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Glacier Bay, AK
•1794: filled by a glacier
• George Vancouver
•1879: it retreated 35km up valley
• John Muir
•William Cooper’s 1916 succession study.
•Pioneer community of herbs, shrubs, tree seedlings
• Succession of dominants:
•Dryas shrubs ⇒ Alder ⇒ Sitka Spruce ⇒ Hemlocks
community of Hemlocks
•Climax
1° Succession On Glacial Till At Glacier Bay &
2° S
Species Richness Increases With Age
•Till is deposited by glaciers
•Climax community took long
time (1,500 yr) to establish.
1° Succession On Glacial Till At Glacier Bay &
2° Succession On Old Abandoned Farm Fields:
Species Richness Increases With Age
•Till is deposited by glaciers
•Climax community took long
•Grasses were pioneer,
time (1,500 yr) to establish.
oak-hickory was climax
• Took 1/10th the time of
Glacier Bay
1° Succession On Glacial Till At Glacier Bay &
2° Succession On Hubbard Brook Clear-cuts:
Biomass Accumulated & Nutrients Changed
Glacier Bay study:
⇑ Organic content ⇓ Density
⇑ Moisture
⇓ pH
⇑ Nitrogen
⇓ Phosphorus
At Hubbard Brook
⇑ biomass
⇓ nutrient export
1° Succession:
Facilitation, Tolerance, or Inhibition
•Facilitation: pioneers Δ habitat facilitating others’ arrival.
• Lichen & moss break down rock; adding their leaf litter to it
creates soil in which seeds of later arrivers can grow.
• Common occurrence
•Tolerance: some spp. of
pioneers become climax
species.
• O’hias (Metrosideros)
• Infrequent occurrence
•Inhibition: colonizers
exclude others.
• Ulva excl. Gigartina
Connell & Slatyer’s
Three Models of Succession
• They have different patterns & processes.
•Facilitation and inhibition are typical
•& can occur together
•Tolerance is uncommon.
Facilitation Tolerance Inhibition
Pioneers
predictable
Climax
predictable
variable
predictable
variable
variable
Inhibition & Facilitation:
Succession On Mt. St. Helens
•Lupines colonizing pumice plains
• inhibited germination of others
but
• facilitated seedling survival of
others by adding nitrogen.
Bottom-Up Control:
Water, Nutrients & Tundra
•Wet tundra meadows have ⇑ productive than dry ones.
•Adding P, N or both ⇑ productivity in either tundra type.
Is 1° Production Controlled from the
Bottom or the Top of the Food Web?
•Bottom-up Effects
•Top-down Effects
•Production affected by •Production affected by
•Grazers
•Predators too?
•Nutrients
•Water
•Sunlight
•Temperature
Consumers & 1° Production:
Trophic Cascades Show Top-Down Control
•Research in 2 lakes
• 1 w/ piscivorous bass A
• 1 w/ planktivores
B
•Predation’s effect
cascades down the
trophic levels.
What if you added herbivores to A?
What controls # of trophic levels?
Community Stability:
Resistance & Resilience
• Stability - the absence of change - results from
1) Lack of disturbance (e.g., fire)
2) Resistance to disturbance:
•
Redwoods not affected by fire
•
Chaparral is fire adapted
3) Resilience - bouncing back in predictable succession:
Community Stability:
Resistance & Resilience
• How does complexity relate
to stability?
Paine’s Food Web Structure
& Species Diversity
•Temperate webs are simple. 13 spp.
•Tropical ones are complex.
•Larger food webs support
proportionately more
predators.
45 spp.
15% are
predators
24% are
predators
• Predators control diversity.
•Let’s see how …
Predators Are Keystone Species:
Robert Paine’s Sea Stars
•He removed them in 2 studies.
•Food web collapsed, because
• prey popns ⇑⇓
• causing ⇑ competition
• leading to competitive exclusion.
•Called them keystone species.
Keystone Species: An Impact
Disproportional To Abundance
• Species whose total impact =
their proportional abundance
fall along the diagonal Y = X.
• Rhinovirus making elk sneeze
has small total impact
• thus isn’t keystone species.
• Parvovirus that kills wolves has
collective magnitude sufficient
for keystone designation.