STREAM BIODIVERSITY, BIOTIC INDEX & ECOSYSTEM
HEALTH
Biotic Index
The simplest measure of environmental quality in streams is the “biotic index” developed
by Scott (1969). The presence or absence of certain types of organisms reflects the level of
pollution and overall health of a stream. In this system, all macroinvertebrates are placed in one
of three categories :
Class 1 Organisms “S” (pollution sensitive)
These are organisms that are highly sensitive to pollution.
Class 2 Organisms “F”(moderately tolerant of pollution)
These are organisms that do fairly well in both clean and polluted streams and are most
abundant where there is a moderate amount of organic input (waste matter).
Class 3 Organisms “T” (pollution tolerant)
These organisms may be found in clean as well as polluted streams, but are highly tolerant
of organic pollution. They tend to be most abundant in such areas, probably due to the absence
of competition from less tolerant organisms.
Calculate the biotic index for your stream section as follows:
The following organisms were found in a section of a stream.
Identify each of the organisms using the classification key, and check the class (1,2 or 3) to
which each critter belongs. Total numbers do not matter in calculating the overall biotic index;
however, do keep track of the total number found. This will give us an idea of the population
density of these organisms and will also be used in evaluating the overall health of the stream.
Organism name
Total #
Class 1
Class 2
Class 3
2. Add up the total number of different Class 1 species, the total number of Class 2 species,
and the total number of Class 3 species in your picture.
# Class 1 species =
# Class 2 species =
# Class 3 species =
3. Compute the Biotic Index as follows:
B.I. = 3 x (number of Class 1 species) + 2 x (number of Class 2 species)
+ 1 x (number of Class 3 species)
B.I. = 3 (
) + 2(
) +1(
)
B.I. =
Judge the water quality as follows :
Excellent (clean stream) --------------------------- B.I. 22+ OR 4+ class 1 (S)
Good (may be slight pollution but generally healthy)---------- B.I. = 17-21 OR 2-3 class 1(S)
Fair (moderately polluted) ------------------ B.I. = 11 – 16 OR 1 class 1 (S)
Poor (grossly polluted) ----------------------- B.I. < 10 OR 0 class 1 (S)
Water quality at your stream section is __________________
In order for you to see how pollutants impact the biodiversity of a stream and the resulting biotic
index, complete the Stream Biodiversity simulation:
http://faculty.etsu.edu/jonestc/Virtualecology/Models/Stream.html
Set your sampling time to 300 sec, set the pollution levels to none and open your seine net to collect
the macroinvertebrates that your stream study team is dislodging from under the rocks in your
stream study area.
Take a screen capture of your data table and graph.
Release the organisms from your net and alter the pollution levels in order to determine the impact
of pollutants on the number and diversity of organisms in your stream area.
Collect enough data/screenshots and compare your data with 1 other group so that you can analyze
your results and draw a conclusion.
POLLUTION = NONE
POLLUTION = MODERATE
POLLUTION = SEVERE
What did you discover about the impact of pollutants on species abundance and species diversity?