HL only: Subtopic 3.2: Human impact on biodiversity. Species extinction contributes directly to the loss of biosphere integrity because permanently removing species reduces biodiversity, disrupts ecosystem processes, and weakens ecosystem resilience. The planetary boundary indicates that extinction and related biodiversity loss have passed a critical threshold, increasing the risk of large and potentially irreversible changes to Earth systems.
The Reasoning
The planetary boundaries model identifies limits to human disturbance of nine processes that maintain the stability and resilience of the Earth system. Biosphere integrity concerns the capacity of living systems to maintain biodiversity, ecological functions, and resilience.
The relationship develops through the following sequence:
- Human activities such as habitat destruction, overexploitation, pollution, climate change, and the introduction of invasive alien species cause population decline.
- Small and fragmented populations lose genetic diversity and become more vulnerable to disease, environmental change, and further disturbance.
- If a species becomes extinct, its ecological interactions (including predation, pollination, decomposition, or seed dispersal) may also be lost.
- These losses can disrupt food webs and feedback mechanisms, reducing the ecosystem's ability to resist disturbance and avoid tipping points.
- When extinction occurs globally at an unsustainable rate, biosphere integrity declines at the planetary scale.
For example, the extinction of the passenger pigeon in North America, driven by commercial hunting and habitat loss, permanently removed a formerly abundant species and altered its ecological interactions. However, one extinction does not by itself cross the global boundary; the boundary reflects the cumulative, worldwide reduction of biodiversity and ecosystem functioning.
A common misconception is that crossing this boundary means that most species are already extinct. It instead means that human pressure has entered a high-risk zone where further losses may cause abrupt or irreversible systemic change.
