# Global Change and the Future of Life Biology I · Life, Its Origin and Its Diversity · https://tryals.app/learn/biology-i/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. ## Practice questions 6 of this lesson's 11 practice questions, with answers. The full set is in the app. ### 1. The background extinction rate is about 1 species per million species per year. If current rates are 500 times higher, how many extinctions per million species per year does that represent? **Answer:** 500 **Why:** **500** per million per year. Even the low end of current estimates puts the rate two orders of magnitude above background, which is the quantitative basis for calling this a mass extinction. Page: https://tryals.app/practice/biology-i/global-change-and-the-future-of-life/the-background-extinction-rate-is-about-1-species-per-million-species ### 2. Habitat loss is currently a larger driver of extinction than climate change. **Answer:** True **Why:** True, habitat loss and fragmentation remain the largest single driver today, though climate change is growing in importance and the two interact: a fragmented landscape leaves species with nowhere to move as conditions shift. Page: https://tryals.app/practice/biology-i/global-change-and-the-future-of-life/habitat-loss-is-currently-a-larger-driver-of-extinction-than-climate ### 3. A population shifts its breeding season earlier in spring as mean temperatures rise, tracking warming locally. Why does this successful phenological adjustment still leave the population exposed to extinction risk? A. It disrupts co-evolved timing if crucial food species advance at different rates B. It increases overall energy demands beyond what local metabolic reserves can support C. It forces the population to shift its geographic range poleward at the same time D. It reduces the genetic diversity available for future evolutionary adaptations to heat **Answer:** A. It disrupts co-evolved timing if crucial food species advance at different rates **Why:** Individual phenological tracking does not guarantee ecosystem synchrony. When mutualists or prey alter timings at different rates, mismatch breaks trophic links regardless of a single species' behavioural or physiological adjustment. Page: https://tryals.app/practice/biology-i/global-change-and-the-future-of-life/a-population-shifts-its-breeding-season-earlier-in-spring-as-mean ### 4. Why are some species far more at risk than others? Sort each characteristic by whether it raises or lowers extinction risk. **Answer:** - Raises risk: A highly specialised diet, A small geographic range, A slow reproductive rate - Lowers risk: A generalist diet, A large geographic range, A short generation time **Why:** Specialisation, small range and slow reproduction all remove options. Generalists with large ranges and fast generations can shift diet, move, or evolve quickly, which is why rats and gulls thrive while specialists decline. Page: https://tryals.app/practice/biology-i/global-change-and-the-future-of-life/why-are-some-species-far-more-at-risk-than-others-sort-each ### 5. Match each driver of global change to its principal effect. **Answer:** - Habitat fragmentation → Isolates populations and blocks movement - Ocean acidification → Dissolved carbon dioxide lowers seawater pH - Invasive species → Novel competitors and predators arrive suddenly - Nutrient runoff → Disrupts the nitrogen and phosphorus cycles **Why:** Each driver acts by a different mechanism, which is why no single intervention addresses them all. Ocean acidification is notable for proceeding directly from dissolved CO₂, it would continue even if warming somehow stopped. Page: https://tryals.app/practice/biology-i/global-change-and-the-future-of-life/match-each-driver-of-global-change-to-its-principal-effect ### 6. Arrange these consequences in the order they typically follow from habitat fragmentation. **Answer:** 1. A continuous habitat is broken into isolated patches 2. Populations in each patch become smaller and separated 3. Gene flow between patches falls and inbreeding rises 4. Local extinctions occur and are not reversed by recolonisation **Why:** Fragmentation divides the landscape, which shrinks and isolates populations, which reduces gene flow, which raises extinction risk. The final step is the critical one: in a connected landscape local extinctions are routinely reversed by recolonisation, and fragmentation removes that rescue. Page: https://tryals.app/practice/biology-i/global-change-and-the-future-of-life/arrange-these-consequences-in-the-order-they-typically-follow-from