AP® Environmental Science review sheet from Aim for Five (aimforfive.com/enviro/units/2/2-5)
Unit 2 · Topic 2.5
2.5 Natural Disruptions to Ecosystems
Natural events like hurricanes, fires, droughts and volcanic eruptions can disrupt ecosystems as much as people do, or more. These disruptions can be periodic, episodic or random, and over long timescales Earth's climate and sea level have shifted many times, forcing species to migrate, adapt or die out.
Key terms
- natural disruption
- periodic
- episodic
- random
- migration
- sea level change
Natural disruptions
A natural disruption is an event not caused by people that changes an ecosystem. Examples include hurricanes, floods, droughts, wildfires started by lightning, volcanic eruptions, earthquakes and tsunamis. Some disruptions can affect ecosystems as much as human impacts, or even more. A large volcanic eruption can bury an entire landscape, and a major hurricane can flatten miles of forest in a day.
Disruptions aren't always bad over the long run. Many ecosystems depend on them. Some pine cones only open in the heat of a fire, and floods spread nutrient-rich sediment over floodplains.
Periodic, episodic and random
These labels describe how often and how predictably an event happens. If you can set a calendar by it, it's periodic. If it happens from time to time, often in certain conditions, it's episodic. If there's no pattern at all, it's random.
| Type | Meaning | Examples |
|---|---|---|
| Periodic | Happens on a regular, predictable cycle | Wet and dry seasons, tides, yearly flooding of a river in spring |
| Episodic | Happens occasionally, at irregular intervals | Droughts, hurricanes hitting a particular coast, wildfires |
| Random | Has no pattern and can't be predicted | Earthquakes, volcanic eruptions, a meteorite impact |
Long-term changes in climate and sea level
Over Earth's history, the climate has swung between warmer and colder periods many times. During ice ages, huge ice sheets covered much of North America and Europe. Over the last million years or so, these glacial periods have come and gone on cycles of roughly 100,000 years, driven largely by small, regular changes in Earth's orbit and tilt.
Sea level rises and falls with these cycles. When ice sheets grow, water is stored on land as ice, so sea level falls. About 20,000 years ago, sea level was roughly 120 meters lower than today, exposing land bridges, like one between Siberia and Alaska. When ice melts, sea level rises and floods coastlines.
Scientists learn about past climates from evidence like ice cores, tree rings, sediment layers and fossils. Today's warming is happening much faster than most past natural changes, which gives species less time to respond.
How wildlife responds
When conditions change, organisms have a few options. They can move to places where conditions are better. Many animals migrate, meaning they move regularly in response to seasons or other changes, like caribou crossing the Arctic or monarch butterflies traveling to Mexico. Over longer times, whole species' ranges can shift toward the poles or up mountains. Populations may also adapt through natural selection if the change is slow enough. If they can't move or adapt, they decline or go extinct.
Worked examples
Try each one yourself first, then open the solution.
- Example 1
Classifying disruptions
Classify each as periodic, episodic or random: (a) a river floods every spring when snow melts; (b) a region has a severe drought about once or twice a decade, at irregular times; (c) a volcano with no history of recent eruptions suddenly erupts.
Show the solutionHide the solution
- Step 1: Ask how regular and predictable each event is.
- Step 2: (a) Happens every year in the same season: periodic.
- Step 3: (b) Happens now and then, but not on a schedule: episodic.
- Step 4: (c) Happens with no pattern and couldn't be predicted: random.
Answer: (a) Periodic, (b) episodic, (c) random.
- Example 2
Sea level and land bridges
Explain why a land bridge between Siberia and Alaska existed about 20,000 years ago but doesn't today, and describe one effect this had on wildlife.
Show the solutionHide the solution
- Step 1: About 20,000 years ago, Earth was in a glacial period. A lot of water was locked up in ice sheets on land.
- Step 2: With less water in the ocean, sea level was much lower (roughly 120 meters), exposing the shallow seafloor between Siberia and Alaska.
- Step 3: As the ice melted, sea level rose and flooded the land bridge.
- Step 4: While it was exposed, animals (and people) could migrate between Asia and North America.
Answer: Ice sheets stored water on land and lowered sea level, exposing the land bridge; melting raised sea level and flooded it. The bridge let animals migrate between continents.
Common mistakes
- Calling every disruption random. Seasonal events are periodic, and occasional ones like droughts are episodic.
- Thinking natural disruptions are always minor compared with human impacts. Some, like large eruptions, can be just as destructive or more.
- Getting sea level backward: more ice on land means lower sea level.
On the exam
- Expect to classify a described event as periodic, episodic or random and justify it with how regular it is.
- If asked how organisms respond to a disruption, give a specific response, like migrating, and explain why it helps.
Connected topics
Videos
Check yourself
4 questions on 2.5 Natural Disruptions to Ecosystems. Pick an answer to see if you got it, and why.
In parts of South Asia, heavy monsoon rains arrive every summer and flood low-lying grasslands for several months. This disruption is best described as
About 20,000 years ago, sea level was more than 100 meters lower than it is today, exposing land bridges between some continents. Which is the best explanation?
On an African savanna, a rainy season and a dry season come every year. Every decade or so, at irregular intervals, the rains fail and a severe drought lasts one to three years.
Each dry season, huge herds of wildebeest and zebras travel hundreds of kilometers to areas where rain has fallen and fresh grass is growing, and they return when the rains come back.
Described scenario
The severe droughts described in the passage are best classified as which type of natural disruption?
The herds' movement during the dry season is best described as
0 of 4 answered