Download ECOLOGICAL SUCCESSION: COMMUNITIES IN TRANSITION

ECOLOGICAL SUCCESSION:
COMMUNITIES IN TRANSITION
• New environmental conditions allow one group
of species in a community to replace other
groups.
• Ecological succession: the gradual change in
species composition of a given area
– Primary succession: the gradual
establishment of biotic communities in lifeless
areas where there is no soil or sediment.
– Secondary succession: series of
communities develop in places containing soil
or sediment.
Primary Succession:
Starting from Scratch
• Primary
succession
begins with an
essentially
lifeless area
where there is
no soil in a
terrestrial
ecosystem
Figure 7-11
Primary Succession
• Pioneer species
– first ones on site
• Ecological development
– accumulation of organic debris allowing
germination of seeds
– orderly sequence of stages occurs as
organisms modify the environment in ways
that allows one species to replace another
Secondary Succession
• Occurs due to disruption of community
– Bare soil colonized by rapidly growing annual
plants
• Plants replace by perennial plants
• Biomass accumulates and allows richer soil,
better shelter, and allows a more complex
community
Secondary Succession
• Occurs due to disruption of community
– Bare soil colonized by rapidly growing annual
plants
• Plants replace by perennial plants
• Biomass accumulates and allows richer soil,
better shelter, and allows a more complex
community
Beginning of secondary succession
Plant colonizing lava flow
Climax woodland community
This is primary succession on a recent lava flow in the
Galapagos Islands, Ecuador. The vegetation near the
center of the photo is mangrove, on the slopes there are
cactus and other desert plants.
This is the northern lower flank of Mt. Saint Helens taken about 24 years
after the eruption. This area was buried under hundreds of feet of ash, so
all the plant life you can see has come in from wind-blown seed. The
plants are things like willows, cottonwoods, grasses and forbes that have
fine, easily wind-transported seeds. In the thin ash area, most of the new
vegetation arises from perennial plants whose roots survived the blast
underground.
This fascinating photo shows plant succession following the retreat of the Emmons Glacier on Mt.
Rainier. The glacier began retreating around the time of the Civil War and has moved up the valley
several miles. The glacier itself is the dark mass (not the white area) that terminates in the lower left
third of the picture. You can see plant succession marching up-valley toward the snout of the
glacier. If you look closely you can see the White River emerging from the snout. This river flows all
the way to Puget Sound at Tacoma. To the right is a beautiful lateral moraine. Plant succession is
visible on the right (north) slope of the moraine but is being impeded by hot dry conditions on the
south (left) slope of the moraine. Most of the pioneering vegetation is alder and willow, with a few
conifers gradually becoming established. The seeds are coming down from the forests above. The
dark old growth forest to the right is mainly mountain hemlock and subalpine fir.
This is reforestation of the blast zone around Mt. Saint
Helens. All of the trees were planted (by Weyerhaeuser)
about 20 years ago.
Climax Community
• Culmination of successional process; is a
stable, complex mature form.
• Equilibrium communities
– Never reach a stable climax because they are
characterized by and adapted to periodic
disruption.
• Example: fire-climax communities (Long-leaf pine
forest)
Forest Ecosystem
• The longleaf/wiregrass community is part
of a fire ecosystem.
• Fire is required to clear brush to allow pine
seedlings to grow
• Wire grass is most likely to bloom and set
seed after a late spring or early summer
burn.
Climax forest on
Stone Mountain
Beech-Maple Forest