Download Interdependence of Plants and Animals

Dangerous Ideas and Forbidden Knowledge- Spring 2005
Lab 7: Soil Invertebrate
Biodiversity
INTRODUCTION
In this field study we will quantify the diversity of the soil invertebrate communities from which we
took soil and leaf litter samples.
Diversity is a measure of the relative representation of species in a community. It is comprised of
two components, species richness (the number of species) and species evenness (the relative number
of individuals of each species). Thus, two communities may have the same number of species (richness)
but one community may have one species that is more numerous than the others (evenness) (Table 1); the
second community would be less diverse. This makes intuitive sense, since the second community would
look to us as though it were made up of almost totally one species and would not look diverse.
Table 1. A comparison of two communities and their apparent diversity.
Number of Individuals
Species
Community 1
Community 2
A
20
92
B
20
2
C
20
2
D
20
2
E
20
2
TOTAL
100
100
There are several ways to quantify diversity. One is simply to calculate species richness. However,
as illustrated in Table 1, this can give a false impression of the relative diversity of two different areas.
Therefore, other measures have been devised by community ecologists. The one we will use is called
Simpson’s index.
Simpson’s diversity index =
D
N N  1
 n i ni  1
where N = total number of individuals of all species in a community and ni = number of
individuals of the ith species or taxon. The summation symbol (∑) means to do the
calculation following the ∑ for each of the species or taxa and then add up the results of all
the calculations.
D ranges from 1, for a community made up of one species (or other taxon), to infinity, for a community
made up of one individual of many species (or other taxon).
Contrary to its outward appearance to the casual observer, soil is a dynamically active living
community. The organisms in the soil are busy using the soil as a place to feed, reproduce, compete, …
live! In the process, they work the soil, making it more fertile, improving its water-holding capability,
increasing the ability of oxygen to enter the soil, and decreasing the soil's susceptibility to erosion. They
therefore play an important role in the formation of soil.
The organisms in the soil interact with one another and with the environment, and thus make up an
ecological community. Energy is transferred through the community, and nutrients are recycled. There
are predators, prey, decomposers, competitors, symbiotic partners. Plant material that drops to the soil
surface, called plant or leaf litter, as well as the bodies of dead organisms, is collectively called detritus,
and becomes food for various organisms called detritivores. They chew it up into smaller pieces, and
leave fecal wastes that then become for other organisms. Some of these organisms are eaten by predators.
The detritus gets worked to smaller and smaller particles, and bacteria and fungi, the decomposers, and
chemical action reduce the organic humus to the minerals that are nutrients used by plants. And the
cycle continues.
Lab
2
In this laboratory exercise, we will collect soil animals and study this complex and highly intriguing
community. This exercise will take several days to complete.
OBJECTIVES
In this study, you will
a) identify various groups of animals that live in the soil.
b) explore concepts of ecosystem ecology.
MATERIALS
Burlese funnel system
dissecting microscope
specimen vials, with alcohol
squirt bottle of alcohol
Guide to common soil animals
dissecting probes/needles
top or bottom of petri dish
PREPARATIONS
1.
In class, we will determine where we want to sample for soil invertebrates. We will then test several
hypotheses together.
FIELD PROCEDURE
1. Work in groups of 3 or 4.
2. Collect a sample of leaf litter and soil from the site, collecting the leaf litter from an area
approximately 1 ft2.
3. Place the samples in the Burlese funnel apparatus. Adjust the lamp so it is about 3 inches above your
funnels.
4. Turn on the light.
5. Fill collecting vials with alcohol, and put each under the appropriate funnel.
6. Let the apparatus "go" for one week.
LAB PROCEDURE (DATA COLLECTION)
Each team will be responsible for analyzing the contents of one of our collecting vials. Note that some of
these vials will be much more difficult to count than others. Jump in and help your neighbors if you are
done early. We can’t analyze our data until all groups have finished their work.
NOTE: Some of these organisms are so small that you will have a hard time seeing them with the naked
eye!
1.
2.
3.
4.
Turn off the light of the Burlese apparatus.
Obtain your sample vial.
Empty the funnels in the trash, but be sure to save the pieces of wire mesh!!
Obtain a dissecting microscope, the top or bottom of a petri plate for each member of your group, and
one or two dissecting needles/probes for each member.
5. Swirl the vial, then pour the contents into enough petri plates so that each member of your group can
be working to identify and count the organisms. If you need to, use the alcohol squirt bottles to wash
out any organisms that remain in the sample vial.
6. Place the petri plate under the microscope and sort the organisms. Start at low power and search for
organisms. Finish at high power, to be sure you have not missed anything.
7. Use the guide to identify what you can.
8. In Table 2, keep track of the number of each type of organism.
c) Sort the organisms by moving similar looking organisms together in groups with the
dissecting needle/probe. Count the number of individuals in each group, using "tick marks"
to keep track instead of counting if there are many individuals, and counting up the tick marks
later.
3
Soil Animals
d) One group member scans the petri plate and call out the identity of organisms they see(e.g.,
“pseudoscorpion, mite, mite, springtail, . . .”, while another group member keeps track of
them by using tick marks. Count up the tick marks when finished.
9. After you have finished, add up the total number of individuals for each type of organism you found
and calculate the Simpson’s Diversity Index for your site.
10. Now compare your calculated Simpson’s index with other groups to test your hypothesis!
DATA & ANALYSIS
Diversity
1. Select one of the hypotheses we tested. Using the information above and your class data, calculate
Simpson’s diversity index for each community sampled to test this hypothesis. Use 2 decimal places.
Hypothesis: ____________________________________________________
Community 1:
D=
Community 2:
D=
2. Which community is more diverse? How would you explain your results: Why is the one community
more diverse than the other?
3. Do these results support your hypothesis? If not, explain what other factors might be influencing these
communities.
4. Compare your results with your class mates. Did their results support their hypothesis? Of the factors
we tested together (shade, impact, etc.), which was the best indicator of biodiversity in this
community? Explain.
Lab
4
Table 2: Your group’s data
Location
(write in location of site)
type of animal
Thysanura (bristletails)
Collembola (springtails)
Thysanoptera (thrips)
Protura
Pseudoscorpions
Ticks
Mites
Centipedes
Millipedes
Spiders
Insect larvae
Beetles
Nematode worms
Others
keep track of numbers here
total number