Grade Level:
Elementary, Middle School, High School
Ecological Concepts: Competition
Arizona Science Standards: Science as Inquiry; Life Science
Materials:
1) Seeds of fast growing plant species
2) Pots, potting soil
3) Trowels*
4) Rulers
5) Writing/drawing materials
*May be borrowed from SCENE.
BACKGROUND
All organisms require certain resources for growth and reproduction. How well an organism accomplishes these goals depends on many factors such as the quality of the habitat, nutrient availability, and the make-up of the community. Organisms that live in a resource-poor habitat, or are living with more organisms than the habitat can sustain, are not likely to do as well as those in better habitats with more resources.
Any factor that is essential for growth and reproduction is a limiting
factor. Too little of it will result in death, or at
the least, very minimal growth. Organisms of the same species are
most likely to compete
for essential resources. Plants in the Sonoran Desert do not usually
have to compete for light, but do often compete for water, a very
limited resource most of the year. Other resources plants compete
for are nutrients such as nitrogen and phosphorus, and space for growth.
GUIDED INQUIRY
Observation/Exploration: Examine the habitat to get
an idea of how well the plants are growing and what the plant density
is. Density is calculated as the number of organisms, in this case,
plants, per unit area. For example, five plants in a 10 m2
plot would equal a density of 0.5 plants/m2.
Group Discussion and Question Period: What resources do plants need most? What happens if plants receive too little of a resource? What happens if lots of plants are growing very close together? Do plants of the same species grow better together than plants of different species?
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 happens if lots of plants are growing very close together?" Our hypothesis could be, "Plants growing closely together will not grow as well as those growing farther apart because of competition for resources."
Sample Experiment Design: Choose a fast growing
desert plant species, such as a wildflower. The independent
variable, or treatment, will be plant density.
Set up ten pots with potting soil. Plant three seeds in each
of five pots for the low density treatment. Plant fifteen seeds
in each of the other five pots for the high density treatment.
This gives five replicates
of each experimental
unit. Control
for factors such as light and water by placing all the pots
in the same location in the habitat and watering them the same
amount. Randomly place the pots so that not all pots of one
treatment are next to each other. Label each pot as to what
treatment it contains, high or low density. Cover the pots with
a fine
mesh netting to keep out herbivores. The netting
will need to be tall enough to accommodate growing plants.
The dependent
variable will be plant growth measured as height.
Let the plants grow for a set period of time, and measure height
again.
Prediction: Plants grown at low density will grow taller than those grown at a high density.
Record Results: Record heights for each plant
in centimeters. Raw data can be presented on a bar
chart with individual plant heights on the vertical
axis and plant density treatment on the horizontal axis.
Sample Analysis of Data and Presentation: Total
the height measurements for all plants in a treatment. Average
the height for plants in each treatment. Compare the averages.
Which plants grew most? Make a bar
graph or frequency
distribution with plant density treatment listed
on the horizontal axis and average plant heights on the vertical
axis. If data were taken multiple times on the same plants,
make a line
graph plotting the heights over time, with days
on the horizontal axis and average plant height each measurement
day on the horizontal axis.
NOTE: If you use seedlings for this experiment instead of growing
the plants from seeds, you will need to record the height of
the seedlings when you start the experiment, and then subtract
that height from the final height measurements at the end of
the experiment in order to get the actual amount of growth.
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) Elementary:
(a) Test a different limiting factor such as water.
Set up a similar experiment but use the same number of plants per
pot. Water five pots with twice as much water as the other five
pots. Compare the heights of the plants as before.
(2) Middle School:
(a) Find the mean,
median,
mode,
and range
of the data.
(b) Run an experiment similar to the one above
with plants at two densities, but leave them uncovered after they
are a reasonable height. Observe for herbivory. Does density affect
how much herbivory a plant suffers?
(c) Calculate plant densities in different areas
of the habitat and in other parts of the schoolyard.
(3) High School:
(a) Calculate the variance
and standard
deviation of the averaged data.
(b) Grow plants of the same species at other multiples
of densities, e.g., 2x, 4x, 10x.
(c) Compare different plant species at the same
densities. Which are the better competitors?
(d) Calculate plant cover. This is a measurement
of how much of the ground is covered by living plants. For example,
a grassy lawn would have 100% coverage, while an area the same size
with only two cholla cacti may only have 10% of the ground covered,
or shaded, by the cactus. For plants above ground level, estimate
the area of plant cover (the area of the ground shaded by the plant)
by measuring length and width of the horizontal canopy of the plant,
and multiplying to get area. Total individual plant cover estimates
and divide by the total habitat area to calculate plant cover.
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