Grade Level:
Middle School, High School
Ecological Concepts: Community, Immigration,
Interspecific interactions
Arizona Science Standards: Science as Inquiry; Life Science
Materials:
1) Magnifying lens, loupes*
2) Plankton net (See directions to make, below)
3) Wading pools or large plastic washtubs/dishpans
4) Writing/drawing materials
5) Cups
6) Fine mesh cloth*
7) Bird netting
8) Water de-chlorination drops (optional)
*May be borrowed from SCENE.
BACKGROUND
Most organisms, at one life
stage or another, immigrate to another area. These migrations, into
or out of an area or habitat, create ecological communities.
Plants move from place to place in the pollen or seed form. Plant
pollen or seeds move by wind, water, and animal carriers. Terrestrial
animals move under their own locomotion, or if small enough, by the
wind or other animal carriers. Aquatic organisms migrate by water,
but also by air or animal carriers. Some aquatic organisms can withstand
periods of drying and thus are capable of being moved from one body
of water to another.
GUIDED INQUIRY
Observation/Exploration Period: Set out some wading
pools in the habitat and fill with de-chlorinated water. (Instructions
for how to de-chlorinate are under Sample Experiment Design, below.
It's easy!) Leave them open. Students can use their eyes and loupes
to observe the contents of the water over time. Students can also
observe other, terrestrial parts of the habitat for signs of changes
in the composition of inhabitants over time.
Group Discussion and Question Period: What causes new areas to become populated with organisms? How long does it take for a new pool of water to have life growing in it? Do the organisms in an area change naturally over time? What causes the numbers or species to change in an area?
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, "What causes
new areas to become populated with organisms?" The hypothesis
could be: "If pools of water are colonized by windblown organisms
and we place open and covered pools in the same area, then the
pools in the open will be colonized sooner than the covered
pools."
Sample Experiment Design: The independent
variable is the type of pool, open to the wind
versus not open. The dependent
variable is the number or density of organisms
that colonize the pools. Place four wading pools near each other
in the habitat. (You can also use smaller pools, such as plastic
washtubs, for space and cost considerations.) Fill each pool
with the same amount of water. The water needs to be de-chlorinated.
Water can be de-chlorinated either with drops for that purpose
purchased from a pet supply store, or by letting the water sit
in the open air for a few days. The chlorine will naturally
dissipate into the air, especially if the water is stirred off
and on. Two pools are left completely open to the air, but covered
with bird netting to prevent birds from entering the water.
The other two pools are covered with a fine mesh net or cloth.
This will allow airflow and should keep the water temperatures
in all pools similar. This way, you will have two replicates
for each treatment
and be controlling
for other factors such as temperature and location.
Sample each pool weekly with a plankton
net. (See All Levels below for instructions on
how to make your own plankton nets.) Collect 0.1L (100ml) water
samples and pour into individual plastic cups. If it looks like
"stuff" (possibly plankton) is sticking to the inside of the
plankton net collecting vial, use about 5-10 ml of fresh de-chlorinated
water to rinse the vial contents into the sample cup. Sort and
categorize the organisms found using a dissecting microscope
or loupe. A pond guide will be helpful. The two basic categories
are zooplankton
and phytoplankton.
For each sample calculate the density (number of organisms/liter
water). Sample the pools the same way each time— swirl
the pool water, take one net full, empty into water filled cup.
Calculate the density of organisms in the pool by dividing the
number of organisms in the sample of a pool by the amount of
water taken for the sample. For example, if the 0.1 L (100 ml)
sample taken from Pool A has 15 organisms then the density is:
15 organisms/0.1 L which equals 150 organisms/liter. To estimate
the total number of organisms in the pool, multiply the number
of organisms per liter by the number of liters in the pool.
Using the above example, this would be: 150 organisms/L x 10L
= 1500 organisms. Empty the sample cup and its organisms back
into each pool to preserve the population numbers close to normal.
Note: Water levels will drop over time due
to evaporation. Mark the pools or tubs at the fill line and
keep them filled with de-chlorinated water.
Sample Prediction: Pools in the open will be
colonized more quickly and by more organisms than the covered
pools.
Record Results: Collect data for four weeks
(more if you have time).
Sample Analysis of Data and Presentation: Make
a bar
graph of the raw numbers with time on the horizontal
axis and density of organisms on the vertical axis. For students
who can divide, calculate the average
density for two pools per treatment. Graph the average number
on the vertical axis.
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. |
MORE:
(1) Elementary:
(a) Try the same experiment with pools in the sun
and pools in the shade. Does light make a difference?
(2) Middle School:
(a) Find the mean,
median,
mode
and range
of the data.
(3) High School:
(a) Calculate the standard
deviation of the averaged data.
(b) Perform a T-test
of species richness. (T-test is a standard statistics test comparing
means of two treatment groups.)
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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