Key Takeaways
- The Shannon-Wiener Index measures uncertainty in species identity - higher values indicate greater diversity
- Simpson's Index represents the probability that two randomly selected individuals belong to different species
- Evenness shows how equally distributed individuals are among species
- A healthy ecosystem typically has high species richness AND high evenness
- Different indices capture different aspects of biodiversity - use multiple measures for comprehensive analysis
What is Biodiversity?
Biodiversity refers to the variety of life in a particular ecosystem or on Earth as a whole. It encompasses three levels: genetic diversity within species, species diversity within communities, and ecosystem diversity across landscapes. Measuring biodiversity helps ecologists understand ecosystem health, conservation priorities, and the impacts of environmental change.
Biodiversity indices provide quantitative measures that allow scientists to compare different ecosystems, track changes over time, and assess the effectiveness of conservation efforts. The most commonly used indices are the Shannon-Wiener Index and Simpson's Index.
Understanding Biodiversity Indices
Shannon-Wiener Index (H')
The Shannon-Wiener Index (also called Shannon Entropy) measures the uncertainty in predicting the species identity of a randomly selected individual. It accounts for both species richness and evenness.
H' = -SUM(pi * ln(pi))
Typical H' values range from 1.5 to 3.5, with most ecosystems falling between 1.5 and 3.5. Values above 3.5 indicate extremely high diversity, while values below 1.0 suggest low diversity or a highly disturbed ecosystem.
Simpson's Index (D and 1-D)
Simpson's Index measures the probability that two randomly selected individuals belong to the same species. It is more sensitive to dominant species than the Shannon Index.
D = SUM(ni(ni-1)) / N(N-1)
Simpson's Diversity = 1 - D
Evenness (Pielou's J)
Evenness measures how equally individuals are distributed among species. An evenness of 1 means all species have equal abundances, while lower values indicate dominance by a few species.
E = H' / ln(S)
Margalef's Richness Index
Margalef's Index adjusts species richness for sample size, providing a standardized measure of species richness that accounts for sampling effort.
DMg = (S - 1) / ln(N)
Comparing Biodiversity Indices
| Index | Range | Sensitivity | Best Use |
|---|---|---|---|
| Shannon-Wiener (H') | 0 to ~4.5 | Rare species | General diversity assessment |
| Simpson (1-D) | 0 to 1 | Dominant species | Community dominance analysis |
| Species Richness (S) | 1 to infinity | Sample size | Quick species counts |
| Evenness (E) | 0 to 1 | Distribution | Abundance patterns |
| Margalef | 0 to infinity | Sample size adjusted | Comparing different sample sizes |
How to Use This Calculator
Step 1: Enter Species Data
Input the number of individuals observed for each species. Species names are optional but helpful for record-keeping. You can add as many species as needed using the "Add Species" button.
Step 2: Calculate Indices
Click the Calculate button to compute all biodiversity indices. The calculator will automatically determine species richness, proportional abundances, and all diversity metrics.
Step 3: Interpret Results
Review the calculated indices and the interpretation provided. Consider the ecological context when evaluating whether diversity values are high, moderate, or low for your ecosystem type.
Practical Applications
Environmental Monitoring
Biodiversity indices are essential tools for tracking ecosystem health over time. Regular monitoring can detect early warning signs of environmental degradation, pollution impacts, or climate change effects.
Conservation Planning
Conservation biologists use these indices to identify high-priority areas for protection, assess the effectiveness of restoration projects, and compare biodiversity across different habitats.
Environmental Impact Assessment
Before and after studies using biodiversity indices help quantify the impacts of development projects, land use changes, or pollution events on local ecosystems.
Frequently Asked Questions
What is a good Shannon Index value?
For most terrestrial ecosystems, a Shannon Index (H') between 2.0 and 3.5 indicates moderate to high diversity. Values above 3.5 are considered very high, while values below 1.5 suggest low diversity. However, "good" depends on the ecosystem type - grasslands naturally have lower diversity than tropical rainforests.
Why use multiple diversity indices?
Different indices are sensitive to different aspects of community structure. Shannon is more influenced by rare species, while Simpson emphasizes common species. Using both provides a more complete picture of biodiversity.
How many individuals should I sample?
Sample size significantly affects species richness estimates. Generally, larger samples capture more species. Use rarefaction or the Margalef Index when comparing samples of different sizes.
Can I compare indices between different ecosystems?
Yes, but with caution. Different ecosystem types naturally support different diversity levels. Compare similar ecosystem types (e.g., temperate forests with temperate forests) for meaningful comparisons.