Welcome to "Human Influences on the Environment"!

Hello future Biologists! This chapter is incredibly important because it connects everything we have learned about ecosystems to the world we live in. Human activities—both industrial and agricultural—have massive effects on natural cycles and ecosystems.

Don't worry if some concepts seem complex; we will break them down into simple, manageable pieces. By the end of this chapter, you will understand how human activities cause air and water pollution, habitat destruction via deforestation, and the enhanced greenhouse effect.

Quick Goal: Master the biological consequences of air pollution, water pollution (sewage and fertiliser eutrophication), the enhanced greenhouse effect, and deforestation.


Section 1: Air Pollution

Air pollution primarily comes from burning fossil fuels (such as coal, oil, and petrol) in vehicles, power stations, and industry.

1. Carbon Monoxide (\(CO\))

  • Source: Formed when fossil fuels burn in an incomplete supply of oxygen (incomplete combustion), especially inside vehicle engines and faulty boilers.
  • Biological Consequence: Carbon monoxide is a poisonous, colourless, and odourless gas. When inhaled, it binds irreversibly to haemoglobin in red blood cells to form carboxyhaemoglobin.
  • This prevents haemoglobin from carrying oxygen, drastically reducing the oxygen-carrying capacity of the blood. This can lead to suffocation, unconsciousness, and death.

2. Sulfur Dioxide (\(SO_2\)) and Acid Rain

  • Source: Released from the combustion of fossil fuels (especially coal and heavy oils) that contain sulfur impurities.
  • Formation of Acid Rain: Sulfur dioxide dissolves in water vapour in clouds to form dilute sulfuric acid (\(H_2SO_4\)). When it rains, it falls as acid rain (pH lower than 5.5).
  • Biological Consequences:
    • Aquatic Ecosystems: Acidifies freshwater lakes and rivers, causing toxic aluminium ions to leach into the water, which damages fish gills and kills aquatic organisms.
    • Forests and Soils: Damages tree leaves and root hairs directly, and leaches essential mineral ions (such as magnesium and calcium) out of the soil, stunting plant growth.
    • Corrosion: Corrodes limestone buildings and metal structures.

Section 2: Water Pollution

1. Water Pollution by Sewage

  • Sewage contains high levels of organic waste and aerobic bacteria, as well as dissolved detergents and mineral ions.
  • If untreated sewage enters a water body, decomposers (aerobic bacteria) multiply rapidly and feed on the organic matter, respiring aerobically.
  • This rapid aerobic respiration uses up large amounts of dissolved oxygen in the water (high Biochemical Oxygen Demand).
  • The sharp drop in oxygen levels suffocates fish and other aquatic animals, leading to widespread death of freshwater life.

2. Eutrophication by Leached Fertiliser

Eutrophication occurs when excess mineral ions (specifically nitrates and phosphates) are washed or leached into rivers and lakes from agricultural fertilisers.

Step-by-Step Mechanism of Eutrophication:
  1. Leaching: Heavy rain washes soluble nitrate and phosphate fertilisers from farm soil into nearby streams and lakes.
  2. Algal Bloom: The high concentration of nutrients causes rapid growth and reproduction of algae on the surface of the water (an algal bloom).
  3. Light Blockage: The dense algal layer blocks sunlight from penetrating to aquatic plants growing beneath the surface.
  4. Plant Death: Submerged plants can no longer photosynthesise, so they die.
  5. Decomposition by Bacteria: Decomposers (aerobic bacteria) feed on the dead plant matter and dead algae, multiplying enormously in number.
  6. Oxygen Depletion (Anoxia): As the bacteria respire aerobically, they consume almost all of the dissolved oxygen in the water.
  7. Death of Aquatic Organisms: Fish and other aquatic organisms die due to lack of oxygen.

Section 3: The Greenhouse Effect and Global Warming

The greenhouse effect is a natural process in which greenhouse gases in the Earth's atmosphere absorb and trap re-radiated infrared radiation from the Earth, maintaining temperatures warm enough to support life.

The Enhanced Greenhouse Effect

Human activities release excess greenhouse gases into the atmosphere. This thicker layer of gases traps too much infrared radiation, enhancing the natural greenhouse effect and causing the average global surface temperature to rise (global warming).

The Five Greenhouse Gases and Human Contributions:
  • Water Vapour (\(H_2O\)): Naturally present; increases in the atmosphere as higher temperatures cause more evaporation from oceans.
  • Carbon Dioxide (\(CO_2\)): Produced by the combustion of fossil fuels in transport and power generation, and by the burning and clearance of forests during deforestation.
  • Methane (\(CH_4\)): Released from cattle and livestock farming (digestive processes), bacteria in flooded rice paddy fields, and decomposition of organic waste in landfill sites.
  • Nitrous Oxide (\(N_2O\)): Released from agricultural soil treated with excess nitrogen fertilisers, vehicle exhaust emissions, and industrial chemical processes.
  • CFCs (Chlorofluorocarbons): Synthetic compounds previously used in aerosol propellants, coolants in refrigerators/air conditioning units, and plastic foam manufacturing.
Biological Consequences of Global Warming:
  • Melting polar ice caps and glaciers: Leading to sea level rise and flooding of low-lying coastal habitats.
  • Climate change and extreme weather: Shifts in precipitation patterns causing severe droughts in some areas and flooding in others.
  • Habitat loss and species extinction: Organisms unable to adapt to shifting climate zones or losing their habitats (e.g., polar species, coral reefs bleaching due to warmer oceans).
  • Shifts in species distribution: Migration of pests, disease vectors (such as mosquitoes carrying malaria), and pollinators to new regions.

Section 4: Deforestation (Paper 2)

Deforestation is the permanent, large-scale removal of trees and forests, primarily to clear land for agriculture (cattle ranching and crop farming) and for timber extraction.

Biological Consequences of Deforestation:

1. Leaching of Minerals

  • Trees intercept rainwater and their roots take up minerals from the soil.
  • When trees are removed, heavy rain falls directly on the soil and washes soluble mineral ions (nitrates, phosphates) deep into the soil and into waterways (leaching), leaving the soil barren and nutrient-poor.

2. Soil Erosion

  • Tree roots bind the soil particles together, and the tree canopy shields the soil from the impact of heavy raindrops.
  • Without tree roots, the loose topsoil is rapidly washed away by rain into rivers or blown away by wind (soil erosion), preventing vegetation from regrowing.

3. Disturbance of the Water Cycle

  • Trees take up water through their roots and release huge quantities of water vapour into the atmosphere via transpiration, contributing to cloud formation and rainfall.
  • Removing trees drastically reduces transpiration, leading to less rainfall and drier, drought-prone local climates (desertification).

4. Disturbance of the Balance in Atmospheric Gases

  • Trees absorb \(CO_2\) during photosynthesis and release \(O_2\).
  • Deforestation means fewer trees to remove \(CO_2\) from the atmosphere via photosynthesis.
  • Furthermore, felled trees are frequently burned (combustion) or left to decay by decomposers (aerobic respiration), releasing stored carbon as \(CO_2\).
  • This imbalance raises atmospheric \(CO_2\) levels and lowers \(O_2\) production, intensifying the enhanced greenhouse effect.

Quick Chapter Wrap-Up and Review

Key Takeaways to Memorise:

  • \(CO\): Binds to haemoglobin to form carboxyhaemoglobin, reducing blood oxygen transport.
  • \(SO_2\): Dissolves in atmospheric water vapour to form acid rain, damaging aquatic life, forests, and soils.
  • Eutrophication: Fertiliser leach -> Algal bloom -> Sunlight blocked -> Submerged plants die -> Bacteria decompose matter and respire -> Oxygen depleted -> Fish suffocate.
  • Greenhouse Gases: Water vapour, \(CO_2\), \(N_2O\), \(CH_4\), and CFCs.
  • Deforestation Effects (P2): Leaching, soil erosion, disturbance of the water cycle (reduced transpiration), and disturbance of atmospheric gas balance (\(CO_2\)/\(O_2\)).