15.4 Ecological Succession

Ecological succession is the gradual, predictable, and directional process by which the species composition of a community changes over time. It proceeds from a simple pioneer community through a series of intermediate stages (seral communities) to a final stable climax community.

---

Key Terminology

---

Types of Ecological Succession

1. Primary Succession

• Occurs in a lifeless area where no soil exists and no previous community was present.
• Examples: bare rock exposed by a retreating glacier, newly formed volcanic islands, sand dunes.
• Begins with pioneer species that can tolerate extreme conditions and initiate soil formation.
• Very slow process (may take hundreds to thousands of years).

2. Secondary Succession

• Occurs where a previous community was destroyed but soil remains intact.
• Examples: abandoned farmland, areas after forest fires, logged forests.
• Faster than primary succession because soil and seed banks are already present.

---

Pioneer Species

Pioneer species are the first colonizers of a barren environment. They are characterized by:

• Tolerance of extreme conditions (drought, temperature extremes, nutrient-poor substrate)
• Ability to modify the environment, making it suitable for subsequent species
• In Xerosere: crustose lichens are the pioneers on bare rock

---

Xerosere (Xerarch Succession)

Xerosere is succession that begins on bare dry rock (xeric = dry). It is the most commonly studied example of primary succession.

Seral Stages of Xerosere:

1. Crustose Lichens (Pioneer stage)

   • Encrust the bare rock surface
   • Secrete organic acids that weather the rock
   • Trap dust and organic matter → initiate soil formation

2. Foliose Lichens

   • Leaf-like lichens colonize as thin soil layer develops
   • Add more organic matter upon death

3. Mosses

   • Colonize the thin soil layer
   • Retain moisture and add more organic matter
   • Accelerate soil development

4. Herbs and Grasses

   • Annual then perennial herbs colonize
   • Root systems further break down rock and deepen soil

5. Shrubs

   • Woody shrubs outcompete herbs for light
   • Provide shade, further modifying microclimate

6. Climax Forest (Climax Community)

   • Trees dominate; species composition depends on climate
   • Stable, self-perpetuating community

---

Hydrosere (Hydrarch Succession)

Hydrosere is succession that begins in open freshwater (e.g., a pond or lake). Over time, the water body fills with sediment and organic matter, eventually becoming dry land.

Seral Stages of Hydrosere:

1. Phytoplankton Stage (Pioneer)

   • Microscopic algae and cyanobacteria colonize open water
   • Begin adding organic matter to the bottom

2. Submerged Aquatic Plant Stage

   • Rooted plants (e.g., Hydrilla, Potamogeton) grow on the bottom
   • Accumulate sediment and organic debris

3. Floating Plant Stage

   • Plants with floating leaves (e.g., water lilies, Nymphaea) colonize
   • Reduce light penetration, suppressing submerged plants

4. Reed Swamp Stage

   • Emergent plants (e.g., reeds, rushes, Phragmites) colonize shallow margins
   • Rapidly fill in the pond with organic matter

5. Marsh Meadow Stage

   • As water level drops, sedges and grasses replace reeds
   • Soil becomes waterlogged but not permanently flooded

6. Woodland / Climax Forest Stage

   • Trees and shrubs colonize the now-dry land
   • Climax community established

---

Climax Community

The climax community is the final, stable stage of succession. Its characteristics include:

• High species diversity and complex food webs
• High niche specialization — species occupy very specific roles
• Dynamic equilibrium — species composition remains relatively constant
• Lower net productivity compared to early successional stages (most energy goes to respiration)
• Self-perpetuating — reproduces itself unless disturbed by fire, flood, human activity, etc.

The type of climax community is determined primarily by climate (temperature and rainfall). For example:

• Tropical rainforest climax in humid tropics
• Temperate deciduous forest in temperate regions
• Grassland climax in semi-arid regions

---

Summary Comparison