Transcript Succession - APESatPVHS

Ecological succession
How communities and
ecosystems change
Balance of nature
• Is there a “balance of nature”?
• What does that even mean?
– Suggests “stability”
– Suggests “lack of change”
• Do ecosystems stay the same?
– Or are they constantly changing?
– May be hard to tell because human time span
is short
Constant change or stability?
• Do ecosystems stay the same?
• Or, they constantly changing?
• Human time scales are short, may not
notice change
Community Change or Stability
Communities are constantly changing, they are in nonequilibrium –
many are in some state of recovery from disturbance
Drought
Flood
Fire
Clearcut
What Is Disturbance?
• A disturbance
– Is an event that changes a community
– Removes organisms from a community
– Alters resource availability
Fire
• A major disturbance in most terrestrial
ecosystems
– Is often a necessity in some communities
Figure 53.21a–c
(a) Before a controlled burn.
A prairie that has not burned for
several years has a high proportion of detritus (dead grass).
(b) During the burn. The detritus
serves as fuel for fires.
(c) After the burn. Approximately one
month after the controlled burn,
virtually all of the biomass in this
prairie is living.
Yellowstone fires, 1988
• Communities can often respond very
rapidly to a massive disturbance
(a) Soon after fire. As this photo taken soon after the fire shows,
the burn left a patchy landscape. Note the unburned trees in the
distance.
Figure 53.22a, b
( b) One year after fire. This photo of the same general area taken the
following year indicates how rapidly the community began to
recover. A variety of herbaceous plants, different from those in the
former forest, cover the ground.
Ecological succession
• Changes in community composition over
time
– In a disturbed community = secondary
succession
– In a new community = primary succession
Succession
• Secondary
– More common
– Where an ecosystem previously existed
– Examples?
•
•
•
•
Flood
Storm
Earthquake
After volcano destroys existing ecosystem
Succession
• Primary
– Occurs where no
ecosystem existed
before or where no
trace remains
•
•
•
•
On a sand dune
On a cliff
After a glacier retreats
On a new volcanic
island
• Where rocks are laid
bare by erosion
Pioneer species
• Species, typically plants or lichens, that
are able to colonize bare ground
• Must disperse easily
• Must live on poor soils
– Hardy organisms, with adaptations such as
long roots, or symbiotic with nitrogen-fixing
bacteria
Pioneer species
• Lichens are important
• Composed of 2
different species:
– Fungus – absorbs
nutrients and holds
water
– Alga – photosynthesis
• On rock, lichen can
grab hold and begin
to break the rock
Succession
• Moraines in Glacier Bay, Alaska
– Follows a predictable pattern of change in
vegetation and soil characteristics
- Some species improve
(a) Pioneer stage, with fireweed dominant
environment for followers
(b) Dryas stage
60
Soil nitrogen (g/m2)
50
40
30
20
10
0
Pioneer Dryas Alder Spruce
Successional stage
(d) Nitrogen fixation by Dryas and alder
increases the soil nitrogen content.
(c) Spruce stage
Ecological Succession
Ecological succession is the set of
changes in community composition that
occur over time in a new or disturbed
community.
Succession after the
Yellowstone fires.
Succession at Mt. St.
Helens.
Retreating
Glaciers at
Glacier Bay
Alaska Make It a
Natural
Laboratory for
Studying
Primary
Succession
Primary succession
occurs when organisms
colonize a barren
environment.
Primary Succession at Glacier Bay, Alaska
A climax community is the stable
community at the final stage of
succession.
Succession
Succession shows some general trends that include:
1) Biomass increase over time.
2) An increase in the number and
proportion of longer-lived species.
3) Increased species diversity.
Succession on Mt. St. Helens – another site of
intense study.
Mt. St. Helens - 1976
Mt. St. Helens – 1980
eruption: May 18, 1980
Meadow vegetation immediately
after eruption
1980
1981
1982
1985
1989
1992
1994
1981
1985
1989
1992
1994
1998
Ecological Succession
Eastern US
Yellowstone wolves
Dying aspen grove
Streamside elk grazing
Hey, what’s that noise?
Yellowstone Ecosystem
Lunch grew back!
• Beaver dams
– Can transform landscapes on a very large
scale
Figure 53.18