Chapter 23: Global Ecology – Human Impacts on the Biosphere

Global Ecology

Introduction to Global Ecology

Global ecology examines ecological phenomena and environmental changes at the planetary scale, focusing on how human activities have transformed Earth's systems. This field integrates atmospheric science, biogeochemistry, and biodiversity studies to understand the interconnectedness of Earth's processes.

• Global changes include alterations in atmospheric composition, nutrient cycles, and land cover.

• Humanity has become a major driver of ecological change, impacting climate, habitats, and species diversity worldwide.

Atmospheric Envelope & Greenhouse Effect

Composition of the Atmosphere

The Earth's atmosphere is a mixture of gases that envelops the planet and supports life. The composition of clean, dry air at the Earth's surface is as follows:

Gas	Percentage (%)
Nitrogen (N2)	78.08
Oxygen (O2)	20.94
Argon (Ar)	0.93
Carbon Dioxide (CO2)	0.003
Ozone (O3)	0.00005

The Greenhouse Effect

The greenhouse effect is a natural process that warms the Earth's surface. Certain gases in the atmosphere trap heat by absorbing infrared radiation emitted by the Earth and reemitting it back to the surface.

• Greenhouse gases include water vapor, carbon dioxide, methane, ozone, nitrous oxide, and chlorofluorocarbons (CFCs).

• Without the greenhouse effect, Earth's average temperature would be much lower, making it less hospitable for life.

Example: Increased concentrations of greenhouse gases due to human activities enhance the greenhouse effect, leading to global warming.

Human Impacts on Global Cycles

Human Alteration of the Nitrogen Cycle

Human activities, especially agriculture and industry, have dramatically altered the global nitrogen cycle. The introduction of synthetic fertilizers and fossil fuel combustion has increased the amount of biologically available nitrogen in ecosystems.

• Natural sources of fixed nitrogen: N-fixing bacteria/plants (100 Tg/yr), marine environments (5–20 Tg/yr), lightning (10 Tg/yr).

• Human sources: 130 Tg/yr (now exceed natural sources).

• 1 teragram (Tg) = $10^{12}$ grams.

Consequences:

• Reduced biodiversity due to nutrient imbalances.

• Decreased ecosystem health (e.g., eutrophication of water bodies).

Changes in Land Cover

Human activities have transformed between one-third and one-half of the ice-free land surface, primarily through agriculture and urbanization. This has led to widespread habitat alteration and loss.

Country/Region	Habitat Type	% Remaining
North America	Tallgrass prairie	1
Sri Lanka	Thorn scrub	25
United Kingdom	Heathland	27
Nigeria	Mangrove	50
Paraguay	Chaco	57
South Africa	Fynbos	67

Tropical Deforestation

Tropical forests are critical for supporting biodiversity and regulating global climate. However, deforestation is a major global problem, especially in the Amazon and Southeast Asia.

• By 1989, tropical rain forests had been reduced to about 8 million km2, less than half their prehistoric cover.

• Annual loss rate: 142,000 km2 per year (1.8% of standing cover).

• Deforestation leads to loss of species, ecosystem services, nutrients, and changes in the hydrological cycle.

Edge Effects: Forest fragmentation exposes edges to more sunlight and wind, making them hotter, drier, and more susceptible to disturbances such as fire, disease, and tree fall.

Human Influence on Atmospheric Composition

Historical Changes in Atmospheric CO2

Ice core data reveal that atmospheric CO2 concentrations have fluctuated over the past 160,000 years, with low levels during ice ages and high levels during interglacial periods.

• Recent centuries have seen a dramatic increase in CO2 due to fossil fuel burning, especially since the Industrial Revolution.

• Current fossil fuel emissions (5,600 Tg/yr) exceed the increase in atmospheric CO2 (3,500 Tg/yr).

Consequences: Enhanced greenhouse effect (global warming), impacts on biodiversity.

Environmental Impacts on Biodiversity

Major Threats to Biodiversity

• Habitat loss: Destruction and fragmentation of habitats reduce species populations and genetic diversity.

• Alien species: Introduction of nonnative species through colonization, horticulture, agriculture, and accidental transport (e.g., zebra mussels) can disrupt native ecosystems, especially on islands.

• Pollution: Includes acid deposition, eutrophication, global warming, ozone depletion, and organic toxins.

• Overexploitation: Harvesting wild populations at unsustainable rates leads to population declines and possible extinction.

• Disease: Human encroachment exposes wildlife to new diseases, threatening population health.

Value of Biodiversity

• Direct value: Medicinal, agricultural, biological pest control, pollination, and consumptive uses.

• Indirect value: Ecosystem services such as nutrient cycling, waste disposal, water provision, soil erosion prevention, climate regulation, and ecotourism.

Working Toward a Sustainable Society

• Protect natural ecosystems.

• Conserve energy and resources.

• Recycle materials.

• Educate others about environmental stewardship.

Additional info: The notes integrate concepts from global ecology, biogeochemical cycles, and conservation biology, providing a comprehensive overview suitable for General Biology students.