Global Change and the Future of Life

The Sixth Extinction

Global change is broader than climate change. It includes habitat loss and fragmentation, pollution, overexploitation, invasive species, and biogeochemical disruption of the nitrogen and phosphorus cycles, with climate change one driver among several, and habitat loss currently the largest.

Extinction is normal. The background rate inferred from the fossil record is roughly 1 extinction per million species per year. Current rates are estimated at 100 to 1000 times that, which is why the present episode is described as a sixth mass extinction: the first with a biological cause.

The species-area relationship quantifies what habitat loss costs:

$$S = cA^z$$

with $z$ typically around 0.25. The exponent is what matters: because it is well below 1, losing area costs proportionally fewer species than you might fear, but the loss is still severe. Destroying 90 % of a habitat leaves $0.1^{0.25} \approx 0.56$, so roughly 44 % of species are lost.

Species respond to a changing climate in three ways, and only three: move, adapt, or die. Ranges are already shifting poleward and upward in altitude, and phenology — the timing of flowering, migration and breeding — is shifting earlier. The danger is mismatch: when a consumer and its food shift at different rates, a partnership that took millennia to tune comes apart in decades.

Not all species are equally at risk. Specialists, those with small ranges, slow reproducers and species at high trophic levels are more vulnerable than generalists with large ranges and fast life cycles.

Mitigation reduces the driver; adaptation reduces the harm. Restoration repairs damaged ecosystems, corridors reconnect fragments so populations can move, and protected areas conserve what remains. These are complements, not alternatives, protecting a fragment that nothing can reach conserves very little.

Common pitfall: treating global change as only warming. Temperature is one axis. Habitat loss remains the largest single driver of extinction today, and ocean acidification proceeds directly from dissolved CO₂ regardless of what the temperature does.