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
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:
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