Biological Kingdoms

1
Abstract of the Connell and Slatyer (1977) models
of successional mechanismsa
a The steps of each model are sequential.
Disturbance can interrupt the process at any
point, but is indicated here only at step
F. b Specific site and species pool determines
disturbance effects in all models.
2
Succession at Glacier Bay, Alaska subsequent to
glacial retreat
moss
Pioneer forbs
Cushion plants
Semi-erect and erect shurbs
Coniferous forest
Occurs in lt 100 years
3
Low Arctic Riparian Succession Example

• Initiation Deposition of sand within a river
• Herbs and stranded plants establish on alluvium
• Increased vegetation on alluvium increases
  roughness and deposition
• Salix alaxensis establishes
• Deposition of fines are accelerated and more
  forbs and grasses establish
• Other Salix spp. and Alnus crispa establish
  resulting in greater shade and ground cover
• Herbaceous species decrease, moss cover increases
  and active layer thickness decreases

4
Low Arctic Riparian Succession Example

• On older gravel bars (later successional stages)
  Salix alaxensis dominates, but with very few
  herbaceous species (2-8) and an increase in moss
  cover (70-100).
• Furthest back from the river there is a shift
  from Salix alaxensis to other Salix spp. Heath
  species increase and there is a futher reduction
  in herbaceous species.
• Increase in the organic leayer and a decrease in
  the active layer (20-30 cm in late July).

5
Low Arctic Succession

• Classical succession is not characteristic
• Directional change characterized by species
  replacement is not typical
• Harsh environment results in species
  establishment and survival rather than replacement

6
Successional Types

• Directional Replacement Succession
• Occurs due to modification of habitat
  (facilitation) and competition

7
Successional Types

• Directional nonreplacement succession
• Occurs in high resistance environments
• Invading species (A, B, C, D) succeed without
  eliminating or replacing each other.
• Space is not limiting
• Typical of polar deserts

8
Successional Types

• Nondirectional nonreplacement succession
• Occurs in extremely resistant environments (e.g.
  High Arctic)
• A very few species arrive and survive while
  fluctuating in cover and biomass
• Other species are unable to invade.

9
Succession in Extreme Environments

• The seed or propagule rain continues to supply a
  site, at a very low rate, from nearby or remote
  locations.
• Some germination may take place, but seedlings of
  most species will not survive.
• Any seedling survival would be of a handful of
  species extremely tolerant to very adverse
  conditions. The main adaptive characteristic of
  these species is miniaturization without loss of
  reproductive capacity.
• Species are able to fit in available safe sites
  and complete their seasonal cycle in a few weeks
  of the short growing season.