Ecological Change and Successional Models

Overview of Ecological Succession

• Definition of Succession: A directional change in an ecosystem that typically leads to increased complexity of community structure and increased biodiversity.

• Key Characteristics:     * It refers to the progressive change in species dominance and composition.     * The community will not return to its former state without a disturbance that resets the process.     * Space-Time Substitution: Researchers study succession by looking at the evolution of plant communities at various sites simultaneously, substituting space for time to understand the progress over centuries.

• Initiation of Succession:     * New Habitat Formation: Occurs when new, unoccupied land becomes available.     * Disturbance: Occurs when an existing community is disrupted.

• Primary vs. Secondary Succession:     * Primary Succession: Relies entirely on colonization as the starting point is completely abiotic (e.g., bare rock).     * Secondary Succession: A biological legacy (seeds, roots, or soil) is already present after a disturbance.

The Prisere and Seral Stages

• Prisere: A complete succession from initial inorganic conditions (dry or wet) to the final climatic climax vegetation.

• Seral Stages: These are the temporary, transitional stages (seres) that develop over time leading toward the final climax state.

• Specific Sere Types (based on initial environment):     * Lithosere: Succession beginning on bare rock.     * Psammosere: Succession beginning on sand dunes.     * Halosere: Succession beginning in salt water/salt marshes.     * Hydrosere: Succession beginning in fresh water (lakes or ponds).

• Environmental Trends during Succession:     * Increasing soil depth and nutrient levels.     * Increasing stability and humus (organic matter) content.     * Lithosere/Psammosere: Decreasing influence of the sea or harsh exposure.     * Hydrosere: Decreasing influence of open water as the site fills with sediment.     * Mesophytic Transition: Communities move from being adapted to extreme dry ($\text{xerophytic}$) or extreme wet ($\text{hydrophytic}$) conditions toward a mesophytic state (balanced moisture), such as an oak climax forest.

Factors and Theories of Succession

• Influencing Factors:     * Autogenic Factors: The plants themselves cause the change (e.g., capturing light, producing detritus/humus, nitrogen fixation, water and nutrient uptake).     * Allogenic Factors: External factors cause the change (e.g., climate change, sea-level changes, volcanic activity, periodic fires, or floods).

• Monoclimax Theory (F.E. Clements, 1916):     * Suggests that for every climatic zone, only one type of climax vegetation (the climatic climax) can evolve.     * The vegetation exists in harmony with the local environment and remains in a steady state once reached.     * The prisere follows a path from pioneer community through various seral stages named after dominant species.

• Polyclimax Theory:     * The replacement for the monoclimax concept.     * It acknowledges that multiple local factors (poly-factors) prevent many areas from reaching a single climatic climax.     * Influencing Factors: Drainage, parent rock material, relief/topography, microclimate, and human activity.

Interruption and Deflection: Subclimax and Plagioclimax

• Subsere: A secondary series of communities that begins after succession is interrupted by a disturbance.

• Arresting Factors: Factors that prevent vegetation from reaching its climatic climax state:     * Biotic: Grazing by animals such as rabbits, goats, or sheep which prevents tree growth.     * Edaphic: Soil conditions, such as high salinity, hindering specific plant growth.     * Hydrological: Significant changes in the water table.     * Topographical: Physical events like landslides.

• Subclimax: A state where vegetation is prevented from reaching its theoretical climax due to a local arresting factor.

• Plagiosere: A succession deflected from its normal course by biotic factors (often human-induced).

• Plagioclimax: A stable plant community maintained by human activity (e.g., deforestation, burning, grazing).

Case Study: Heather Moorlands (U.K.)

• Historical Context: Following the Ice Age, primary succession occurred in the lower hills, moving from mosses and lichens to oak forests.

• Human Deflection: Humans cleared trees for farming, leading to soil leaching and loss of fertility. The resulting acid soils were conducive to heather growth.

• Maintenance of Plagioclimax:     * Grazing: Sheep graze on young heather shoots, preventing the regrowth of shrubs and trees.     * Muirburn: Periodic burning removes old wood, releases nutrients, and encourages new shoots.

• Pathways to Heather Moorland:     * Oak forest $\rightarrow$ Deflection by humans (cutting/burning) $\rightarrow$ Shrubby heath $\rightarrow$ Heather moorland.     * Overgrazing and burning of grassy heaths or blanket bogs can also result in heather moorland.

Lithosere (Succession on Bare Rock)

• 1. Lichen Stage (Pioneer):     * Crustose Lichens: Species like graphis, rhizocarpon, rinodina, and lacanora tolerate extreme desiccation ($\text{drying out}$).     * Chemical Weathering: Organic acids produced by lichens corrode rock, releasing minerals.     * Foliose Lichens: Species like parmelia and physcia follow; they have leaf-like structures that retain more water and trap wind-borne soil particles.

• 2. Moss Stage (Pioneer):     * Species like polytrichum and grimmia grow in soil accumulated in rock depressions.     * Moisture-loving mosses like hypnum and bryum follow as organic matter increases.

• 3. Herb Stage (Pioneer):     * Death of mosses creates a mat of organic matter.     * Hardy annual grasses germinate, followed by perennial grasses as decomposition continues.

• 4. Shrub Stage (Intermediate):     * Accumulation of soil allows for herbaceous plants, shrubs (brambles, gorse), and shade-intolerant trees (rowan, alder, birch) to grow.     * Roots penetrate rock, furthering weathering and soil formation.

• 5. Forest Stage (Climax):     * Hardy trees are replaced by larger shade-tolerant trees like oak and ash.     * This process can take hundreds of years.

Psammosere (Sand Dune Succession)

• 1. Embryo / Fore Dune (Pioneer):     * Environment: Alkaline (due to sea shells), high salt, drought-prone, and unstable.     * Species: Sea rocket, frosted orache, saltwort, and sea couch.     * Adaptations: Salt tolerance, deep tap roots, waxy leaves, and low (prostrate) growth habits to avoid wind.

• 2. Yellow to Grey Dune (Building Stage):     * Yellow Dunes: Dominated by marram grass ($\text{Ammophila arenaria}$). It traps sand and binds it with roots/rhizomes. Marram thrives on being buried.     * Grey Dunes: As marram decays, humus increases, sand stabilization improves, and pH drops from roughly $\text{pH 8}$ to $\text{pH 7.5}$. Plant cover reaches $\text{80\,\%}$.

• 3. Dune Slacks:     * Low-lying areas between ridges that intersect the water table. Soil is acidic and supports moisture-loving plants like willow, moss, reeds, rushes (phragmites), and bog cotton.

• 4. Mature Dune (Climax):     * Sheltered with nutrient-rich, acidic soil (high organic content).     * Species include heather, sea buckthorn, birch, alder, and oak.

Hydrosere (Freshwater Succession)

• 1. Phytoplankton Stage (Pioneer): Algae and diatoms. Dead material forms humus on the bottom.

• 2. Submerged Stage: Rooted species like ceratophyllum, elodea, vallisneria, and hydrilla grow as water shallows.

• 3. Floating Stage: Plants like nymphaea (water lily) and potamogeton. Broad leaves shade out submerged species; decaying matter forms organic mud.

• 4. Reed-Swamp Stage: Emergent plants like typha (bulrush) and phragmites (common reed). Creeping roots knit mud together, creating resistant reed peat.

• 5. Sedge-Meadow Stage: Water level drops; habitat occupied by juncus, carex, and poaceae. Soil is no longer totally waterlogged.

• 6. Woodland Stage: "Carr" woodland develops with trees like alder, willow, and birch. Transpiration lowers the water table further.

• 7. Climax Community: Varies by climate—forest (humid), grassland (sub-humid), or desert (arid).

Halosere (Salt Marsh Succession)

• 1. Pioneer Stage:     * Eelgrass and algae grow on mudflats.     * Salicornia (glasswort) and spartina (cord grass) trap silt and clay, raising the ground level.

• 2. Intermediate Stage (Competitive):     * Inundation becomes less frequent. Vegetation becomes "closed" with less bare ground.     * Species like sea aster appear. Competition for space and light leads to species like sea purslane and thrift replacing pioneers.

• 3. Intermediate Stage (Stabilizing):     * Ground height increases significantly; species like sea lavender and scurvy grass appear.

• 4. Climax Stage:     * Immersion only occurs during exceptional storm tides (1-2 times per year).     * Upper marsh supports rush, sedge, and red fescue. If freshwater influence exists, it transitions into scrub and then forest (oak/ash).