Hilsenhoff Biotic Index

The Hilsenhoff Biotic Index (HBI) estimates the overall tolerance of the community in a sampled area, weighted by the relative abundance of each taxonomic group (family, genus, etc.). Organisms are assigned a tolerance number from 0 to 10 pertaining to that group's known sensitivity to organic pollutants; 0 being most sensitive, 10 being most tolerant. An example of how one would use this equation to calculate a biotic index:
Group (i) Tolerance Value (a) Specimens (n) Product (n x a)
 Perlodidae
2
5
10
 Baetidae
4
3
12
 Uenoidae
3
7
21
Total
15
43

To calculate the Hilsenhoff Biotic Index number (HBI), multiple the tolerance values times the number of specimens (n x a). The sum the products (sigma, ∑). The total is divided by the total number of specimens in all groups (N). In this example:

HBI = 43 ÷ 15 = 2.87

How to Interpret the Results
• Insect taxa are assigned tolerance values based on the ability of the particuar insect to live under a variety of stressful conditions, such as low oxygen content in the water. Although tolerance values can be applied to a family, species within that family may vary. • After all of the observed groups have been identified, an average can be calculated using the HBI equation.

• HBI values range from 0 to 10. Low HBI values reflect a higher abundance of sensitive groups, thus a lower level of pollution. The HBI in the above example was 2.87. What does that say about the health of the stream?