Bouncing back: Every ecosystem faces a disturbance sooner or later. Its resilience decides whether it recovers. You met resilience in topic 1.2; here it is tied to succession.
The points to remember
- Resilience: the ability to resist a disturbance or recover and return to a stable state.
- It is about coping with and recovering from a disturbance, not stopping it happening.
- Mature ecosystems sit in a dynamic equilibrium: small ups and downs, balanced overall.
- Negative feedback pulls a disturbed ecosystem back to its equilibrium.
- Disturbances can be natural (fire, storm, drought, disease) or human (felling, grazing).
Negative feedback
- Works against the change: negative feedback
- Keeps the ecosystem stable
- Helps it recover
Positive feedback
- Makes the change bigger: positive feedback
- Pushes it away from balance
- Too much lowers resilience
Real example: the Great Storm of October 1987 blew down about 15 million trees in south-east England. Many woods were left alone, and within a few decades they had regrown from seeds and stumps: a dynamic equilibrium restored.
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Whether an ecosystem tolerates a disturbance depends on the size of the disturbance and on how resilient the ecosystem is.
The points to remember
- A disturbance within its resilience: the ecosystem recovers to the same state.
- A disturbance too big or too frequent: it passes a tipping point into a new state.
- Positive feedback then pushes it further away, and the old state may not return.
- So how much an ecosystem can tolerate depends on its diversity and resilience.
Real example: fires in 1988 burned about a third of Yellowstone National Park, USA. The lodgepole pines' serotinous cones held a huge store of seeds, so young pines covered the burnt ground within a few years.
Resilient is not untouchable: A resilient ecosystem still gets damaged. What marks it out is that it recovers.
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Diversity is the main reason some ecosystems cope better. Many species and genes give redundancy, back-up routes for energy and more ways to survive.
The points to remember
- More species mean more complex food webs: energy has several pathways.
- Redundancy: if one species is lost, another with the same role takes over.
- Genetic diversity lets some individuals survive a disease, drought or fire.
- Evenness: when no single species dominates, one loss does less harm.
- Niches are filled, so invasive species find it harder to move in.
Real example: at Cedar Creek, Minnesota, scientists grew grassland plots with different numbers of species. In the 1987-88 drought, plots with many species lost a much smaller share of their biomass, and recovered faster, than plots with only a few.
Remember it as: Many species, many back-ups.
Succession builds diversity, storages and soil, so it builds resilience too. A climax community is usually more stable than the stages before it.
The points to remember
- More species, habitats, niches and genetic diversity at the climax.
- Higher gross productivity and more biomass stored: a buffer to regrow from.
- More complex food webs and energy pathways.
- More established nutrient cycling and deeper, more fertile soil.
- More favourable abiotic conditions: shade, moisture, shelter.
- More established negative feedback, so the climax stays in equilibrium.
- Early stages are less resilient: few species, small storages, thin soil.
Real example: Białowieża Forest, on the border of Poland and Belarus, is one of Europe's last old-growth lowland forests. Thousands of species, deep soils and huge stores of living and dead wood mean that when a storm fells some trees, the forest keeps working.
Compare the stages: When asked why a climax is more stable, compare it with the earlier stage: 'more species than the intermediate stage', not just 'it has many species'.
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Human interference can undo what succession builds. People often arrest a succession, or reset it, and that lowers diversity and resilience.
The points to remember
- Humans can reset a succession: deforestation sends it back to an early stage.
- Or arrest it: farming and grazing hold land at a field or grassland stage.
- Urbanisation stops it altogether under concrete.
- Monocultures cut species and genetic diversity, so one pest or disease can wipe them out.
- Pollution, overfishing and fire suppression remove key species or storages.
- Each lowers diversity and storages, so the ecosystem tolerates less disturbance.
Reset
- Clear-felling a forest
- Starts again from early stages
Hold
- Fields, pasture, grazing
- Kept at a simple early stage
Stop
- Towns and roads
- No succession under concrete
Real example: in 1840s Ireland, most poor families lived on one potato variety, the Lumper. When potato blight arrived in 1845, it destroyed crop after crop; about a million people died. One crop with no genetic diversity could not tolerate the disease.
Because succession rebuilds diversity and storages, people can use it to rebuild resilience.
The points to remember
- Restore degraded land by letting succession run, or helping it with the right next-stage species.
- Knowing the stages lets managers predict recovery time after a disturbance.
- Rewilding: step back and let succession and natural grazing do the work.
- Invasive species often take hold in early stages, so they are managed early.
- Some ecosystems need fire or grazing; managers keep them, rather than stopping them.
Real example: since 2001, the Knepp estate in Sussex, England, has let its old farmland go wild, with free-roaming ponies, cattle and pigs. Scrub and young woodland spread, and rare nightingales, turtle doves and purple emperor butterflies returned.
Link back to diversity: In an answer, say what the action does to diversity or storages, and so to resilience.
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How this comes up: Paper 2, Section A: a diagram of succession stages, then why the climax is more stable [2].
Figure 1 shows the stages of a succession after a fire in an oak forest.
Outline two reasons why the mature oak forest is more stable than the young pine and oak stage.
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