Ponder this….
Do communities change?

Ponder this….
Do communities change?
yes, of course
What makes them change?

Ponder this….
Do communities change?
yes, of course
What makes them change?
something about the environment changes, may be huge or small, natural or man-made, sudden (a disturbance) or gradual (abiotic factors change as the community changes)
How do they change?

Ponder this….
Do communities change?
yes, of course
What makes them change?
something about the environment changes, may be huge or small, natural or man-made, sudden (a disturbance) or gradual (abiotic factors change as the community changes)
How do they change?
gradual change over time as species are replaced by other species

Ponder this….
Do communities change?
yes, of course
What makes them change?
something about the environment changes, may be huge or small, natural or man-made, sudden (a disturbance) or gradual (abiotic factors change as the community changes)
How do they change?
gradual change over time as species are replaced by other species
For a given area, is change predictable?

Ponder this….
Do communities change?
yes, of course
What makes them change?
something about the environment changes, may be huge or small, natural or man-made, sudden (a disturbance) or gradual (abiotic factors change as the community changes)
How do they change?
gradual change over time as species are replaced by other species
For a given area, is change predictable?
sort of…. If you could stand in a cleared field in Georgia for
500 years, what would you expect to see?

disturbance
This change in a community over time is called ecological succession

• Definition: a gradual process of change and replacement of the types of species in a community
• The sequence of changes is somewhat predictable
• May take place over hundreds or thousands of years

Two types of ecological succession
Primary succession: occurs on a surface where no ecosystem existed before (no soil)
Secondary succession: occurs on a surface where an ecosystem has previously existed but has been disturbed (soil is present)

Primary Succession
• On new islands created by volcanic eruptions
• On sand dunes

Primary Succession
• Where glaciers have retreated

Primary Succession
• Paved areas that are abandoned

Primary Succession
Begins with pioneer species (defined as the first species to colonize a disturbed area), like lichens, that can grow directly on rock; begin the breakdown of the rock to form soil

Primary Succession
Once rocks begin to weather and crack, soil forms and small plants can take root

Primary Succession
Larger plants continue to move in until the bare ground is covered with life

Secondary Succession
Definition: type of ecological succession that occurs on a site where an ecosystem previously existed (soil is present)
Due to a disturbance to the ecosystem – something upsets the natural balance
May be natural or man-made (anthropogenic) disturbances
Think of examples of each….

Examples of natural disturbances tornadoes, floods, trees falling, fire, hurricanes, disease

Examples of man-made (anthropogenic) disturbances deforestation, pollution, urbanization, harvesting

‘General Ecology’, D.T. Krohne

Secondary Succession
After a disturbance, pioneer species move in, followed by other species in a somewhat predictable sequence

Typical stages of secondary succession in woodlands
• Early: plants typically small with short lifecycles
(annuals, grasses), rapid seed dispersal, environmental stabilizers (pioneer species)
• Middle: plants typically longer lived, slower seed dispersal, and usually larger
• Late: plants and animal species are those associated with older, more mature ecosystem: larger, slower dispersal, better competitors, slower growth
• Climax community: final stage of succession that will last as long as there is no disturbance

• Each stage of succession brings about changes in the ecosystem that enable or prevent certain species from moving in – better soil, less light, more competition as ecosystem matures
• Some changes may help other species – called facilitation
(example: dogwood trees grow well under the shade of larger trees)
• Some changes may hinder other species – called inhibition
(example: black walnut produces a chemical that keeps other plants from growing near it)

Even though the stages can be generally predicted, stages of succession can vary, depending on local conditions
Amount of rain, temperature, terrain, latitude, altitude, soil type, human impact

Succession in two forests: What causes the difference in these climax communities?
For example, eastern U.S. climax species are oak, hickory, maple
Succession in the boreal forest (for example in Canada) – climax species are fir, spruce, hemlock

Even in one location, the climax community depends on many factors

Current way of thinking: instead of one stable, final climax community, there are patches that are constantly changing within an ecosystem (called patch dynamics )

Identify the “patches” around this school in the southeastern U.S. Although in the
“deciduous forest”, it is not that simple….

We can see changes in the animal communities over time, too

Ecological Succession can occur in aquatic ecosystems
Fig 10.7 Diagram of bog succession.
© 2003 John Wiley and Sons Publishers

Fig 10.8 Graphs showing changes in biomass and diversity with succession.
Changes in Biota
© 2003 John Wiley and Sons Publishers

Changes in Abiotic Factors
Fig 10.9 Changes in soil nitrogen and phosphorus.
© 2003 John Wiley and Sons Publishers