Continued from previous lecture

1
Continued from previous lecture
2
Keystone species
3
2nd level Predators
1st level Predators
Herbivores
Trophic cascade effect on primary production
Reference
4
Interaction Web
Possible interactions ( species)2 Connectance
actual interactions / possible
interactions 10/49 0.20
5
Development of Communities

• Succession gradual changes in the species
  composition of a community
• Primary succession when plants become
  established on substrates lacking organic matter.
• Secondary succession plants become established
  on substrates previously occupied by plants.

6
(No Transcript)
7

• Early view succession was deterministic
• a predictable sequence of communities leading to
  a final stable community resistant to change.
• Each transitional community in succession was
  called a seral stage
• First colonizers represent the pioneer stage
• Succession progresses toward a climax stage with
  K-selected species dominating the site.
• Climax community attained dictated by climate and
  edaphic factors.
• Early key assumption
• each invading species makes the environment more
  suitable for a replacement species.
• This process is known as facilitation

8
Example of Primary Succession Glacial retreat
(Glacier Bay) across 200 years Tills and moraines
- serving as rudimentary soil low in nitrogen
and organic matter
9

• Pioneers a black crust of cyanobacteria
  lichens, liverworts, and horsetail.
• Cyanobacteria fix nitrogen modest increase in
  soil nitrogen but organic matter is minimal.
• Willow sp., Dryas, Alder sp., and spruce
  seedlings occur, but distribution is sporadic.
• Dryas drummondi (nitrogen fixation) dominates
  after about 40 years
• Progressive increase in soil nitrogen and organic
  matter.
• Dense, closed thickets of nitrogen-fixing alder
  at ca. 60 years.
• Nitrogen surpluses accumulate in the soil and
  litter increases.
• Invasion of spruce seedlings.

10

• Spruce-needle litter turns the soils acidic
• Shading causes competitive exclusion of alder and
  many original, understory species.
• Hemlock begins to flourish.
• Ca. 200 years
• 1) Well drained soils a climax of spruce and
  hemlock results
• 2) Poorly drained soils forest invaded by
  Sphagnum mosses, which accumulate water and
  further increase soil acidity
• Muskeg bog

11
(No Transcript)
12

• Facilitation
• Dryas and alders increase soil nitrogen levels
• Facilitates the invasion of spruce
• Inhibition
• But, alder shades out Dryas
• Eventually spruce shades out alder
• Facilitation, originally thought to control the
  entire process of succession, important in early
  stages.
• Inhibition was important in the later phases of
  succession

13
Secondary Succession

• Piedmont Plateau, North Carolina
• Old growth forests cut and replaced by
  agricultural crops.
• Eventually, fields were abandoned
• Seral stages revealed by fields abandoned at
  different times.
• 35 native species (pioneers) 1 year after fields
  were abandoned
• Seedlings of colonizers have to be able to
  withstand exposure and dry soil. Two species
  dominated crabgrass and horseweed

14

• 1st year self inhibition of horseweed
• 2nd year, previous species still present, but 26
  new species were also thereaster and ragweed
  dominant.
• Broomsedge can outcompete aster for soil moisture
• 3rd year, Broomsedge became dominant, and
  maintained this position for several years.
  Species richness declined.
• Seeds of pines arrive by wind.

15

• 5th year, pines became established, producing a
  closed canopy by year 10
• Pine seedlings do poorly under a pine canopy
• (self-inhibition), but hardwoods thrive. Gradual
  process of replacement.
• Random-colonization model chance involved in
  which hardwoods become established.
• By 100 years, there are equal numbers of
  hardwoods and pines.
• By 200 years, only scattered pines remain in a
  typical oak-hickory forest climax.

16

• In secondary succession
• Many species are present at the outset
• Which species germinate first, or can propagate
  from roots, have a role of determining, by
  chance, the pattern of succession.

17
(No Transcript)
18
Succession can be detected
Huerfano Butte, Pima County, Arizona
1902
1969