Margalef’s Richness (DMg)

Understanding Margalef's Richness Index (D_Mg)

Margalef's Richness Index (D_Mg) is a measure of species richness that attempts to account for the effect of sample size (total number of individuals). It is calculated based on the total number of species (S) and the total number of individuals (N) in a sample.

Formula:

DMg = (S - 1) / ln(N)

• S is the Species Richness (total number of different species present).
• N is the Total Number of Individuals of all species in the sample.
• ln is the natural logarithm.

Interpretation:

• Higher D_Mg values generally indicate greater species richness for a given sample size.
• The index increases as the number of species (S) increases and decreases as the total number of individuals (N) increases (for a fixed S). This means that for the same number of species, a smaller sample size will yield a higher Margalef's index, reflecting the idea that finding S species in a smaller sample implies higher density of species.

Assumptions and Limitations:

• Assumes that the sample is representative of the community.
• Highly sensitive to N, especially at small sample sizes. The term ln(N) means the index is undefined if N=1 (as ln(1)=0) or N=0. It's generally used for N > 1.
• If S=1, D_Mg = 0 (a community with only one species has zero Margalef's richness beyond that single species). If S=0, the index is undefined.
• Like simple species richness (S), it does not account for species evenness (relative abundance of species).

How to Use:

1. Enter the number of individuals (count) for each species identified in your sample.
2. Use the "Add Species" button to create more input rows as needed.
3. Click "Calculate Margalef's Index" to see the results.