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Falling Arthropods and Citizen Science

Originally appears in the Summer 2016 issue

“Open the trap away from yourself. That way if anything jumps out it will land on your too-close partner, not you.” With those words I begin another session of pitfall trapping. For 20 years, my students have been digging holes in the dirt, planting open cups, and waiting 48 hours for insects and other arthropods to fall into these traps. We call it “pitfalling,” and it has been a central part of our 20 year citizen science effort to better understand our Rio Grande’s riverside forest, known by its Spanish name, bosque.

I am lucky enough to have two jobs wrapped into one. I am a classroom teacher and I co-direct a citizen science program for 10,000 students, teachers, and others. Our network of 32 river-side monitoring sites provides me with the opportunity to support and see all sorts of student field work. Arthropod trapping is one of several techniques we use in the Bosque Ecosystem Monitoring Program (BEMP) to track environmental change over time. This article describes just one way teachers can replicate BEMP-style research and engage their students in authentic field-based data collection.

Arthropod Pitfall Trapping
Our local area has over 500 vertebrate species, but when I step outside with a class of students, birds flitter away quickly, and mammals, amphibians, and reptiles tend to avoid human encounters altogether. So I find that arthropods, a phylum of animals with jointed legs and lacking backbones, are the wildlife easiest to introduce to my students. They tend to be abundant, diverse, and found in all types of habitats. When students open a pitfall trap and discover a centipede, they are being introduced to a neighbor in their own ecological community. Shrieks of excitement are the norm. Sometimes there are comments like, “That’s disgusting.” Whatever their initial reaction, they become engaged with questions about why is this creature living in our neighborhood and how is it making its living? My students can’t seem to ignore iridescent green beetles and multi-legged millipedes found in pitfalls in our desert city.

We dive into coming to know our local arthropods through the circular opening of a simple plastic drinking cup. Catching animals in an open pit is an archaic skill. Tens of thousands of years ago our Neanderthal cousins were catching animals using pits dug into the ground where creatures fell in and could not escape. Our technique uses Neanderthal technology and a few modern petrochemical products: two plastic drinking cups, a small board, and a few wood screws or some tiny stones. All these plus a hole in the ground make an arthropod pitfall trap.

Within the Bosque Ecosystem Monitoring Program (BEMP) there are 32 different 5 acre (2 hectare) plots we call “BEMP sites.” At these sites, students from age 7 to 18 work with staff to track environmental changes caused by the ecological drivers of flood, fire, climate, and human alteration. Those ecological drivers act like different chefs in similar kitchens. Each environmental force takes the ingredients at hand – cottonwood trees, sandy soil, the river itself, to name just a few – and creates something different. The changes that emerge are documented by classes of students. At least once a month, these young citizen scientists measure variables like the ratio of native to exotic plants, depth to groundwater, and weather conditions. Three times a year, they also investigate local forest arthropods through pitfall trapping.

All of our 32 sites are on public or tribal land. Most sites were established at the request of a government agency to track the effectiveness of different land management strategies. Our staff secure access permits and then partner with schools at a local tract of riverside land. Some students walk to their sites; others take short bus rides. All are supported with science and environmental education curriculum provided by our staff. Young students gather samples and do simple sorting, such as grouping beetles, isopods, and ants. Further identification is done by BEMP interns and staff. Graduate students write research papers on the findings. All the work is matched to student ability and a quality control program is in place to guarantee the scientific validity of the work.

All BEMP data is delivered to the University of New Mexico (UNM) and becomes part of a citizen science effort to provide information and analysis. BEMP research informs river policy and multi-million dollar restoration projects along the Rio Grande by federal, tribal, and local governments. BEMP students take pride in helping UNM biologists better understand ecological change. Ondrea Hummel, an ecologist with the US Army Corps of Engineers stated, “We use BEMP data in evaluating our Middle Rio Grande Restoration Projects. BEMP’s vegetation and arthropod data helps give us a sense of the ecological health of a site before we begin a restoration project and during the restoration process.” Students know that their research has a positive impact on their local environment.

When students do research that connects them to their community they take care in their measurements and work. There is a seriousness of purpose. Although we send university students into the field to accompany the younger students and to do quality control, it is rare that the university students need to intervene. Pitfall trap field collection is preceded by an overview of humane handling procedures and considerations and safety guidelines.

What Students Discover
All of the focus and seriousness, however, rarely restricts squeals of delight and surprise when pitfall traps are opened. At a recent pitfall collection, in the heart of our state’s largest city, 11-year-old students swooped down on their assigned traps and poured the contents of their cups into labeled plastic bags. As they looked at the bags, they began simple counting and sorting. “13, no, 14 stink beetles and 3 roly polys,” said Everett. Elliott consulted a printed guide to figure out what “this baby cricket thing is.” And so it went; each cup was poured into its own bag until the contents of all 20 traps were collected.

Kids are kids, so when Anelé and Hala finished their collection ahead of the rest, they pulled some tall plants over themselves and created a shelter. As they said, “We sat in our fort and dissected a sunflower.” A critical part of our approach to learning is time and the opportunity for play and exploration, side by side with science.

Class reconvened when Everett, Elliott, and the other students were joined by Anelé and Hala, who climbed out from under the shelter. Atop a fallen log the students told each other what they found in their traps. Differences and similarities were noted. The discussion shifted to habitat variation. Most of the forest where their BEMP site is located burned the year they were born, and in the case of two students, the fires took place in their birth month. After comments by a few, Emiliano offered his view that, “Yeah, the fire killed stuff. But it made room for other things. There was a chain of events. Many of the native cottonwood trees died. Exotics, like Tree of Heaven and Russian olive, took over. Now our bugs and other arthropods have had to adapt or die. Maybe some don’t live here anymore since the fire.”

One advantage of being part of a 20-year citizen science effort is that we can review the pre- and post-fire arthropod data from that area and test Emiliano’s hypothesis.

As I went about my rounds that week, wearing my BEMP co-director title and checking on other classes, I found myself 100 miles (160 km) north of where Everett, Anelé, and their classmates had set their traps in the city. I was at another BEMP site on the Ohkay Owingeh Native American Pueblo, where for many hundreds of years people of this First Nation have farmed along the banks of the Rio Grande. Jennifer, a member of that community, emptied a pitfall trap full of beetles into a bag. Somehow the conversation shifted to beetles as toad food. She talked about each summer seeing huge Woodhouse toads by her house, a short 10 minute walk from where we stood. We talked about how the environment links us all – farmer, river, plants, beetle, toad, scientist – to each other.

Ecological Knowledge
Throughout the week, at 32 different sites across 350 miles (560 km), in 20 trap sets, thousands of arthropods were gathered by students from 32 different schools. Each small animal serves as a data point and an opportunity to more fully understand ecological trends across time and space. Individual species serve as bio-indicators of landscape change. The western harvester ants were abundant in the city BEMP site but not so much at Ohkay Owingeh. Western harvester ants prefer dry soil and the Ohkay Owingeh site was frequently flooded. Tiny, armored Eurasian immigrant roly poly isopods were abundant at both sites, and the story of how exotic roly polys displaced native crickets as forest decomposer served as a conversation gateway for how Europeans displaced Native Americans across the continent.

Pitfall traps have limitations. They will not catch all types of arthropods. Some arthropods fly or jump out and others will not enter the pit. Pitfalls tend to capture only ground-dwelling arthropods. Even if there are thousands of a beetle species on a tree above a trap, it is quite possible none of them will end up in the trap. However, pitfall trapping does produce an index of relative animal numbers of particular species at different times and locations. So recording data is important, both numerically (how many of each type of arthropod are found in different traps) and visually through photographs or drawing.

While all BEMP arthropod collection and associated data is used by ecologists at the University of New Mexico, you do not need to be part of an official partnership such as this to share pitfall results with others. Collected arthropod data can become part of larger, ongoing research efforts through a number of citizen science efforts. For example, anyone can share their species observation on websites such as

Back in the Classroom
There are many ways a class can continue building upon their field pitfall work. Through graphic data representation and interpretation, both using computer-based spreadsheets and traditional paper and pencil, students can visualize and understand ecological trends.

Pitfall trapping can be tied to literary endeavors too. For older students, reading “The Little Things that Rule the World” by biologist E.O. Wilson can give insight into the importance of arthropods and their kin to human survival. For younger students, A.A. Milne’s “Winnie the Pooh” chapter 5 “In Which Piglet Meets a Heffalump” is all about a pitfall trap. Recently we had elementary students create mini magazines, or zines, to write and illustrate their own stories of what had plopped into their pitfall traps. There are numerous ways to weave authentic data into the writing process.

These rich experiences create a stronger connection to the often forgotten or overlooked happenings in the neighborhood. Attention to these details helps students see the world anew. Pitfalling in one’s backyard is just part of discovering one’s ecological place in the world.


Building and Setting Arthropod Pitfall Traps

There are several pitfall trap variations that are used by ecologists and other scientists. The very simplest consists of placing a cup in the ground with the rim of the cup either level with or just ever so slightly lower than the surrounding ground. A cover, consisting of a flat rock, a board, or other slender object positioned about a finger’s width above the cup and supported by pebbles, wood screws, or similar items will keep out the rain, shade captured animals, and protect the animals in the trap from predators. To reduce puncture wound danger, file or cut off the sharp ends of wood screws or nails.

Materials List
Small hand trowel for digging holes

Materials per trap
• Two 16 ounce (475 ml) plastic drinking cups
• One ¼ inch board measuring approx. 4×4 inches (10 x 10 cm)
• Four wood screws or nails, one placed at each corner of the board, or three or four tiny stones, to create a 1 inch (2.5 cm) gap or less between the earth and the cover board

To allow for a cup to be pulled from the ground and then placed back in position without soil collapsing into the hole, a second cup can be used to hold the form. Cup A has pencil diameter drain holes poked into its bottom and goes in first. Cup B has no holes and nestles into Cup A. The rims of both cups are flush or below ground level. When it is time to remove the trap’s contents, use a finger to hold Cup A in place in the dirt as Cup B with its contents of arthropods is lifted out of the hole.

Larger cups or buckets work too, but the chances of catching mammals (like mice and shrews) and reptiles (like lizards and snakes) greatly increases. Therefore smaller cups are not only less expensive and easier to install in the ground but are less likely to catch unintended species.

To increase capture rates, small “walls” or “fences” can be constructed of hand-width high aluminum flashing or similar material extending out from the edge of traps, though this can draw unwanted attention to traps and increase vandalism.


Humane Handling of Animals

• Check traps frequently. Once opened, there is a humane responsibility of checking a pitfall trap at frequent intervals. The optimal time between opening and checking is 24 hours. That gives arthropods time to amble in, but not too much time if a non-targeted species, such as a lizard, is captured and needs to be released. For maximum capture success, traps can be opened and then checked several days in a row. Within our Bosque Ecosystem Monitoring Program (BEMP), we leave traps open for 48 hours.
• Do not create wet traps. Wet traps contain a solution, such as alcohol, designed to trap, kill and preserve specimens. Wet traps will likely, even if inadvertently, catch vertebrate species that will suffer an inhumane death. Therefore, they are incompatible with simple ecosystem monitoring done with school children.
• Increase cover and habitat within traps. Adding a bit of forest duff, such as leaves, in the bottom of the cup provides shelter and moisture for captured animals. Too much duff added to a cup allows for easy escape. Small pieces of polystyrene can be added to provide floating rafts for captured animals in times of rainfall.
• Deactivate traps when not in use. Just as important as checking traps on a regular basis is deactivating traps once the capture period ends. If trapping will not be repeated in the near term, deactivate the trap by removing the cup from the ground. If a new trapping session will start shortly, applying a fitted drinking cup lid and then pressing the outside cover board or rock directly on top of the cup and its lid can temporarily deactivate a trap. Buried and covered cups are easy to lose and become pollution, so it is important to know all trap locations with specific measurements from a fixed point and be ready to retrieve them all at the end of trapping sessions.
• Have a plan for captured arthropods. Arthropods captured and examined in the field can easily be identified to at least the taxonomic order level and sometimes to the family level. The simplest, easiest, and most humane way to process caught arthropods is to identify and count arthropods in a plastic zipper closure bag and then release them in the area where they were caught. In BEMP, after students’ preliminary field identifications, all arthropod samples are frozen and sent to a university lab for detailed identification and preservation in its collection.


Safety Guidelines

• Open traps away from people. Imagine that the cover board has a hinge. “Swing” the cover board open so that the edge furthest from you opens first.
• Allow vertebrates to escape. If a vertebrate species, such as a shrew, lizard, or mouse has been captured, place a few sticks in the cup so that the animal can use it as a ladder to scurry up and out. In the case of capturing a mouse or other mammal, once the trap is empty, the cup should be thrown away in a sealed plastic bag to avoid the remote possibility of transfer of disease from the captured mammal to humans.

• Cover boards with nails or wood screws should always face down to the ground so hands and feet are not accidentally impaled.

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Daniel Shaw teaches at the Bosque School in Albuquerque, New Mexico. Along with his students, his research includes radio-collaring porcupines, habitat issues in urban landscapes, and threats to amphibian survival. His publications include Southwest Aquatic Habitats: On the Trail of Fish in a Desert and Eco-tracking: On the Trail of Habitat Change, both of which were published by UNM Press. Learn more about the Bosque Ecosystem Monitoring Program at

1. Milne, George, Alan Alexander Milne, and Ernest H. Shepard. The world of Pooh: The complete Winnie-the-Pooh and the house at Pooh corner. McClelland and Stewart, 1989, 1977.
2. Wilson, Edward O. “The Little Things That Run the World (The Importance and Conservation of Invertebrates).” Conservation Biology 1.4 (1987): 344-346.