Calculating beta diversity helps us understand the difference in species composition between different locations or habitats. One classic method, proposed by Whittaker (1960), involves comparing local (alpha) diversity to regional (gamma) diversity.
According to Whittaker (1960), given a set of N plots, beta diversity can be summarized as the ratio of two inventory diversities measured at different scales. Specifically, β = γ/α, where α is the average diversity of the N plots and γ is the total diversity of all N plots combined (regional diversity).
Understanding the Components: α, γ, and β Diversity
To calculate beta diversity using Whittaker's ratio, you first need to determine the alpha and gamma diversity values for your study area.
- Alpha Diversity (α): This represents the average species diversity within a specific area or plot. It's the number of species found in a single local community, averaged across multiple such communities if you have them.
- Gamma Diversity (γ): This is the total species diversity across all the areas or plots within a larger region. It's the total number of species found when you combine all the local communities being studied.
- Beta Diversity (β): As calculated by Whittaker's ratio (γ/α), this measures the species turnover or the difference in species composition between the local communities across the region. A higher beta diversity value indicates greater differences in species composition between sites.
Here's a simple breakdown:
| Diversity Type | Scale | Description | Calculation Context (Whittaker) |
|---|---|---|---|
| Alpha (α) | Local | Average diversity within individual plots/sites | Average species count per plot |
| Gamma (γ) | Regional | Total diversity across all plots combined | Total unique species in region |
| Beta (β) | Turnover | Difference/Turnover between plots/sites | γ / α |
Step-by-Step Calculation (Whittaker's Method)
Let's walk through the calculation using a simplified example.
Imagine you surveyed three plots (Plot 1, Plot 2, Plot 3) and recorded the species present:
- Plot 1: Species A, B, C
- Plot 2: Species A, C, D, E
- Plot 3: Species B, E, F
-
Calculate Alpha Diversity (α):
- Plot 1 diversity = 3 species
- Plot 2 diversity = 4 species
- Plot 3 diversity = 3 species
- Average alpha diversity (α) = (3 + 4 + 3) / 3 = 10 / 3 ≈ 3.33 species
-
Calculate Gamma Diversity (γ):
- List all unique species found across all plots: A, B, C, D, E, F
- Total unique species (γ) = 6 species
-
Calculate Beta Diversity (β):
- Using Whittaker's formula: β = γ / α
- β = 6 / (10/3) = 6 * (3/10) = 18 / 10 = 1.8
In this example, the beta diversity of 1.8 indicates there is almost twice as much diversity at the regional scale (gamma) compared to the average local scale (alpha). This suggests a notable turnover of species between the plots.
Why Calculate Beta Diversity?
Calculating beta diversity is crucial for:
- Conservation Planning: Identifying areas with high species turnover can highlight regions important for preserving overall biodiversity.
- Environmental Monitoring: Changes in beta diversity over time can indicate habitat degradation or recovery.
- Understanding Ecological Patterns: It helps explain how species are distributed across landscapes and the factors influencing this distribution (e.g., environmental gradients, dispersal limitation).
While Whittaker's ratio is foundational, it's important to note that other metrics exist that quantify beta diversity in different ways, such as those focusing purely on species composition differences (dissimilarity indices). However, the method based on the α and γ relationship remains a key concept in biodiversity studies.
