Transcript Introduced Species

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
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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: