Chapter 16 - Lecture

Chapter 16: Species Abundance and Diversity

Overview

• Focuses on the relationship between species abundance and diversity in ecological communities.

• Explains that most species are moderately abundant, with few being extremely abundant or rare.

Key Concepts

• Concept 16.1: Most species are moderately abundant; few are very abundant or extremely rare.

• Concept 16.2: Species diversity defined by both the number of species and their relative abundance.

• Concept 16.3: Higher species diversity in complex environments.

• Concept 16.4: Intermediate levels of disturbance enhance species diversity.

Community Structures

• Community: A collection of interacting species in a defined area.

  • Attributes include number of species and relative species abundance.

• Guild: A group of organisms utilizing similar resources.

  • Example: Seed-eating animals in a desert.

• Life-form: A classification based on structure and growth dynamics of plants.

Species Abundance

General Patterns

• Regularities in species abundance: few are very abundant, few are rare, and most are moderately abundant.

• Preston studied these patterns and proposed concepts of commonness and rarity.

The Lognormal Distribution

• Graphs of species abundance show a bell-shaped curve when plotted on a log2 scale.

• Most lognormal distributions only show part of the curve.

• Sample size significantly influences the observed distribution, requiring significant effort to capture rare species.

Species Diversity

Defining Factors

• Species Richness: The total number of different species in a community.

  • Example: Tide Pool A with 4 species has greater richness than Tide Pool B with 2 species.

• Species Evenness: The relative abundance of each species.

  • Measures how equally individuals are distributed across species.

Comparative Community Analysis

• Community 1 has an even distribution among species (25% each).

• Community 2 shows low evenness, where one species dominates (68.75% abundance).

Quantitative Measures

• Shannon-Wiener Index: A quantitative index to measure species diversity, incorporating both richness and evenness.

  • H' = -Σ(Pi log(Pi)), where Pi is the proportion of the ith species.

• Rank-Abundance Curves: Show relative abundance and allow comparisons of species diversity across communities.

  • A lower slope indicates higher evenness, while a steeper slope indicates less evenness.

Environmental Complexity

Impact on Species Diversity

• Generally, species diversity increases with environmental complexity.

• MacArthur's Study: Found a positive relationship between foliage height and warbler diversity.

• Environmental complexity aids in the coexistence of species by providing varied niches.

Paradox of the Plankton

• Describes coexistence in phytoplankton communities that compete for limited resources despite a simple environment.

• Similar patterns observed in terrestrial plant communities due to environmental complexity.

Nutrient Availability in Plant Niche

• Tilman's Findings: Coexistence of diatoms depends on nutrient ratios (silicate and phosphate).

• Variations in limiting nutrients affect the diversity of algal and plant communities.

Research Studies on Complexity

• Lebo et al. studied variability in nutrient concentrations in Pyramid Lake, revealing non-uniform phytoplankton growth conditions.

• Robertson et al. studied nitrogen and moisture variations in abandoned fields, indicating the importance of spatial variability for community structure.

Disturbance and Diversity

Role of Disturbance

• Intermediate levels of environmental disturbances increase species diversity.

• Disturbance types can be abiotic (fires, floods) or biotic (predation, disease).

Intermediate Disturbance Hypothesis

• Proposed by Connell, stating that both high and low disturbance levels reduce diversity.

• Intermediate levels allow for a wider variety of species to thrive without competitive exclusion.

Case Studies

• Intertidal Zone Studied by Sousa: Found diversity peaks at intermediate disturbance levels of boulders.

• Prairie Dogs in Grasslands: Their burrowing behavior creates disturbances leading to increased biodiversity.

Human Impact on Diversity

Urbanization Effects

• Urban environments show varied impacts on species richness and diversity.

• Intermediate land use intensity can create peaks in bird species richness.

Conclusion

• Understanding species abundance and diversity is crucial for conservation efforts and ecological studies.

• Highlights the importance of maintaining environmental complexity and managing disturbance levels to promote biodiversity.