18.3.1—Extinction causes

Extinction is the end of a species. Environmental change and human activity can reduce survival and reproduction until a population becomes unable to persist; several pressures may act together.

• Climate or environmental change — changed temperature, sea level, ocean conditions or ice cover can make a habitat unsuitable and reduce access to food.
• Competition — limited food, water, habitat or mates can reduce population size; competition may be within or between species.
• Hunting and overexploitation — removing too many individuals, especially from a vulnerable population, can leave too few to survive.
• Habitat degradation or loss — destruction, pollution or reduction of the habitat removes the resources and conditions on which the species depends.
• Introduced invasive species — a non-native species may lack natural predators, competitors or pathogens, increase rapidly, and then reduce native populations through competition or disease.

Use the causal chain: direct pressure → reduced resources, survival or reproduction → smaller population → increased extinction risk. Habitat loss can also force individuals into the remaining area, increasing competition. A small population may then experience more inbreeding and genetic drift, reducing genetic variation and fitness; this feedback can increase mortality and reduce reproduction further.

Do not treat one pressure as necessary in every extinction event, or confuse a falling population with immediate extinction. This card explains causes and the small-population vulnerability feedback; conservation methods, IUCN status and assisted reproduction are separate cards.