In-depth Notes on Community Structure and Interactions

Community Structure

Definition of Community: A community consists of a group of species living in a specific area, interacting directly or indirectly.

Spatial Concept: Communities are spatially defined, existing within specific boundaries.

Narrow Use: Community may refer to groups sharing similar environmental responses (e.g., plant community, bird community).

Attributes of Community

Key Attributes:

• Species Number and Relative Abundance: Measures how many species are present and their population sizes relative to one another.

• Physical Structure: Often defined by growth forms of plants.

• Interactions Among Species: Includes predation, competition, mutualism, etc.

Biological Structure Defined by Species Composition

Species Composition: Defines the biological structure of a community based on various species.

Measurement of Individuals: The number of individuals can be counted to assess community composition.

Relative Abundance: More informative than sheer count:

pi = ni / N
Where:

• pi: Proportion of individuals of species i

• ni: Number of individuals of species i

• N: Total number of individuals in all species

Rank Abundance Diagrams

Plot Characteristics: Rank abundance diagrams plot relative abundance against ranks, providing insights about community structure.

Key Features:

• Species Richness (S): Total number of species in a community.

• Species Evenness: Distribution equality among species.

• Curve Interpretation:

  • Longer curves indicate greater species richness.

  • More gradual slopes suggest higher evenness in species distribution.

Species Diversity Indices

Simpson's Diversity Index (D):

D = Σ pi^2
Measures probability that two randomly selected individuals belong to the same species.
Ranges from 0 (infinite diversity) to 1 (no diversity).

Simpson's Index of Diversity:

1 - D: Higher values indicate more species diversity.

Shannon Index (H): A widely used measure of species diversity.

H = -Σ (pi * ln pi)

Ranges from 0 (only one species) to ln(S) (all species equally abundant).

Species Evenness (E):

EH = H / Hmax
Ranges from 0 to 1, indicating complete evenness when all species are equally abundant.

Keystone Species

Definition: Species with a disproportionately large impact on their community compared to their abundance.

Roles of Keystone Species:

• Habitat creation/modification.

• Influencing interactions among species.

Example: The coral species Oculina arbuscula creates essential habitat.

Predator Examples: Sea otters in kelp forests illustrate the keystone concept through their role in regulating sea urchin populations, which in turn affects kelp biomass.

Food Webs and Species Interactions

Food Chain: Represents feeding relationships in a community.

Complexity of Food Webs: Actual relationships are more complex than simple chains, comprising numerous interlinked chains.

Key Components of Food Webs:

• Basal Species: Autotrophs that do not eat but provide energy to other species.

• Intermediate Species: Herbivores or carnivores that serve as prey for others.

• Top Predators: Feed on intermediate species but are not predated upon themselves.

Analyzing Food Web Structure:

• Links between species indicate predation and feeding relationships (Direction of the arrow indicates energy flow).

• Connectance (C): C = L / S²
  Where L is the actual number of links and S² is the maximum possible links.

• Linkage Density (LD): Average number of links per species: LD = L / S
  Indicates the complexity of interactions.

Community Physical Structure

Definition: Communities have biological and physical structures, influencing habitats and species distributions.

Aquatic Community Layers:

• Lakes exhibit stratification (epilimnion, metalimnion, hypolimnion) and varying temperatures and oxygen levels.

Photosynthetic Layers:

• Photic Layer: Light penetration for photosynthesis.

• Aphotic Layer: Insufficient light for photosynthesis.

• Benthic Layer: Bottom sediments with decomposers.

Zonation in Community Structure

Definition: Zonation refers to spatial changes in community structure across landscapes.

Example of intertidal zones (supratidal, intertidal, subtidal) highlighting species adapted to different tidal conditions.

Overall Summary

Communities are defined by species composition and interactions, influenced by physical structures and spatial changes in the environment. Key metrics like species richness, evenness, and various diversity indices help in assessing biodiversity and community health.