0.375
37.5
%
0.625
40
species
60
%
50
%
0.375
37.5
%
0.625
40
species
60
%
50
%
The Jaccard Similarity Index Calculator measures how similar two ecological communities are based on shared and unique species. The Jaccard Index ranges from 0 (no species in common) to 1 (identical species composition). It is one of the most widely used similarity coefficients in community ecology, biogeography, and conservation biology.
By comparing species lists between two sites, habitats, or time periods, ecologists can quantify turnover, assess habitat connectivity, and evaluate the impact of environmental changes on community composition.
The Jaccard Similarity Index is calculated as:
J = C / (A + B - C)
Where:
The Jaccard Distance (1 - J) measures dissimilarity and is commonly used in cluster analysis and ordination.
Inputs
Results
The union is 25+30-15=40. With 15 shared out of 40 total unique species, J = 0.375 indicates moderate similarity.
Inputs
Results
The union is 20+22-18=24. J = 18/24 = 0.75 indicates high similarity between these communities.
A Jaccard Index of 0 means the two communities share no species at all. They have completely different species compositions, which may indicate very different environmental conditions, geographic isolation, or distinct habitat types.
Both measure similarity based on shared species, but the Sorensen Index gives more weight to shared species by doubling them in the numerator. The Jaccard Index uses the union as the denominator, while Sorensen uses the sum of both sets. Sorensen values are always equal to or greater than Jaccard values for the same data.
Use the Jaccard Index when you have presence-absence data for species in two communities. It is appropriate for comparing species lists, evaluating habitat similarity, tracking community changes over time, and assessing the effectiveness of wildlife corridors or reserves.
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