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: