Follow the disruption link by link. The right answer explains every column in the table.
A disruption rarely stops where it lands. Add a species, remove a predator, pour in fertilizer, or cut a forest, and the change travels along who eats whom and who needs what. Recovery depends on what's left behind.
Key terms
- Invasive species — an introduced species that spreads fast, often because the predators, parasites, and competitors that limited it at home are absent.
- Trophic cascade — a change at one feeding level that ripples through others, reaching species it never touches directly.
- Eutrophication — nutrient overload in a lake or stream, usually nitrogen or phosphorus from fertilizer or sewage.
- Habitat fragmentation — splitting one habitat into small, isolated patches with roads, farms, or clearings.
- Succession — a community rebuilding after disturbance. Primary starts on bare rock with no soil; secondary starts where soil survives.
How each disruption spreads
- An invader free of its usual enemies → multiplies fast → eats or outcompetes natives → the loss ripples through the food web.
- Top predator removed → herbivores boom → plants get eaten down → species that need those plants decline. Bring the predator back and the chain can reverse.
- Fertilizer runoff → algal bloom → algae die → bacteria decompose them, using up dissolved oxygen → fish and other animals suffocate.
- Clear-cutting → no living roots to take up nitrogen or hold soil → nitrate from ongoing decay washes into streams, with eroded sediment.
- Roads and clearings → small, isolated populations → less gene flow, more drift and inbreeding → rare alleles lost. Corridors that allow occasional crossing help.
- Warming climate → many species' ranges shift poleward and upslope → those that can't move or adapt fast enough decline.
Why it matters
Expect before-and-after tables with several columns. Find the one mechanism that explains every column, and reject any choice that fails even one. Use the controls the design hands you: the years before an invader arrived, or an intact forest beside a cut one.
Exam traps
| Trap | Actually |
|---|
| Algae use up the oxygen when they photosynthesize. | Photosynthesis releases oxygen. The crash comes when bacteria decompose the dead bloom, so the oxygen low follows the bloom, not the fertilizer. |
| A burned forest recovers slower than bare rock, since fire sterilizes soil. | If soil, seeds, and roots survive, it's secondary succession: years to decades. Bare rock must build soil first, which takes far longer. |
| A predator only affects the animals it eats. | Through a cascade it reaches further: fewer herbivores → more plant cover → more of the animals that nest or feed there. |
| A road changes where animals move, not their genes. | Less movement means less gene flow. Each smaller patch drifts, losing rare alleles that no migrant replaces. |
Worked example
An invented shrub from East Asia is spreading through North American forest edges, where it grows about twice as tall as it does at home. Researchers grew it in both regions and caged some plants to keep insects off. Mean height after two years: native range, open 0.8 m; native range, caged 1.6 m; North America, open 1.6 m; North America, caged 1.7 m. What best explains the shrub's success in North America?
- Native range: caging doubles growth, 0.8 → 1.6 m. Insects there are holding the shrub back.
- North America: caging barely matters, 1.6 → 1.7 m. The insects that limit it at home are missing or do little harm.
- Compare the caged plants: 1.6 m at home, 1.7 m in North America. With insects kept off, both places grow it about equally well.
It escaped the insect herbivores that limit it in its native range, so it grows far more freely in North America.
The cages separate enemies from everything else about each place. The near-miss is 'North American soil and climate suit it better.' If so, caged plants would grow much taller in North America. They don't. Only the open plants differ, and that difference comes from the missing insects.