Communities change all the time, but some disruptions are fast and large: an invasive species arrives, a habitat is cleared, a pollutant spreads, a disease sweeps through, or the climate shifts. Community structure tells us how effects spread through a web. Population genetics tells us which populations can adapt.
- Invasive species: brought to a place outside their range, they can outcompete or eat native species. Zebra mussels, for example, spread through North American lakes and rivers after arriving from Eurasia.
- Habitat loss and fragmentation: clearing land and splitting habitats into small patches shrinks and isolates populations.
- Pollution: excess nutrients cause algal blooms; some chemicals build up in food chains.
- Disease: a fungal blight from Asia, first recorded in New York in 1904, killed roughly four billion American chestnut trees in the first half of the 1900s.
- Changes in climate: warmer seas can make corals expel the algae that live in them (coral bleaching), and shifting temperatures change where species can live.
Key idea
Disruptions can change populations fast. They do not hit every species equally: species with narrow niches, small ranges or little genetic variation are most vulnerable. A population with more variation is more likely to include individuals that can survive a change, so it can adapt by natural selection.
Why it works
Natural selection can only choose among the variants a population already has. If every individual is nearly identical, as in a field of one crop variety grown from cuttings, a new disease that kills one kills them all. In a varied population, a few individuals may happen to resist it. They survive and leave offspring, and the population recovers with the resistance now common. That is why genetic variation is called the raw material of adaptation, and why bottlenecks (Genetic Drift) leave populations at risk.
Worked examples
Worked example: Who is hit hardest?
A warm spell damages a mountain meadow. Which is most at risk: a plant found only on that mountain's cool peak, a widespread grass, or a weed that grows almost anywhere?
- The peak plant has a narrow niche and nowhere cooler to move.
- The grass and the weed are generalists with wide ranges.
Answer: The plant found only on the peak. Some generalists may even gain from the change.
Worked example: Variation and survival
In a population of 2,000 snails, 3% carry an allele that protects them from a new parasite. The parasite kills 95% of unprotected snails and none of the protected ones. How many snails survive, and what share of the survivors carry the allele?
- Protected: 3% of 2,000 = 60; all survive.
- Unprotected: 1,940; 5% survive = 97.
- Survivors: 60 + 97 = 157; protected share: 60 / 157 ≈ 38%.
Answer: 157 survive, and about 38% of them carry the allele, up from 3%: selection in one generation.
Common mistake
Thinking a disruption affects every species equally. Species with narrow niches or little genetic variation are hit hardest, while some generalists may gain. Where people disagree about how to respond, for example how to balance land use and conservation, the science describes the effects; choosing among the trade-offs is a decision people weigh together.