Community Structure and Composition

Topic 17: Community Structure

Overview of Community Ecology

• Previous Topics Covered:
    - Species interactions, including:
      - Interspecific competition (T15)
      - Other interspecific interactions (T16)

• Upcoming Topics: How to characterize a community includes:
    - Determining the number of species present.
    - Assessing how common or rare species are.
    - Identifying the factors that determine community makeup, such as:
      - Competition
      - Predation
      - Commensalism
      - Neutralism
      - Amensalism

Learning Objectives for Topic 17

• Upon completion of this lecture, you should be able to:
    1. Define species richness and species evenness, and describe how they are determined.
    2. Explain species diversity and calculate it using the Shannon-Weiner Diversity Index.
       - Reference to Activity 4 for practical application.
    3. Discuss the factors that affect species richness and diversity, including:
       A. Species Interactions (refer back to T15 & T16)
            - Keystone species (discussed in Topic 18)
            - Functional Redundancy
       B. Productivity (Topic 18)
       C. Ecosystem size (area) and proximity to source populations [Island Biogeography]; species-area hypothesis.
       D. Ecological Succession: different types including Primary and Secondary Succession.
       E. Heterogeneity (spatial and temporal) & Disturbances (related to the Intermediate Disturbance Hypothesis).
    4. Apply specific terms related to succession and use the Glacier Bay example to illustrate these concepts (including facilitation, inhibition, & competition).
    5. Assess changes in species richness/diversity throughout succession.
    6. Describe the significance of community diversity.

Properties of a Community

• Species Richness:
    - Defined as the total number of different species in a community.

• Evenness:
    - Describes the relative abundance of individuals among the different species present in a community.
      - Example Analysis:
        - Community A shows a richness of 4 with evenness similar across species.
        - Community B shows a richness of 4 with uneven distribution among species.

Measuring Species Diversity

• Species diversity encompasses both species richness and species evenness (relative abundance of each species).

• It offers a more comprehensive understanding of community structure and heterogeneity.
    - Calculation complexities may arise, such as:
      - Mathematical difficulties or abstract integrations.

Shannon-Weiner Diversity Index

• A method for quantifying species diversity by integrating evenness and richness into a single value:
    $H = - ext{∑} (p_i imes ext{ln} (p_i))$
    - Where:
      - $H$: Diversity index value.
      - $p_i$: Relative abundance of species (between 0 and 1).
        - $p_i = rac{ ext{number of individuals of a specific species}}{ ext{total number of individuals}}$
    - The natural log (denoted as ln) differs from log10 calculations.
    - Higher values of H correspond to higher biodiversity.

Example Calculation of the Shannon-Weiner Diversity Index

• Assuming relative abundances of four species are:
    - $p_1 = 0.25$, $p_2 = 0.25$, $p_3 = 0.25$, $p_4 = 0.25$

• Calculating natural log for each gives:
    - $- ext{ln}(0.25) = -1.39$.

• Multiplied by $p_i$ gives:
    - Values computed:
      - For each species, $p_i imes ext{ln}(p_i)$: -0.35, -0.35, -0.35, -0.35, -0.18, -0.15, -0.15, -0.23.

• Total diversity index values found:
    - $H = 1.4$ for one scenario.
    - $H = 0.71$ for another data point.

Factors Affecting Species Richness and Diversity

Main Factors Identified:

• Species interactions.

• Productivity levels within the ecosystem.

• Effects of Island Biogeography (area, proximity).

• Impact of Time (Succession).

• Influence of Heterogeneity and Disturbance.

Interspecific Interactions in a Community

• Types include:
    - Competition
    - Predation
    - Commensalism
    - Amensalism
    - Neutralism

• Example of significant impact on community structure using Keystone Species:
    - Species whose influence on the community is disproportionately large relative to their population size.
      - Pisaster sea stars as a keystone species affecting mussel populations that suppress other species in intertidal ecosystems.

Productivity's Influence on Species Richness

• Higher productivity generally results in increased species richness.

• Productivity patterns are influenced by:
    - Evapotranspiration (reflecting temperature and precipitation levels).

• Biomes with highest temperatures and significant precipitation tend to support greater species richness.

Island Biogeography Concepts

• Applied to diverse types of 'islands', including:
    - Alpine peaks, forest patches, and actual oceanic islands.

• Species-area hypothesis: Larger areas boast greater species diversity due to:
    - Increased habitat range and environmental diversity.
    - Enhanced possibilities for niche partitioning.

Main Factors of Island Biogeography:

1. Size: Larger islands have better immigration rates and lower extinction rates.
      - In contrast, greater distances from the mainland increase extinction rates and decrease immigration probabilities.

2. Proximity: The closer an island is to the mainland, the higher the potential for species richness.

3. Species richness is a dynamic balance between immigration (colonization) and extinction rates.

Ecological Succession

• Primary Succession:
    - Begins in lifeless areas such as bare rock formed from glacial retreat or volcanic eruptions. Species incident begins.

• Climax Community: The ultimate and stable association of species resulting from succession.

Stages of Succession in Glacier Bay:

1. Pioneer Stage: Colonizers like lichens and mosses appear on newly exposed rock.

2. Dryas Stage: Dryas plants emerge, outcompeting fireweed and enhancing soil fertility via nitrogen fixation.

3. Alder Stage: Alder trees shade out competitors while boosting soil fertility further.

4. Spruce Stage: Sitka spruce take over, further enhancing soil conditions to inhibit other species.

• Secondary Succession: Begins following disturbance of an existing community while maintaining soil integrity (e.g., post-fire scenarios).

Types and Stages of Early and Late Succession:

• Early Successional: Characterized by quick-growing, weedy plants.

• Mid Successional: Herbaceous plants and shrubs begin to proliferate.

• Late Successional: Eventually forms into a mature forest ecosystem.

Importance of Diversity

• Diversity relates closely to ecosystem health:
    1. Promotes ecosystem productivity.
    2. Enhances resilience against environmental stressors.
    3. Aids in resisting invasive species.

• Functional redundancy emphasizes that many species may perform similar roles; in a complex ecosystem, loss of one may have minimal impact.

• Conversely, simpler ecosystems may suffer greater impacts from species loss.

Summary of Learning Objectives (Reiterated)

• Understanding species richness/evenness, diversity calculations, and factors affecting these metrics in a community holds paramount significance in community ecology.

• Reviewing these concepts and applying them through practical examples, such as Glacier Bay, reinforces their utility in interpreting community dynamics.

Further Research Reference

• Study on ecological succession and community structure emphasized through genetic profiling of microbes using advanced techniques such as DNA metabarcoding in Antarctic soils.