Topic 2.5 Natural Disruptions to Ecosystems: describe natural disruptions to ecosystems and explain their short-term and long-term effects on populations and biodiversity.

What this topic is asking

The College Board (Topic 2.5) wants you to describe natural disruptions to ecosystems, classify them by how regularly they occur, and explain their short-term and long-term effects on populations and biodiversity. The key idea is that disturbance is a normal part of ecosystems, and its effects depend on its scale, frequency and intensity.

What counts as a natural disruption

Examples range from fast events such as fires and storms to slow processes such as plate tectonics and long-term climate change.

Classifying disruptions by timing

The frequency matters because organisms can evolve adaptations to periodic, predictable disturbances (such as fire-adapted plants) but are more often devastated by rare, random ones.

Short-term and long-term effects

• Short-term effects: a fire, storm or flood kills organisms, removes vegetation, exposes soil and sharply reduces population sizes. The ecosystem is suddenly simplified.
• Long-term effects: disturbances can permanently change the landscape and climate, drive local or global extinctions, or create new habitat. A volcanic eruption can destroy an ecosystem but later leave fertile soils or new islands that are recolonised over decades.

Very slow disruptions matter too. Plate tectonics rearranges continents and oceans over millions of years, separating and joining populations and driving speciation and extinction. Natural climate change over thousands to millions of years shifts biomes and the species they hold.

Disturbance and biodiversity

A subtle but important point is that disturbance is not always harmful to biodiversity. Moderate, periodic disturbances can actually increase diversity by opening up space, releasing resources and resetting succession, which prevents any single competitively dominant species from taking over. Many ecosystems are adapted to and even depend on regular disturbance: some pine forests need periodic fire for their cones to release seeds, and floodplain ecosystems depend on seasonal flooding. Suppressing natural disturbance can therefore reduce biodiversity and allow fuel or sediment to build up, making a later disturbance far more severe. Topic 2.5 connects directly to ecological succession (Topic 2.7), because disturbance is what restarts the successional process, and to adaptations (Topic 2.6), because surviving disturbance is a major selective pressure.

Try this

Q1. Identify whether a hurricane that strikes a coast irregularly is a periodic, episodic or random disruption. [1 point]

• Cue. Episodic (occasional and irregular).

Q2. Explain how suppressing all natural fires in a fire-adapted ecosystem could harm it. [2 points]

• Cue. Fuel and dominant competitors build up, so the ecosystem loses the diversity that periodic fire maintains, and a later fire may be far more intense and destructive.