Community

Succession and Community Dynamics

• Succession: The directional (or sequential) change in species composition (or structure) over time.

  • Historical Reference: Gough (1793) identified how lakes convert to dry land via deposition of organic matter.

  • LeDuc (1810) described the six stages of lake to wetland.

  • 1860: The term "succession" was coined by Thoreau.

  • Early 1900s: This period saw significant debate regarding community development over time.

Primary Succession

• Introduced in subsequent sections but not explicitly detailed in the transcript.

Factors Influencing Succession

• Allogenic Factors: External abiotic factors that direct ecosystem development.

  • Concept: Individualistic or continuum concept.

• Autogenic Factors: Internal biotic factors that direct ecosystem development.

  • Concept: Unidirectional deterministic concept.

• Community Composition Implications: Initially related to community composition, crucial for understanding successional dynamics.

Understanding Community

• Definition: A recognizable association of interacting plant and animal species (biotic) and their common environment (abiotic) recurring across the landscape.

• Community plays a pivotal role in succession.

  • Initial debates centered around whether communities are identifiable in time and space.

  • This debate extended similarly to the term "ecosystem".

Deterministic Concept of Succession

• Clements (1916): An advocate of the autogenic perspective.

  • Proposed plant communities behave like a "supraorganism", indicating clear associations between species.

  • Succession follows a predictable trajectory:

  • Pioneer communities ➜ Seral communities ➜ Climax community.

  • The climax community is biologically mediated through interactions and processes that lead to observable zonation.

  • Characteristics:

  • Sharp, clear community boundaries.

  • Recognition that climax communities are dynamic, not static.

  • Clements proposed that disturbances could lead to “disclimax” communities, which would subsequently regenerate toward climax after disturbance.

Individualistic or Continuum Concept of Succession

• Gleason (1926): A proponent of the allogenic viewpoint.

  • Argued that organisms distribute along gradients according to their individual tolerances.

  • Key mechanisms in succession:

  • Immigration and environmental selection.

  • Key Features:

  • Blurry community boundaries, with little association between species.

  • Succession is non-linear and can experience regression or cyclic processes.

  • There exists significant abiotic and biotic heterogeneity over time and space.

  • This concept builds upon Cowles' early works (1899).

Definition of Population

• Population: A group of individuals of the same species that inhabit a specified area.

• Relation to Succession:

  • Different species have unique physiological or morphological traits facilitating persistence in specific environments.

  • Hence, community structure is significantly influenced by population distributions.

Conservation Implications

• Clements View: If communities are tightly integrated systems of co-evolved species, the conservation of ecosystems is justified.

  • Preservation of boundaries is key.

• Gleason Perspective: If communities are loosely organized, the conservation value diminishes as primary environmental constraints become unclear.

  • Community boundaries are often fuzzy in this model.

Ongoing Debate in Wetland Literature

• The discourse surrounding Allogenic vs. Autogenic continues to evolve in wetland studies.

  • Beyond simplified criticisms of Clements' "supraorganism" model, substantial literature supports both perspectives:

  • Continuum (Autogenic): Citations include Odum (1971), Chabreck (1972), Glaser et al. (1981), Rochefort et al. (1990), De Steven and Toner (2004).

  • Individualistic (Allogenic): Citations include Whittaker (1967), van der Valk (1981), Jensen (2004).

Complexity in Wetlands

• Wetlands exhibit high variability; thus, a single explanatory paradigm is challenging.

  • Plant community zonation is generally distinct and corresponds with temporal and spatial dynamics.

  • Studies illustrate patterns linking community change over time (associated with autogenic succession) and distinguish sharp environmental gradients in space (linked to allogenic succession).

  • No singular succession paradigm is universally applicable to wetlands; instead, allogenic and autogenic processes should be assessed along the hydrologic continuum (HGS, riparian continuum, HGM, or components of hydroperiod).

  • Allogenic Factors: Many wetlands are initially restricted by these external conditions, which govern their ecological state.

  • Autogenic Processes: If hydrologic settings are suitable, autogenic processes may subsequently dictate biological communities,

  • Example: Peatlands transition from allogenic to autogenic as organic matter accumulates.

Pielou's Hypothesis on Community Boundaries

• Centrifugal Organization Hypothesis by Wisheu and Keddy (1992): Discusses the relationship between community organization and environmental gradients.

  • Community Unit Hypothesis: Expectation of clustering boundaries; upper and lower boundaries should fluctuate together along gradients.

  • Individualistic Hypothesis: The average number of boundaries should be consistent along different gradients, with independence between upper and lower boundaries.

van der Valk's Approach

• Critique of van der Valk’s methods and oversights include:

  • Addressing the simplification of Clements' views.

  • The effect of consequences arising from competition could classify models as autogenic.

  • What if local biotic interactions primarily constitute disturbances?

Keystone Species

• Definition: A keystone species is one whose ecological impact on the community or ecosystem is significant and disproportionate relative to its abundance or biomass.

  • Discussion whether these species fit within allogenic or autogenic categories, especially in context of ecological models.

Implications of Competition and Community Structure

• Competition: Organizes communities along environmental gradients.

  • Centrifugal Models: State competitive hierarchies inherent in core habitats versus peripheral areas, influenced by stressors and ruderals.

Predictions and Anthropogenic Impact

• Wisheu and Keddy’s Prediction: Rare species typically found in peripheral habitats.

  • Implications for wetland plant richness due to anthropogenic factors like eutrophication spread from edges into wetland areas.

Contributions by Mitsch and Gooselink (2001)

• Notable assertion: Common endpoints of successional processes in bogs versus previous terrestrial forest states.

  • Discourse surrounding whether these processes are still directional or predictable in nature.

Peatland Development Dynamics

• Two critical processes for peatland formation:

  • Positive water balance.

  • Peat accumulation exceeding decomposition rates.

  • Importance of hydroperiod, intensity, duration, and type of hydrological inputs.

  • Noteworthy statement: Once established, bogs exhibit resilience to alterations in water balance.

Succession Types in Peatlands

• Terrestrialization: Detailed as the infilling process of shallow hydrological features, with unique forms of succession associated:

  • Quaking Succession: Associated with specific vascular plant frameworks and hydrological regimes.

  • Paludification: Upland vegetation undergoes change following terrestrialization.

Complexity of Throughflow Succession

• Throughflow Succession: Directly linked to surface water variability over time.

  • Shifts in inundation promote peat formation during low intensity conditions.

Transition in Intermediate Properties

• Changes in local environmental gradients alter as peatlands form, mediating landscape factors.

  • Metrics include landform characteristics, conductivity, and topography.

Vegetational Response to Environmental Gradients

• Vegetation adapts in relation to changing environmental factors such as nutrient availability and water-level fluctuations as regulated by autogenic processes.

  • Evidence from various studies highlights species richness and their correlation with nutrient concentrations (e.g., HCO3-).

Exotic Species and Their Impact

• Exotic Species: Definition of sources and classifications.

  • Invasive Species: Define these as exogenous or indigenous species that displace others and alter ecosystems.

Conservation and Exotic Species Dynamics

• Addressing ecological distinctions between exotic and invasive species, with references to their ecological impact.

  • Importance in understanding that many exotic species are symbiotic or benign.

  • Acknowledgment that the majority of introductions fail to naturalize.

Impacts of Invasive Species on Biodiversity

• Statistics highlighting the impact of introductions on native species, particularly endangered ones.

Case Studies of Invasive Species

• Competition: A review of Purple Loosestrife (Lythrum salicaria), termed the ”Purple Plague”, detailing its introduction and profound ecological impact on wetlands.

• Predation: Discuss the introduction of the Brown Tree Snake (Boiga irregularis) and its significant effect on native bird populations in Guam.

• Introduced Mammals: Overview of Feral pigs and their historical and ecological consequences in wetland environments through predation and habitat disruption.

Disease Dynamics in Ecosystems

• Chestnut Blight (Cryphonectria parasitica): Discussed as an introduced fungus responsible for widespread destruction of the American Chestnut.

• Detail the cycle of infection, regeneration, and how it leverages historical introduction impacts.

Hybridization Concerns

• Discusses how hybridization affects native species populations, pointing to cases such as the Phragmites australis and the implications of cryptic invasive strategies on native wetlands.

  • Emphasizes difficulties in controlling invasives due to their aggressive strategies in mixed communities.