Community Change – Chapter 21

Community Change

• Sit in an open field or wooded lot, and you will see the community change

• If we designate a prairie as a protected area and keep out grazers and fire, the grassland will go from prairie shrubland forest

• Succession – the development of the community by the action of the vegetation on the environment, which leads to additional species being established

Two Types of Succession

• Primary succession – occurs on new sterile land, such as that uncovered by a retreating glacier or created by an erupting volcano

• Secondary succession – the recovery of disturbed sites

Mount St. Helens – Primary Succession

Mount St. Helens – Primary Succession

Chance events tend to drive early primary succession – little biological interaction

Concepts of Succession• Relay floristics – an orderly hierarchical system

of change in the community

Monoclimax hypothesis – the development of the community is a gradual and progressive change from pioneer species to the climatic climax (community determined by the climate).

Subclimax community – results from particular soil types, fire, or grazing (refers to the climatic climax)

Facilitation model – early species facilitates the arrival of the later species.

Concepts of Succession• Initial floristic composition – development at any

one site depends on who gets there first

First species there either inhibits or has no effect on the next species. Eventually, longer lived species will become the stable community.

Concepts of Succession• Random Colonization Model – succession only

involves the chance survival of different species and the random colonization of new species

No facilitation or inhibition. Succession can move in any direction

The Climax State• The final or stable community in a successional

series– Self-perpetuating and in equilibrium with the

physical and biotic environment

• Monoclimax – only one possible climax state in each region– Take away time and disturbance, will always end up with

the same community

• Polyclimax – many different climax states can be found in the same area– Climaxes controlled by soil moisture, nutrients, animal

activity, or some other factor

The Climax State

• Criteria is a steady state over time– Abstract idea, climate is always changing on a

geologic time scale

Patch (Gap) Dynamics

• Space in a forest = light– A large tree falls in the forest and a ‘gap’ in the

canopy opens up and light floods the floor (Even if no one hears it!)

• A stable community at a regional level may be a mosaic of patches at a local level