Rucete ✏ Campbell Biology In a Nutshell
Unit 8 ECOLOGY — Concept 54.3 Disturbance Influences Species Diversity and Composition
Ecological communities are dynamic, not static. Disturbances like fire, storms, or human activity play a critical role in shaping species diversity and composition over time. Recovery after disturbance involves ecological succession, a natural progression of community change.
1. The Role of Disturbance in Communities
- Disturbance = any event that removes organisms or alters resource availability
- Examples: storms, fires, floods, droughts, human impact
- Nonequilibrium model: most communities are constantly changing due to disturbances
- Shifts focus from the outdated idea of stable, climax communities
2. Intermediate Disturbance Hypothesis
- Moderate levels of disturbance → highest species diversity
- High disturbance: excludes slow-growing species
- Low disturbance: allows dominant species to monopolize resources
- Supported by research (e.g., New Zealand stream study): invertebrate richness peaks at intermediate flood frequency
3. Small vs Large Disturbances
- Small-scale disturbances: create habitat mosaics → promote diversity
- Large-scale disturbances: natural in many ecosystems (e.g., fires in Yellowstone)
- Example: lodgepole pine cones open after fire; seeds thrive in cleared, nutrient-rich soils
4. Ecological Succession
- Succession: species gradually replace one another after disturbance
- Primary succession: occurs in lifeless areas (e.g., after glaciers or volcanic eruptions)
- Starts with microbes, lichens, mosses → grasses → shrubs → trees
- Example: Glacier Bay, Alaska: Dryas → alder → spruce → hemlock → moss bog
- Nitrogen-fixing species (Dryas, alder) improve soil fertility
- Secondary succession: occurs where soil remains (e.g., abandoned farmland, after fire)
- Pioneer herbaceous plants → shrubs → trees
5. Human Disturbance
- Human activities are now the dominant source of disturbance
- Agriculture, deforestation, urbanization alter ecosystems drastically
- Example: ocean trawling scrapes the seafloor like clear-cutting a forest
- Human-caused disturbances often:
- Reduce species diversity
- Disrupt succession
- Create habitats unsuitable for recovery
In a Nutshell
Disturbance is a natural and essential part of ecological communities, driving changes in species composition through succession. Moderate disturbance supports biodiversity, while extreme or absent disturbance can reduce it. Today, human impact is the leading cause of disturbance, with widespread ecological consequences.