explain why populations and species can become extinct as a result of: climate change, competition, hunting by humans, degradation and loss of habitats

Conservation

Extinction is the permanent loss of a species from Earth. In the A-Level curriculum, students must understand the main drivers that can lead to population decline and eventual extinction. The following notes cover four key drivers: climate change, competition, hunting by humans, and degradation or loss of habitats.

1. Climate Change

Climate change alters temperature, precipitation patterns, and the frequency of extreme events. These changes can shift the distribution of species, disrupt phenological synchrony, and increase physiological stress.

1. Temperature rise → altered metabolic rates: \$E = mc^2\$ (energy demand increases).
2. Shift in suitable habitat ranges: species must migrate or adapt.
3. Phenological mismatch: e.g., pollinators emerging before flowering.
4. Increased frequency of droughts, floods, and storms → direct mortality.

When a species cannot keep pace with rapid environmental change, its population size declines, reducing genetic diversity and increasing extinction risk.

2. Competition

Competition occurs when two or more species vie for the same limiting resource (food, space, mates). Competitive exclusion can lead to the local or global extinction of the less competitive species.

• Interspecific competition: introduction of a superior competitor can outcompete native species.
• Intraspecific competition: high density leads to resource depletion and lower reproductive success.
• Competitive exclusion principle: two species cannot occupy the same niche indefinitely.

Competitive displacement often results in reduced population sizes, especially when coupled with other stressors.

3. Hunting by Humans

Direct removal of individuals by hunting reduces population size and can alter demographic structure.

1. Selective removal of large, reproductive individuals → skewed sex ratio.
2. Reduced genetic diversity due to loss of rare alleles.
3. Disruption of social structure (e.g., removal of dominant males).
4. Population decline below the Allee threshold, where reproduction becomes unsustainable.

Unsustainable hunting, especially of charismatic megafauna, has historically led to many extinctions.

4. Degradation and Loss of Habitats

Habitat degradation (e.g., pollution, fragmentation) and outright loss (e.g., deforestation, urbanisation) reduce the area available for species to live and reproduce.

• Fragmentation → isolated populations with limited gene flow.
• Edge effects → altered microclimate and increased predation.
• Loss of critical resources (food, nesting sites).
• Increased exposure to invasive species and diseases.

When habitat area falls below the minimum viable population threshold, extinction becomes inevitable.

Extinction Risk Assessment – IUCN Red List Categories

Suggested Diagram

Understanding these drivers enables students to evaluate conservation strategies, such as habitat restoration, legal protection, and climate mitigation, to prevent further extinctions.