Do Spiders Limit Prey Population Numbers?
Spiders prey upon insects. Do they keep insect
numbers down? What do we need to know in order to answer this question?
A "Mark and Recapture" study is one method scientists use to estimate
the number of individuals in a population when it is impossible to find
and count all of the individuals. They may be hard to find for various
reasons. What are the reasons they may be hard to find?
- field measuring tape
- vials or cup
- acrylic paint
- paint applicator
- Each class will be assigned to look at a specific
area on campus.
- One team will be in charge of measuring the area
(length x width), describing the habitat, and creating a map.
- The other teams will be assigned to a plot within
the study site. Within that plot, students will look for and mark
every spider they can find. Mark the spiders by placing a dot of paint
on their back. Keep count of how many spiders you mark. Each team
will have their own color of paint for their plot.
- Meet with the measurement team to have your plot
area outlined on the map.
- Return to the classroom and write the information
(how many spiders were marked in your study site) on the board. The
measurement team will copy down this information on a separate piece
of paper and turn it in with the map.
- 3-4 days later, we will return to the study site.
Teams will work in the same plot as before. Find and count all spiders.
Record how many were marked and with what color and how many spiders
were unmarked. The measurement team will continue to add detail to
the map of the study site. Report your data to the measurement team
to be included on the map.
- In class, information (# of spiders
recaptured, colors and # of marked spiders, and # of unmarked spiders
for each plot) is placed on the board. Make a table to show the data.
- Calculate the number of spiders that
live in the study site using the Lincoln Index:
- N = number of spiders that live in the study site
- n1 = number of spiders captured on first day (by
- n2 = number of spiders captured on second day (by
- m = number of marked spiders that were captured
on the second day
3. Calculate how many insects are being
eaten per week by spiders on the study site using feeding rates determined
with our spiders in class. How many insects are eaten in your study
area monthly? yearly?
Results and Conclusions:
- Check the locations on the map made
by the measurement team. Did any of the marked spiders move to different
plots? What are some reasons why spiders may move to new areas? Were
any webs occupied by a new spider?
- Describe the distribution of spiders
in the study area. What factors may account for this distribution?
- Are spiders limiting the numbers of
insects in the study area? Do you think spiders are a benefit to farmers,
gardeners, and humans? Why or why not?
- Further discuss your results on the
impact of spiders on insect populations. Explain how your class data
support your conclusions.
- What assumptions are being made in a
Mark and Recapture study?
Write a one paragraph (5-7 sentence) abstract
summarizing this experiment and the information gathered by doing this