5. Human Impact on Ecosystems

Human activities have dramatically altered Earth's ecosystems, especially since industrialization. The GED® tests your ability to identify these impacts and their ecological consequences.

Habitat destruction and fragmentation:

• The single largest driver of biodiversity loss.
• Deforestation: clearing forests for agriculture, lumber, or development. Destroys habitat, releases stored carbon, increases erosion, reduces water retention.
• Habitat fragmentation: large contiguous habitat is broken into isolated patches. Populations become smaller and isolated, increasing extinction risk (reduced gene flow, harder to find mates or food).
• Wetland drainage: wetlands filter water and control flooding — draining them removes these services.

Invasive (non-native) species:

• Introduced to new ecosystems (intentionally or accidentally) where they have no natural predators.
• Can outcompete native species, alter food webs, and reshape habitats.
• Examples: zebra mussels (Great Lakes), kudzu vine (southeastern US), cane toads (Australia), Burmese pythons (Florida Everglades), brown tree snakes (Guam — caused extinction of most native birds).

Overexploitation:

• Harvesting species faster than they can reproduce.
• Overfishing has depleted many ocean fish stocks.
• Hunting drove the passenger pigeon to extinction; demand for ivory threatens elephants.

Pollution and eutrophication:

• Agricultural fertilizer runoff carries excess nitrogen and phosphorus into lakes and rivers.
• This nutrient overload causes algae blooms (eutrophication).
• When algae die and decompose, bacteria consume oxygen — causing hypoxia (oxygen depletion) that kills fish and other aquatic life. The Gulf of Mexico "dead zone" near the Mississippi River is a major example.

Introduced diseases:

• White-nose syndrome fungus has killed millions of North American bats.
• Chytridiomycosis fungal disease has driven dozens of frog species to extinction.

⚠️ All these impacts are interconnected. Deforestation increases runoff → eutrophication; fragmented habitats make populations more vulnerable to disease; invasive species spread faster in disturbed ecosystems.