Endpoint? : Revisited Climax community “Relatively steady-state condition with no directional changes in species composition” Traditional view

Equilibrium View time Successional state “Climax”

time Successional state Equilibrium View? “Climax” Disturbance

Definition: “The interruption of tranquility, peace, rest, or a settled condition; interference with the due course of any action or process.

time Successional state Equilibrium View? “Climax” Disturbance

time Successional state Non-equilibrium View

Disturbance Definition: “The interruption of tranquility, peace, rest, or a settled condition; interference with the due course of any action or process. Disturbance is viewed as something disrupting natural events tending to equilibrium. Equlibrium view Disturbance is viewed as a natural component of a community, producing change over time. Small disturbances are always occurring, but do not necessarily lead to successional processes. Non-equlibrium view

Many Small Disturbances a study by Jane Lubchenko Coastal tidal pools

Elements in study Disturbance agent Litorina littorea Plant species (periwinkle snail) – density varies among pools – herbivore Chondrus crispus (Irish moss) – red alga – only eaten as last resort Eneteromorpha – green alga – relatively small – Tender and highly edible Numerous ephemeral algae species

Disturbance rate Disturbance Effects (herbivory) Disturbance rate Species richness Species diversity Low snail density Enteromorpha smothers Chondra Intermediate snail density Competitive exclusion reduced due to con- sumption of Entero- morpha by snails High snail density All but Chondrus eaten

Conclusion Disturbance is a constant influence that defines community structure

Intermediate Disturbance Hypothesis Michael Huston 1977

Two Species Succession High growth rate; poor competitor Low growth rate; good competitor Competitive exclusion

Impact of disturbance Disturbance removes 25% biomass Competitive exclusion halted or delayed

Potential Reversal of Fortunes

Multiple species extension Eventual winner

Multiple species extension Competitive exclusion is delayed Species diversity maintained at a higher level

Multiple species extension At high disturbance rates diversity declines again

Intermediate Disturbance Hypothesis Frequency of disturbance Diversity Highest diversity will be found at intermediate disturbance levels

A Field Test of the Intermediate Disturbance Hypothesis Wayne Sousa 1979

Study Location

Rocky Intertidal Plant Community

Boulder Size and Disturbance Small……….………………Large Newtons of pull = weight Low disturbance Medium disturbance High dist.

Diversity and Disturbance High MediumLow Disturbance rates Maximum diversity

Predicting Species Distributions mid-successional species

Mid-succession species and disturbance Distribution as predicted

Assessing impact of disturbance Early succession Mid-succession Late succession

Conclusion Results of this experiment are consistent with the intermediate disturbance hypothesis

Further studies examining diversity-disturbance relationships Mackey and Currie 2001

Meta-analysis pattern predicted by intermediate disturbance hypothesis Disturbance response

Conclusion The intermediate disturbance hypothesis is only one of many possible relationships observed between diversity and disturbance

Ecology at the Global Scale

Global Climate Change: Impacts on Vegetation

Part #1: The Driving Force behind Climate Change

The Greenhouse Effect CO 2 (Carbon dioxide) NH 4 (Methane) N 2 0 (Nitrous oxide) O 3 (Ozone) CFCs

Mauna Loa: CO 2 Measurements 4,170 m

Mauna Loa: [CO 2 ] Summer minimum Winter maximum Mean trend

Does this represent a long term trend?

Fossil AtmospheresMauna Loa Further Evidence