The sixth mass extinction is being caused mainly by human activities that rapidly reduce populations, destroy ecosystems and prevent species from adapting or recovering. Human population growth is an overarching cause because it increases demand for land, food, energy and other resources.
The Reasoning and Mechanisms
Unlike earlier mass extinctions, the current biodiversity crisis is primarily anthropogenic, meaning caused by humans. Several pressures often act together, so a species weakened by habitat loss may become more vulnerable to disease, invasive species or climate change.
| Cause | How it increases extinction risk |
|---|---|
| Habitat loss and fragmentation | Deforestation, agriculture and urbanization reduce available habitat and isolate small populations, limiting reproduction and gene flow. |
| Overexploitation | Hunting, fishing and harvesting remove organisms faster than populations can replace them. |
| Pollution | Pesticides, plastics, excess nutrients and toxic chemicals reduce survival or reproductive success. Some pollutants undergo biomagnification. |
| Invasive alien species | Introduced species may outcompete, prey on or transmit pathogens to endemic species that lack suitable defences. |
| Climate change | Rapid changes in temperature, rainfall, ocean chemistry and habitat distribution may exceed species’ tolerance limits. |
| Emerging diseases | Human transport and ecological disruption increase the spread of pathogens into populations with little resistance. |
These pressures reduce genetic diversity, species diversity and ecosystem diversity. Small, isolated populations are especially vulnerable because inbreeding, random events and reduced variation can make recovery less likely.
A common misconception is that extinction itself is unusual. Extinction occurs naturally at a background rate; the crisis is the unusually rapid, global increase in extinction caused predominantly by human activity.