2.5 - Natural Disruptions to Ecosystems

Learning Objectives

The central goal is explaining how natural disturbances, both short term and long term, impact habitats and ecosystems. The accompanying skill is describing patterns or trends found within data.

Natural Disturbances and Impacts

A natural disturbance is an event that occurs naturally and disrupts the basic structure and/or function of an ecosystem.

Effects: These events interrupt the normal cycling of energy and matter. They may displace or totally destroy organisms and reshape or alter the entire habitat.

Examples: Tornadoes, hurricanes, fires, and asteroid strikes. The asteroid that eliminated the dinosaurs about 66 million years ago is a highly destructive example.

Severity: It’s critical to recognize that natural disturbances can be even greater in scale and impact than disturbances caused by humans, like clear cutting or farming.

Example: When Mount St. Helens erupted in 1980, its sideways blast flattened or killed the forest across about 600 km² (230 square miles) in a few minutes.

Time Scales of Disturbances

Natural disturbances can be categorized by how frequently and predictably they occur.

Time Scale	Frequency and Predictability	Examples
Periodic	Occurs with a regular frequency and is somewhat predictable.	The predictable shift between wet and dry seasons in ecosystems like the African savanna.
Episodic	Occurs somewhat frequently but doesn’t follow a regular interval, so it’s less predictable.	Hurricanes, droughts, and fires. We expect them in certain seasons, like summer fires in California, but the precise timing is uncertain.
Random	Occurs without warning and is entirely unpredictable.	Earthquakes, or an asteroid striking Earth.

Natural Climate Change

Earth’s global climate has naturally varied a lot over long geological periods thanks to non-human factors.

Orbital and tilt changes: Slight, natural shifts in Earth’s orbit and its axial tilt as it revolves around the sun cause recurring periods of both ice ages and warmer periods. These shifts in orbital eccentricity occur in cycles of every 100,000 to 400,000 years.

There are three cycles: the shape of the orbit (eccentricity, about every 100,000 and 400,000 years), the angle of Earth's tilt (obliquity, about every 41,000 years), and the wobble of Earth's axis (precession, about every 26,000 years). Together they're called Milankovitch cycles.

Natural $\text{CO}_2$ variation: Carbon dioxide levels in the atmosphere have naturally risen and fallen over time. These natural variations can be caused by oceans warming, which makes them release $\text{CO}_2$ , or by periods of increased plant growth, which takes in $\text{CO}_2$ .

Volcanic eruptions: Big eruptions shoot sulfur dioxide into the stratosphere, where it forms tiny droplets that reflect sunlight back to space. After Mount Pinatubo in the Philippines erupted in 1991, Earth's average surface temperature dropped by roughly 0.5°C (about 1°F) in 1992 before recovering. Over much longer times, periods of very heavy volcanic activity have released enough $\text{CO}_2$ to warm the climate.

Changes in the sun's output: The sun's energy output rises and falls slightly, for example over the 11-year sunspot cycle. These changes are small, but they have played a part in some past climate shifts.

Plate tectonics: Over tens of millions of years, moving continents change where land sits and how ocean currents carry heat around the planet, which shifts climate. Big ice sheets can only build up on land near the poles.

Perspective on current change: Natural climate change exists, but current atmospheric $\text{CO}_2$ levels far exceed any natural variation or peak seen in the last 800,000 years.

Ice cores show $\text{CO}_2$ swung between about 180 ppm (ice ages) and about 300 ppm (warm periods) over the last 800,000 years. NOAA's yearly average at Mauna Loa, Hawaii, was about 427 ppm in 2025.

Natural Sea Level Fluctuation

Sea level has varied considerably throughout geological history, largely due to natural changes in Earth’s temperature.

Ice cap melting: Warmer periods coincide with increased $\text{CO}_2$ and lead to melting polar ice caps. That meltwater flows into the ocean and increases sea level.

Only ice on land (mountain glaciers and the ice sheets on Greenland and Antarctica) raises sea level when it melts. Floating sea ice already displaces its own weight of water, so it barely changes sea level when it melts, the same way melting ice cubes don't make a glass overflow.

Thermal expansion: When ocean water warms, water molecules naturally spread out, which increases the total volume of the ocean and raises sea level slightly. This is called thermal expansion.

Ice ages lower sea level: At the peak of the last ice age, about 20,000 years ago, so much water was locked up in ice sheets that sea level was about 120 m (400 feet) lower than today. That exposed land that is underwater now, including a land bridge between Siberia and Alaska (Beringia) that animals and early humans used to move between Asia and North America.

Environmental Change and Habitat Disruption

Major environmental changes cause significant habitat destruction and alteration.

Impact of sea level rise on estuaries: Rising sea levels cause the loss of coastal estuaries. These brackish habitats (part salt, part fresh), which are crucial breeding grounds, become completely flooded.

Habitat loss: The flooding removes the necessary terrestrial (land) aspect of the estuary.

Changing conditions: The flooding also increases water depth and salinity.

Plant community change: Increased depth means submerged aquatic plants get less sunlight, which makes it impossible for certain species to survive.

Outcome: Species that rely on these coastal habitats are forced to migrate or face death.

Natural example (the Green Sahara): About 11,000 to 5,000 years ago, a slow shift in Earth's orbit strengthened the African monsoon, and much of the Sahara was grassland dotted with lakes. When the monsoon weakened, the region dried out into the desert it is today, and the animals and people living there had to move or adapt.

Forced Migration

Natural environmental changes and disturbances frequently force species to migrate.

Predictable migration: Some migration follows predictable patterns, like wildebeest tracking rainfall across the Serengeti to ensure access to water and plant growth.

Short-term migration: Animals leave during a disruption and come back afterward. When Tropical Storm Gabrielle hit Florida in 2001, young blacktip sharks being tracked in a nursery bay swam out to deeper water as the air pressure dropped, then returned after the storm passed.

Long-term migration: A species' whole range shifts over many generations. As the ice sheets melted at the end of the last ice age, trees like spruce and oak gradually spread hundreds of kilometers north into land the ice had covered.

Climate driven shifts:

Ocean species: Ocean species are migrating further north as equatorial waters become too warm for them to tolerate.
Birds and insects: Climate change causes insect hatching, like caterpillars, to occur earlier in the spring due to warming temperatures. As a result, bird migration and breeding patterns have to shift earlier in the year so that their hatchlings’ peak food demand lines up with peak insect availability.

Big picture: The Earth’s system operates on two speeds. There are rapid, unpredictable natural disturbances (an earthquake or hurricane) that act like sudden punches to an ecosystem, and geological shifts (orbital changes or tectonic plate movement) that act like a massive, slow moving tide. Current human driven climate change is concerning because it forces ecosystems to react to the slow, massive changes at the speed of a rapid, sudden disturbance.

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