Grade Level:
Middle School, High School
Ecological Concepts: Plant defense, Herbivory
Arizona Science Standards: Science as Inquiry; Life Science
Materials:
1) Magnifying lenses/loupes*
2) Writing/drawing materials
*May be borrowed from SCENE.
BACKGROUND
Plants are
autotrophs, producing their own food supply through photosynthesis.
Plants are sessile,
literally rooted to the ground. Therefore, unlike most animals, plants
cannot move away when threatened by a predator
trying to eat it. A plant predator is called an herbivore.
Instead of moving, many plant species have developed other defenses.
Spines, thorns, a nasty taste, and toxins are some defenses against
herbivory. Others are thick, waxy cuticles,
hairy leaf surfaces, or being ephemeral.
Plants that live and reproduce very quickly avoid herbivory by not
being around long enough for many herbivores to find them.
Plants are eaten whole or in parts by various mammalian, bird, reptilian
and arthropod
species. In schoolyard habitats, likely herbivores are rodents (rabbits,
mice), birds, and insects.
GUIDED INQUIRY
Observation/Exploration Period: Examine the plants
in the habitat. What types of physical structures might provide plants
a defense against being eaten? Look for spines, thorns, hairs, sticky
substances, and others you may discover. Do some plants show signs
of herbivory? Signs of herbivory include partially or totally eaten
leaves and stems, cocoons, eggs on the plant leaves or stems, mines
on the leaf surface, galls
on leaves or stems, or insect frass (feces). You might even find tracks
of birds, mammals, or lizards in the soil around the plant. This is
indirect evidence of what might be preying on a plant.
Group Discussion and Question Period: Are some plants eaten more than others? Why do some plants or parts of plants have more signs of being eaten than others? Do plants have characteristics that attract or deter insects? Why don't we see many insects on some plants but lots on others? Does time of day or year change how much herbivory occurs?
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 plants
have characteristics that attract or deter insects?" Let's
consider the fact that the leaves of some plants are smooth
and leaves of other plants are hairy. Our hypothesis might be,
"Plants without a hairy surface will show more signs of
herbivory because the herbivores can feed more easily on the
leaf tissue."
Sample Experiment Design: Compare hairy leaves
to smooth leaves. The independent
variable will be plant type, hairy leaves or smooth
leaves. The dependent
variable will be number of signs of herbivory detected
on the plants. Randomly
choose four plants of a species with a hairy surface and four
of a species with similar-sized leaves without a hairy surface.
This will give four replicates
of each experimental
unit.
Look at the leaves and stems for signs of damage caused by herbivores.
These would be holes in leaves, torn leaves, leaves completely
gone, broken stems, mines
or galls
on leaves or stems. Depending on the size of the plant, survey
and count signs of herbivory on every branch and leaf, or randomly
choose two to five branches on which to count all signs of herbivory.
If you know where all the plants are in the habitat and one
is completely gone, that might be another instance of herbivory.
Sample Prediction: The plant species with a
smooth leaf surface will have more signs of herbivory than the
plant species with a hairy leaf surface.
Record Results: Record the number of herbivory
instances per plant on the data sheet. If counting only a few
branches per plant, add the numbers together for each individual
plant to yield a number per plant. Total the number of instances
per plant type.
Sample Analysis of Data and Presentation: Make
a bar
graph or histogram,
with plant type on the horizontal axis and herbivory instances
on the vertical axis. Calculate the average
number of insects per plant type. 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) Middle School:
(a) Find the mean,
median,
mode,
and range
of the data.
(b) Collect potential insect herbivores in the
habitat. These could be beetles, caterpillars, ants, etc. Prior
observation of the plants and insects will give you an idea which
insects to use. Place captured insects on a branch of each plant.
Enclose the branch using fine
mesh netting. Use at least two plants of each defense
type (e.g., plant species with hairy leaves, and plant species without
hairy leaves). Choose branches free of herbivory signs. After a
set amount of time (a day, a week), remove the netting, count the
remaining insects and the number of herbivory instances per plant.
Average the data per plant type.
NOTE: The variable of interest in this experiment is herbivory,
but it is good to know how may insects are still alive at the end
of the experiment because if, for example, you place five insects
on each of the branches, and on some branches all or most of the
inscts die, while on others all or most live, that could explain
the differences in instances of herbivory among the branches.
(c) Determine other possible plant defenses. Test
these in similar experiments.
(2) High School:
(a) Calculate the variance
and standard
deviation of the averaged data.
(b) Conduct the experiment described in Guided
Inquiry, above, at two different times of day or year. Test for
a correlation
between the two variables, number of insects and time of collection.
Draw a scatter
plot and calculate the sample correlation
coefficient.
(c) Measure the height, length, and depth of each
plant. Calculate the volume of each plant (height x depth x length).
Use the volume
of a sphere if plant structure more closely matches a sphere shape.
Volume will mathematically correct for differences in herbivory
instances that may be due to differences in plant size. (You cannot
correct for differences in herbivory due to size of the herbivore
unless you know what the herbivores are.) This means, if Plant A
is twice as big as Plant B, that size difference may be the only
reason Plant A had more instances of herbivory. By calculating the
plant volume available for herbivory, and then dividing the number
of herbivory instances by that volume, the resulting number, in
# herbivory instances/volume will give a more accurate representation
of which plant type really endures more herbivory.
(d) Do a T-test
of herbivory instances per plant unit volume area. (T-test is a
standard statistics test comparing means of two samples.)
|
