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Transcript
Ecosystem Responses to Disturbances AP Env Sci 2011-2012 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. 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 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 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 Aquatic Succession 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 Akureyri 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 Resilience • Resilient ecosystems will continue functioning even through a disturbance • Fire vs. Overgrazing example: