ED-STEEP: Education Solutions to Environmental
and Economic Problems
Biodiversity of Ground-Dwelling Arthropods
Subjects
Biological Sciences, Environmental Sciences, Agricultural Sciences
Introduction
Biodiversity refers to the abundance and variety of organisms, genes, and communities in nature,
and is an essential concept for students to understand. In this lesson, students will define
diversity, discuss its importance, and conduct an experiment to compare the biodiversity of two
habitats. Students will collect ground-dwelling arthropods with pitfall traps, sort their catches
into species or groups, calculate diversity, and analyze their data. Pitfall traps are inexpensive
and easy to set up. Students will be fascinated by what they collect in their traps, and will be
surprised by the diversity of arthropods. This is an excellent activity to collect specimens to
teach classification. Related lesson plans include the following:
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Soil Organic Matter and Biodiversity
Biodiversity of Soil Invertebrates
Organic Matter
Soil Chemistry and Physics
Relevant STEEP Research Projects
Scientists at University of Idaho have been studying the importance of biodiversity in sustainable
agricultural systems. Here are some highlights of their research:
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Pitfall traps were used to compare populations and communities of invertebrates in no-till
and conventionally tilled fields in Latah and Nez Perce counties, Idaho. The scientists
wanted to find out if the use of no-till planting would decrease pest populations and increase
natural enemy populations in crop fields.
Overall, more beetles, but fewer species, were found in conventionally-tilled fields.
Spiders and seed-feeding ground beetles were more common in no-till fields.
Species diversity and composition of invertebrates were more similar between native prairie
and no-till habitats, than between prairie and conventionally-tilled systems; thus no-till
enhances biodiversity across the agroecosystem.
Examination of tillage factors, crop type, soils and non-crop habitat upon soil fauna and
ground dwelling predators in a small inland PNW watershed, J. Johnson-Maynard, T.D.
Hatten, N.A. Bosque-Perez, and S.D. Eigenbrode. 2004 STEEP Annual Progress Report.
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Assessing the impact of direct seeding (no-till) and conventional-till on crop, variety, soil,
and insect responses. S.O. Guy, N.A. Bosque-Perez, S.D. Eigenbrode, and J. JohnsonMaynard, 2004 STEEP Annual Progress Report.
Objectives
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Students will discuss the causes, consequences, and importance of biodiversity
Students will classify arthropods into species or morpho-species
Students will estimate and compare the diversity of arthropods in two different habitats
Major Concepts
Biodiversity, Classification, Invertebrates, Scientific Method, Terrestrial Ecosystems
Standards
AAAS Benchmarks
Idaho State Science Standards
Oregon State Science Standards
Washington State Science Standards
Materials
Print Resources
 Biodiversity Fact Sheet
 Using Pitfall Traps to Collect Ground-Dwelling Arthropods
 Diversity Data Analysis
 Scientific Experiments and Publication Formats for Lab Reports
 Constructing Bar and Line Graphs
 Student Data Sheet
 Any general book on insects and spiders for identifying specimens
Collecting Ground-Dwelling Arthropods
 16 and 14 ounce plastic cups (about 25 per habitat). The smaller cup should fit snugly
into the larger cup.
 a 50:50 mixture of water and propylene-glycol based antifreeze (this is a more
environmentally safe antifreeze; do not use the standard ethylene glycol antifreeze).
 Small trowels for putting the cups in the ground
 Wire flags or ribbons to mark the location of traps
 A sieve for washing the arthropods
 A quart mason jar for storing the insects
 70% ethyl alcohol for storing the insects until sorting (alternately, they can be kept in the
refrigerator if used within a couple of weeks)
Sorting Insects
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Forceps for picking up arthropods
Trays for sorting and organizing arthropods into groups
Dissecting microscopes (optional)
Web Resources
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Ecological and Ecosystem Diversity, National Biological Information Infrastructure
Biodiversity Fact Sheet, Ecological Society of America
Life Under Your Feet: Measuring Soil Invertebrate Diversity
General Procedures
Ideally, students should be allowed to place the pitfall traps in the ground and collect their own
arthropods. However, this depends on the availability of suitable and readily available habitats,
and the time of year. Many different kinds of habitats can be used, including old fields,
grasslands, alfalfa fields, no-till crop fields, and forests. Typically, the greater the plant diversity
of the habitat, the greater diversity of arthropods. It takes about 1 minute to put a trap in the
ground. The time of year is also critical and is best done before October and after mid-May.
Other times will also work depending on the local climate. If habitats are not readily available
and students can’t collect their own insects, then the teacher can place the pitfall traps in the
ground and collect the arthropods. If the soil can be worked easily, the placement of pitfall traps
will take one person about 1 hour to complete. Collecting and processing the insects will take
about 1 hour, also. Specimens can then be kept for other labs (e.g., classification, arthropods,
invertebrates).
Procedures
1. Plan on 2-3 weeks to complete the activity (actual class time is 1-3 periods)
2. Follow the instructions in Using Pitfall Traps to Collect Ground-Dwelling Arthropods to put
the traps in the ground
3. Present students with information on biodiversity (see Biodiversity Fact Sheet)
4. Working in groups, have students write down a testable hypothesis relating habitat to
biodiversity. For example:
a. biodiversity of ground beetles is greater in more diverse and complex habitats
because there are more food resources available
b. biodiversity of arthropods is greater in forest habitats than crop fields because there
are more plant species in forests to support more arthropods
5. After 1-2 weeks, have the students empty the pitfall traps into a quart mason jar – 1 jar will
hold the contents of 10 traps.
6. Teacher: place the contents of the mason jar into a medium–mesh sieve and rinse thoroughly
under tap water to remove the antifreeze. Place the arthropods back into the mason jar and
either place in the refrigerator or store in 70% ethyl alcohol.
7. Have students sort the contents of the jar into “morpho-species” or groups that look similar.
8. Have the students record the number of individuals within each group in the data table (the
data table in the Student Data Sheet will need to be adjusted according to the exact
experiment).
9. Have the students calculate species richness (# of species) and the Shannon Index (see:
Diversity Data Analysis); record the data in the table.
10. Have the students construct a bar-plot (histogram) of the number collected for each species,
in order of abundance (typically, you’ll see a log-normal distribution with a few abundant
species and a lot of species represented by few individuals).
11. Have students discuss results and conclusions (alternately - formal lab report)
Assessment
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Student Handout
Student Lab Report
Questions:
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What is a “morpho-species” and why is it used instead of real species?
Which habitat had the most and least individuals?
Which habitat had the greatest and least species richness?
List 3 hypotheses to explain why species diversity differs between habitats.
Is the data representative of the total diversity of the habitats? Why or why not?
What is the advantage and disadvantage of using species richness as an estimate of
diversity?
Is there a relationship between diversity of a particular group (e.g., beetles), and the
diversity of the entire habitat?
What would happen if the diversity of arthropods was suddenly reduced by 50%? 95%?
Does this type of experiment have a control?
Notes:
Morpho-species are often used in biodiversity studies for two main reasons. First, many
specimens can not be identified to the species level because experts are not available to identify
species. Second, many individuals can’t be identified to the species level because the taxonomy
of many groups has simply not been worked out. In fact, most insects in the world haven’t been
identified to species. Often, paraprofessional taxonomists, generally from the region where the
insects are collected, are trained to recognize distinct groups.
Clearly, it is extremely difficult to estimate the diversity of organisms in a particular habitat
because of the spatial, temporal, and taxonomic variability of organisms, the difficulty in
identifying individuals (particularly immature organisms), and the difficulty collecting
organisms. Biodiversity researchers often try to correlate the diversity of one group to the
diversity of other groups. For example, the diversity of plants in a region can be correlated with
the diversity of grasshoppers.