Grade Level:
Middle School, High School
Ecological Concepts: Biodiversity,
Species richness
Arizona Science Standards: Science as Inquiry; Life Science
Materials:
1) Plankton net, optional (see directions to make, below)
2) Dip nets*
3) Plastic trays*
4) Writing/drawing materials
5) Sweep nets
6) Pipettes*
7) Forceps*
8) Field measuring tape*
9) Pond field guides*
10) Hand lenses *
*May be borrowed from SCENE.
BACKGROUND
Insects are six-legged arthropods
with exoskeletons.
They are found in almost every ecosystem worldwide. Some are carnivores,
others herbivores,
omnivores,
parasites,
or parasitoids.
Insects occupy many niches
and microhabitats.
They are not always obvious, but every habitat has insects of some
sort.
Insects inhabit almost all areas of the Earth. Which types and species
occupy an area will vary depending on the biotic
and abiotic
characteristics of the habitat. Also, the number and types of insects
found in a habitat change over time. Potential factors causing these
changes are birth, death, immigration,
and emigration.
These factors in turn are influenced by what is available in the habitat
for the insects to be able to live and reproduce.
Some aquatic insects spend their entire life in the water, while others
spend only part of their life there, usually the larval
stage. Most species with aquatic larvae spend their terrestrial adult
stage near water, feeding, mating, and laying eggs. Aquatic
insects are found on submerged plants, free in the water itself, around
rocks, on and in the sediment (mud) of the body of water. In each
of these microhabitats
particular insect species will be found, filling a niche
in the aquatic environment. Within a habitat the organism finds necessary
resources such as food and safety from predators.
GUIDED INQUIRY
Observation/Exploration: Examine the water habitat for insects
or signs of insects or other arthropods such as spiders, mites, or
amphipods
and isopods.
Signs include spider webs, cocoons, eggs on the plant leaves or stems,
exoskeletons, and dead insects. Look on and under plants, inside leaves,
in the air using a sweep
net, in plant debris (leaf litter) on the sediment, in
the water, and in the mud.
Group Discussion and Question Period: Why do some plants or parts
of plants have more insects than others? Do we see many insects out
in the water? What insects are in the mud? How many different kinds
of insects are in the habitat? Does time of day or year change what
insects will be here? What insects are in our pond? Do some
places in the pond have more insects than other places? What do these
animals eat? Which ones live their whole life in the water? How do
aquatic insects avoid predators like other insects, fish, and birds?
Important aspects of guided inquiry are encouraging students to
generate multiple
hypotheses, and letting students make decisions about
what data are important and create their own data sheets.
Keeping these ideas in mind, the sample in the box below illustrates
how ONE OF MANY possible investigations around this topic might develop.
Sample
Hypothesis : Let's use the question: "Do
some areas of the pond have more insects than other areas?"
Our hypothesis could be: "As plant density increases, insect
density will increase." Or, "Areas of the pond with
more plants will have more insects because the food or safety
they need is there."
Humans may have determined the original placement of plants
in the schoolyard pond, but where insects will be found, how
many, and what species are unknown until a survey is done. To
get beginning, or baseline, data, we need to set up a method
for collecting it. Test the hypothesis that insects prefer places
with more plants by setting up an experiment in which the
independent variable is plant density and
the dependent
variable is numbers/species of insects found.
Initial Survey: Sketch the pond, drawing in
the plant locations. Divide the pond (on paper) into sections
that are approximately equal in size and shape (see Figure 1.
below). Use stakes or flagging
along the edge of the pond to mark the sections after measuring
with the field
measuring tape. Each section will be searched
for insects. Divide students into groups. Each group will search
one section of the pond for insects and other athropods. Each
group will need a white tray, a dip net, a hand lens, forceps,
pipette, paper and pencil, and any other aquatic insect collecting
equipment that is available. Students will search their section
of the pond for aquatic insects on and around plants, in the
mud, in the open water, and in the air, and record the number
and types of insects found. Be careful, for your safety and
for that of the insects. Keep the animals wet and release them
once the data are collected.
Figure 1. Diagram of pond divided into eight
sections and marked with flagging.
Sample Prediction: The types and numbers of insects will
be higher in areas of the pond with more plants.
Record Results: Record the number of plants and insects
found in each pond section. Group insects according to general
body characteristics, e.g., beetles, dragonflies, damselflies,
or other arthropods. At this point it is not necessary to know
the names of the insects. Count how many insects of each type
were found.
Sample Analysis of Data and Presentation: Graph the total
number of insects for each section of the pond, and the numbers
of each type of insect on a bar
graph, with plant density on the horizontal axis
and insect numbers on the vertical axis. Or, you can graph the
total number of insects on one graph, and numbers of individual
types of insects on a separate graph.
Discussion: Was your hypothesis supported? If yes, go on
to test other hypotheses. If not, why not? What did happen?
Why? This is a great opportunity to revise your hypothesis and
do another test.
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MORE:
(1) Middle School:
(a) Find the mean,
median,
mode
and range
of the data.
(b) Test what would happen to insect density if
the types of plants were changed.
(c) Use a plankton
net to capture zooplankton
in the open water and near the edges. Compare the types and numbers
found in the two areas, open water versus edge. Use a pond guide
to identify the zooplankton.
(2)
High School:
(a)
Calculate the variance
and standard
deviation of the averaged data within the low and high
plant density sections.
(b) Conduct the insect counts at different times
of the year. Test for a correlation
between the two variables, number of insects and time of year. Draw
a scatter
plot and calculate the sample
correlation coefficient. This will yield data indicating
what changes are occurring over time in the community
of insects. Map the data onto a diagram of the pond.
(c) Calculate species
richness of insects for high versus low plant
density sections.
(d) Do a t-test
of species richness. (T-test is a standard statistics test comparing
means
of two samples).
(e) If the pond
is uniformly dense with plants on all the edges, set up a manipulative
experiment where sections of the pond are planted with more plants
to increase the density. Split the pond (on paper) into eight sections.
Use stakes or flagging along the edge of the pond to mark the sections
after measuring with the field measuring tape. Increase the plant
density in every other section to get four high-density replicates
and four low-density replicates. (You might be able to just move
plants around to change density. Aquatic plants are often easily
moved.) Survey the sections of the pond before doing the manipulation
to get baseline data. Put the plants in, give them time to take
root, and time for the aquatic animals to acclimate
to the change. Survey each section as before, trying not to
be too disruptive. Depending on the shape of the bottom of the pond
and the material it is made of, you may be able to temporarily place
a triangle of solid walls to divide each section while surveying.
This will keep animals in their respective section during the survey,
resulting in a more accurate count.
All levels:
Making your own plankton nets is easy, and much
cheaper than buying them from a biological supply house.
Supplies needed:
Metal or plastic embroidery hoop
Old bed sheet
Small plastic pill vial
2 pieces of strong, flexible wire, each 10 cm longer than hoop diameter
6 meters of nylon fish line
1 rubber band
To make the net: Experiment with a pattern to create
one that will form a cone when one side of the cloth is sewn all the
way. Sew the bed sheet cloth into a cone shape. Make the top as big
around as the hoop and the other end as narrow as the pill vial opening.
Wire yoke: lay the 2 wires next to each other. Bend them in half together.
Twist the two wires together in the middle to create an opening. This
will be for tying the fishing line on to.
Assemble the net: Put the hoop onto the top of the net by separating
the two parts of the hoop, placing the cloth over the inner hoop and
the larger hoop over the cloth and inner hoop. Drop the pill vial
into the bottom of the cone with the bottom of the vial coming out
the opening of the cone. Secure it in place with the rubber band over
the cloth. Attach the wire in four places evenly around the large
opening. Poke the wire ends through the cloth, pull through and twist
back on themselves. Tie the fishing line to the loop at the top of
the wires.
To use the net, throw it underhand out into the water, let it sink
a little bit, and then slowly pull it back in. Hold it upright until
all the water except what is in the vial has drained. Invert the net
and pour the contents of the vial into a plastic cup.
Figure 1. Assembling a plankton
net.
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