Patterns of Succession

Succession is:

• Non-seasonal, directional change in plant species living in a particular place through time.

• Involves:– Colonization– Establishment– Ultimate local extinction

• Initiated by a disturbance that opens up space and resources

Types of Succession

• Primary: Occurs when new bare rock is generated by geologic activity (no organic material)– Mt. St. Helens, Krakatoa, glacial retreat

• Secondary: vegetation invades areas that have been previously occupied, but has been removed due to disturbance (fire, agriculture, etc)

Primary Succession after glacial retreat

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Example of primary succession:

lichensmoss small forbsshrubstrees

Lichens and moss growing on rock (primary succession)

Secondary Succession

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Secondary Succession in YNP after 1988 fires

Classic Study: Billings (1938)

• Studied succession from old field to oak forest (150 years)

– Annual plants = pioneer species– Biennial plants, grasses– Perennial plants, shrubs– After about 15 years: softwood trees– After about 50 years hardwood trees, primarily oak

“Old-field Succession”

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Successional Progression• Annuals: Pioneer Species

– Tolerate low nutrient soils, need high light– Good dispersers – Grow rapidly, short life span (1 year)– Produce many offspring

• Biennial plants, grasses– Share attributes with pioneer species, but longer lived, grow

more slowly

• Perennials and shrubs– Slower growing, longer lived– Can reproduce asexually as well as sexually– Better competitors

• Softwood trees (Pines)– Slow growing– Eventually produce canopy– Shade out light-loving shrubs and perennials in

understory– Also shades out new baby pines

• Hardwood trees (Oaks and hickory)– Longer lived– Better competitors– Eventually dominated forest as softwood trees

died.

Oak forest in this case = climax community

Climax Communities• Final successional stage: self-replacing,

persistant over very long periods of time

• Idea of climax community controversial these days because of recognition of role of disturbance– Shifting mosaic steady state model:

• Majority of patches in habitat in some stage of recovering from disturbance

• Landscape is in a steady-state because roughly constant portions of landscape are in each stage of succession

• Thus there is a dynamic equilibrium across the landscape

Disturbance Specialists: Often Weeds

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Mechanisms of SuccessionConnell and Slayter, 1977

1. Facilitation- Changes in abiotic conditions are

caused by the plants currently occupying an area

- These changes favor new invaders over the current occupants (current occupants pave the way for future occupants) Examples: Lichens make soil, nitrogen-fixing plants

enrich soil, nurse plants support others

2. Tolerance• Plants in different stages of succession

don’t have much impact on each other

• Position in the scheme of succession depends on their different life histories

• Example: Old field Succession

3. Inhibition

• Members of one stage of succession resist invasion of later stages

• Succession proceeds when one stage dies

Example: Allelopathy

Succession and Chemical Cycling

• Biomass, production, diversity and chemical cycling change during succession

• Biomass and diversity peak in mid-succession, increasing at first to a maximum, then declining and varying over time.

K-selection and r-selection

• Pioneer species generally have particular set of characteristics - all together these are called r-selected traits

• Climax species generally have different set - represent a different life history strategy

Succession and Plant Diversity

Succession and Diversity

Just focusing on woody plants:

Corresponding changes in animal species

Succession in the Intertidal

Diversity through Succession in Intertidal

Succession in Streams

Points to Consider:• Change through time is a natural

characteristic of most ecosystems - when thinking about conservation need to incorporate understanding of natural disturbance regimes

• Mosaic of Patches in different stages of succession maximizes habitat heterogeneity and thus diversity

Ecological Restoration

• Attempt to return impacted land to natural state

• What exactly should be returned?• Increasingly are trying to replace the original

disturbance regime so that succession can occur naturally– Need to restore disturbance and succession

processes both temporally and spatially– What area of land is big enough to encompass

these processes?

Stream Restoration• What ecosystem functions/services need to

be restored?– Habitat– Conduit– Barrier– Filter– Source– Sink

• Processes need to be restored, not just individual components– Hydrologic functions:

• How much flow, how deep, timing

– Geomorphological functions • Stream profile, sedimentation patterns, erosion

– Physical and Chemical Properties of Water• Toxins?• Nutrients• PH

– Biological Processes (eg large woody debris, habitat structure, heterogeneity in stream)

– Role of Disturbance/Dynamic Equilibrium