The student will:
- Perform a simulated survey that demonstrates
how biological indicators are used to gauge
the health of a stream.
- Perform six different calculations with 100%
- Assess the “water quality” of
their sample with 100% accuracy.
- Comparatively determine the sample site that
indicates high pollution.
A - Mystery of the Missing Bugs Worksheet
Large bag of M&Ms
Bag of macaroni noodles
Box of Cheerios
Bag of beans
Small plastic bags
Animals are often more finely-tuned than humans
to changes in an environment. At one time, coal
miners used canaries to detect the presence of
dangerous gases because these birds are sensitive
to shifts in air quality. Similarly, aquatic macroinvertebrates
have differant tolerances to water pollution.
The health of a stream can be judged by the macroinvertebrate
species that are present and may indicate shifts
in an environment before such changes are easily
recognizable to humans.
Both macroinvertebrate species that are sensitive
to and those tolerant of pollution can live in
a stream with good water quality. With increased
pollution however, fewer sensitive species will
be found. If the stream’s water quality
is very poor, only those macroinvertebrates tolerant
of pollution will survive. EPT is an abbreviation
for Ephemeroptera, Plecoptera, and Trichoptera
(mayflies, stoneflies, and caddisflies), insect
groups that are largely intolerant of pollution.
If many EPTs are present, the water quality is
probably good. Macroinvertebrates capable of surviving
in very polluted water include aquatic worms,
leeches, and midge larvae.
The Hilsenhoff Biotic Index, or HBI, measures
the health of a stream by assigning tolerance
values to macroinvertebrates which range from
0 for organisms that are very sensitive to organic
wastes to 10 for organisms that are very tolerant
of organic wastes. The biotic index of a stream
site is found by collecting a sample of benthic
macroinvertebrates, then multiplying the number
of individuals within each separate species by
the tolerance value of the species. The resulting
products are then added together and this sum
is divided by the total number of organisms in
Students will be given the problem of locating
the most polluted site. M&Ms will represent
species very sensitive to pollution, Cheerios
will represent species semi-sensitive to pollution,
noodles will represent species semi-tolerant of
pollution, and beans will represent species tolerant
The instructor will:
- Prepare small plastic bags (one for every
two students) filled with a variety of M&Ms,
Cheerios, noodles, and beans. There should be
at least 30 pieces in each bag. All but one
of the bags should be composed of each of the
four groups, though care should be taken to
insure that the bags do not contain a majority
of beans or noodles. One bag, however, should
contain very few M&Ms, few Cheerios, and
be composed of a majority of noodles and beans.
- Tell students that the health of a stream
is often measured using macroinvertebrates as
bio-indicators and give an explanation of the
Hilsenhoff Biotic Index as provided in the Background
- Tell students that pollution has been detected
in a small stream near their home and samples
have been taken from a number of different sites.
Their job is to locate the most polluted site
based upon the “macroinvertebrates”
collected from the different sample sites.
- Divide the students into groups of two and
distribute a copy of Attachment A and one bag
to every group. Tell students that each bag
represents a different sample site and the M&Ms,
Cheerios, noodles, and beans represent different
macroinvertebrates found in the sites.
- Write on a blackboard that for the purposes
of this exercise, the M&Ms represent species
that are very sensitive to pollution and have
a tolerance value of 1. Cheerios represent species
semi-sensitive to pollution and have a tolerance
value of 4. Macaroni noodles represent species
semi-tolerant of pollution and have a tolerance
value of 6. Beans represent species that are
tolerant of pollution and have a tolerance value
- Direct the students to empty their bags and
divide their contents into four different groups:
Group 1: Sensitive to pollution, Group 2: Semi-sensitive
to pollution, Group 3: Semi-tolerant of pollution,
Group 4: Tolerant of pollution. Write the numbers
in each group on Attachment A.
- Direct the students to multiply the number
of items in each group by their respective tolerance
values, then add the four products together.
Students should record this information on Attachment
- Instruct students to calculate the biotic
index of their sample by dividing the sum of
the products in Step 6 by the total number of
“macroinvertebrates” in their sample.
This number should be recorded on Attachment
- Ask students to compare the biotic index of
their sample with the chart on Attachment A
and identify the water quality of their sample
- Direct each group to share the biotic index
and water quality of their sample site with
the rest of the class.
- Ask students to record on their worksheets
which group has the sample from the most polluted
site based on information provided from all
of the Missing Bugs Rubric (41KB pdf)
If time and resources allow, use this exercise as
an introduction, then conduct an actual macroinvertebrate
stream assessment. Numerous identification keys
are available on the Web, i.e. http://www.epa.gov/bioiweb1/html/benthosclean.html