Lecture #10 Disturbance and Succession

Disturbance

• Discrete events in time causes abrupt change in ecosystem; removes all parts of a community

Succession

• Directional change in plant community over time

Primary succession

• Happens after disturbance

• Disturbance removes soil and all organisms

Secondary succession

• Disturbance removes some or all organisms (depends) but leaves intact soil

• May alter soil, but will not remove

Disturbance regime

• Type

• Severity

• Size

• Frequency

• ^^Characteristics of disturbance

Disturbance regime: Volcano eruption (primary and secondary)

• Physical removes/kills living plants and animals - Burry or kill

• Removes soil - Yes by scour or burial

• Changes resources availability (e.g., light, water, nitrogen, phosphorus) - More light, less water, N and P

Disturbance regime: Wind (secondary)

• Physical removes/kills living plants and animals - Yes, localized area

• Removes soil - Does not remove soil

• Changes resources availability (e.g., light, water, nitrogen, phosphorus) - More light, maybe more N

Disturbance regime: Fire (secondary)

• Physical removes/kills living plants and animals -Burns plants

• Removes soil - Does not remove soil

• Changes resources availability (e.g., light, water, nitrogen, phosphorus) -More light, N and P, less water

Intermediate disturbance hypothesis

• Posits that species diversity should be highest at intermediate levels of disturbance

• Competitive exclusion reduces species diversity at low levels of disturbance

• Suggests that local species diversity is maximized when ecological disturbance is neither too rare nor too frequent

What are the three models of succession

• Facilitation

• Inhibition

• Tolerance

Facilitation

• Barren ground is uninhabitable by all but most stress-tolerant plants (yellow flower)

• Early stress-tolerant colonists make environment suitable for successive species (purple flower) by increasing nutrients, developing soils, providing

  shade, etc.

• Most competitively dominant species (orange flower) no longer facilitates growth of other species

Inhibition

• All species arriving on an unoccupied site can survive

• A colonist that becomes established inhibits growth of later arrivals bymonopolizing space/resources

• New colonists can invade/grow only if space/resources become available

• Short-lived early species die more frequently, thus succession progresses slowly from short- to long-lived species

Tolerance

• All species arriving on an unoccupied site can survive

• Later species arrived later or grew more slowly

• Late arriving species tolerate early species because they are better competitors for light/nutrients

• Early-successional species have no effect on late-successional species

Resistance

• How much does the community change due to disturbance

Return time

• How quickly did the community recover from this change?

Resilience

• How closely did the post-recovery community resemble the pre-disturbance community?

  • maintain basic structure/function