Chapter 23: Global Ecology – Human Impacts and Environmental Change

Global Ecology

Introduction to Global Ecology

Global ecology examines ecological phenomena and environmental changes at the scale of the entire planet. It focuses on how human activities have altered Earth's systems, including the atmosphere, land, and biodiversity.

• Global changes: Large-scale transformations in Earth's systems due to natural and anthropogenic factors.

• Human impact: Humanity has significantly changed the Earth's environment, especially in recent centuries.

Atmospheric Envelope & Greenhouse Effect

Composition of Earth's Atmosphere

The Earth's atmosphere is a mixture of gases that play crucial roles in climate and life support.

• 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 trap heat by absorbing infrared radiation emitted by the Earth and reemitting it back to the surface.

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

• Mechanism: Solar energy reaches Earth, some is reflected, and some is absorbed. Greenhouse gases absorb and reemit infrared radiation, maintaining Earth's temperature.

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

Human Activity and the Global Nitrogen Cycle

Alteration of the Nitrogen Cycle

Human activities, especially agriculture and industry, have greatly increased the amount of fixed nitrogen entering ecosystems.

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

• Human sources: 130 Tg/yr (Vitousek, 1994).

• 1 teragram (Tg): $10^{12}$ grams.

Consequences:

• Reduced biodiversity due to nutrient overload (eutrophication).

• Decreased ecosystem health and stability.

Example: Excess fertilizer runoff leads to algal blooms in aquatic systems, depleting oxygen and harming aquatic life.

Changes in Land Cover

Human Alteration of Land Surfaces

Humans have transformed 1/3 to 1/2 of the ice-free land surface, mainly through agriculture and urbanization. This has led to habitat loss and fragmentation globally, affecting both forest and non-forest habitats.

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 vital for supporting biodiversity and regulating global climate. Deforestation is a major global issue, especially in regions like the Amazon.

• Importance: Home to at least half of Earth's species; influence climate through carbon storage and water cycling.

• Deforestation rates: In 1989, tropical rainforests were being destroyed at a rate of 142,000 km2 per year (1.8% of standing cover).

Consequences:

• Loss of species and ecosystem services (nutrient cycling, water regulation).

• Fragmentation and edge effects.

Edge Effects

Edge effects occur when forest fragments are exposed to increased sunlight and wind, altering the physical environment at the edges.

• Abiotic changes: Edges are hotter, drier, and have higher solar intensity.

• Susceptibility: Edges are more prone to disturbances such as fire, disease, and tree fall.

Example: Fragmented forests in the Amazon show increased vulnerability to environmental stressors.

Human Influence on Atmospheric Composition

Changes in Atmospheric CO2

Human activities have dramatically increased atmospheric CO2 concentrations, especially since the Industrial Revolution.

• Historical record: Ice core samples show fluctuations in CO2 over 160,000 years, with recent sharp increases.

• Industrial Revolution: Marked rise in CO2 due to fossil fuel burning (5,600 Tg/yr).

• Interruptions: World War I, Great Depression, World War II temporarily slowed CO2 emissions.

Consequences:

• Global warming due to enhanced greenhouse effect.

• Impacts on biodiversity and ecosystem stability.

Environmental Impacts on Biodiversity

Major Threats to Biodiversity

Biodiversity is threatened by several human-induced factors, leading to species loss and ecosystem degradation.

• Habitat loss: Conversion of natural habitats for human use.

• Alien species: Introduction of nonnative species through colonization, agriculture, and accidental transport (e.g., zebra mussels).

• Pollution: Includes acid deposition, eutrophication, and global warming.

• Overexploitation: Excessive harvesting of wild populations, leading to population declines.

• Disease: Spread of new diseases due to human encroachment.

Alien Species and Island Ecosystems

Islands are particularly vulnerable to invasive species, which can disrupt native communities and outcompete or prey on native species.

• Example: Mongooses introduced to Hawaii to control rats also prey on native birds, causing declines in native populations.

Pollution and Biodiversity

• Acid deposition: Sulfur and nitrogen compounds from fossil fuels lower pH of soils and water, harming organisms.

• Eutrophication: Nutrient enrichment leads to algal blooms and oxygen depletion in aquatic systems.

• Global warming: Rising temperatures threaten sensitive ecosystems, such as coral reefs.

Value of Biodiversity

Direct and Indirect Values

Biodiversity provides both direct and indirect benefits to humans and ecosystems.

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

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

Working Toward a Sustainable Society

Strategies for Sustainability

To maintain Earth's ecological balance, society must adopt sustainable practices.

• Protect natural ecosystems.

• Conserve energy.

• Recycle resources.

• Educate others about environmental stewardship.