1. Ecological Succession

2. Ecological Succession
Ecosystems are constantly changing in response to both natural and human disturbances. Examples: forest fires, strip mining, glacier recession, volcanic eruption, etc.
It occurs when one community (animal and plant species) in an area gradually replaces another community over time.
Each community has its own dominant plant and animal species and ecological succession results in certain species becoming less abundant and other species becoming more competitive and dominant over time.

3. Each successive (new) community is more favorable for new species
Changes in stages until a climax community is established

4. Climax Community
It is the final community that will exist in an ecosystem – usually consists of trees
Ecological succession occurs in two forms:
1. Primary Succession
2. Secondary Succession

5. Composition of the community changes with time
Past
Community
Future
Community
Present
Community
Some species in the
past community were
out-competed, and/or
did not tolerate altered
abiotic conditions
Modifies such abiotic
factors as:
Light intensity
Wind speed
Air Temperature
Soil composition
Light quality
Wind direction
Soil water
Humidity
Changes allow new
species to become
established.

6. Primary Succession
Occurs in an area where NO community had previously existing
Bare rock present
Receded glaciers
Volcanic regions with eruptions of lava rock and ash
First organisms to appear in these areas are called pioneer species (or colonizers)
Climax
Community
Lichens, bryophytes
And annual herbs
Slower growing
broadleaf species
Grasses and
Small shrubs
Fast growing
trees
Bare Rock
years

10. Pioneer Species
First organisms to live in a new community. Usually brought in by wind or animals

11. Primary Succession
Begins very slowly because the physical environment must be changed to produce soil needed for the growth of plants
Lichens (consisting of fungus and algae) and mosses are examples of pioneer species
Lichen grows on rock and helps to break apart rock to form soil

12. Mount St. Helen 1980 – Washington State
Another example of primary succession

13. Secondary Succession Takes place after a land clearance
Fire, landslide, forest
clearing, severe weather,
human agriculture
Soil is already there
More rapid than primary
Climax
Community
Open pioneer
community
(annual grasses)
Mature
woodland
mainly oak
Primary
bare earth
Grasses and
low growing
perennials
Scrub: shrubs
and small trees
Young broad-
leaved
woodland
150+
1-2 years
3-5 years
16-30 years
31-150

14. Secondary Succession
Farmers field over a 20 year period

15. Secondary Succession
After a forest fire

16. Gap Succession
Example: forested area with a thick canopy has reduced light on soil
When a tree falls sunlight fills forest floor
New communities are formed