1. Ecosystem Responses to Disturbances AP Env Sci 2011-2012

2. Equilibrium vs. Non-Equilibrium Systems Once popular idea: “Balance of Nature” –Natural systems in balance, slow changes in ecosystem –Eventually “climax” vegetation covers ecosystem Newer thinking: continuous change and instability –Ecosystem is a mosaic of vegetation patches at various stages.

3. Ecological Succession Natural changes in the biotic community over time Why? –Modification in physical environment caused by species themselves –Leaves door open for next biotic community, example: –Pioneers (crabgrass) –Colonizers (deciduous hardwoods) Moving Towards “Climax Ecosystem” –Continues for relatively long time –Change if: climate changes, introduced species, removal of species

4. Primary Succession If area has not been occupied previously Example: bare rock –Moss captures soil particles –Seeds of larger plants can thrive –Fallen leaves collect, enrich soil –Trees grow, moss is eliminated

5. Secondary Succession Area was previously occupied, but cleared by natural or human activities Why is it secondary different than primary? –Secondary succession starts with soil already in place –Crabgrass, weeds/grasses, pine trees, deciduous trees

7. Aquatic Succession

8. Disturbance and Resilience Disturbance: fire, flood, windstorm, drought –Remove organisms –Reduce populations –Create opportunities –Enhance biodiversity Where do early stage species come from? Nearby! All stages of succession are likely to be represented in the ecosystem – NOT CLIMAX only Elimination of a number of species – may block succession –Iceland

9. Akureyri

10. Fire and Succession Major Disturbance Ecosystems that depend on fire: –Fire climax ecosystems (some grasslands and pine forests) –Pine forest: Periodic fires reduce deadwood on ground Less fuel, fires burn along ground without harming trees Lodgepole pine

11. Resilience Resilient ecosystems will continue functioning even through a disturbance Fire vs. Overgrazing example: