water, leading to excessive algal growth, death of aquatic plants and oxygen depletion.
• Algal bloom: a rapid and excessive growth of algae, often triggered by high nutrient levels.
• Leaching: the washing of dissolved nutrients out of the soil by water.
• Soil erosion: the removal of topsoil by wind or water, particularly when roots are no longer present.
• Evapotranspiration: the combined loss of water from the soil surface (evaporation) and from plant leaves (transpiration).
• Deforestation: the large-scale removal of trees from an area.
• Carbon monoxide: a toxic gas produced by incomplete combustion that binds irreversibly to haemoglobin.

Connected Pages¶

• 4a The Organism in the Environment
• 4c Cycles within Ecosystems
• 5a Food Production
• IGCSE Biology Key Terms Glossary