North Olympic Peninsula Skills Center
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Natural Resources

North Olympic Peninsula Skills Center

Final Products

Elwha River Pitfall Trap Project

SIt was like Christmas in July. Each time we checked our pitfall traps, we never knew what was lurking in them. Because of the recent removal of the Elwha Dam and the subsequent drainage of Lake Aldwell, we choose to do an investigation to discover which insects would be found at different locations at a particular site along the Elwha River. We thought it would be fascinating to also see which insects preferred which habitats. The sediment level has increased so much in the past year that we thought that a project dealing with the new build-up of sediment would certainly fit into the trending topic right now.


Pitfall traps are slick plastic cups placed in another cup, which lay in a hole in the ground. The cups can be any size or color. Our procedure includes digging a small pit in the ground, deep enough that the rim is even with the edge of the ground, so that insects would easily fall into our trap. We placed 3 or 4 medium-sized rocks at the rim of the cups, to hold up the piece of bark or large rock acting as the cover. We put the traps in three different locations. They included a site 33 meters from the water (Site 1), which was near a lot of vegetation and had several older log jams and small depressions around it. Our second site was 15 meters from the water (Site 2) and was near a semi-large tree stump habitat. Our last site (Site 3), was 4 meters from the water, and was sandy, slightly rocky, and very close to the river.


Site 1

Site 2

Site 3

Trial 1

  • 1 Beetle; unknown; alive
  • 0 found
  • 1 Arachnid; Harvestman; alive

Trial 2

  • 1 Arachnid; Wolf Spider; alive
  • 1 Centipede; dead
  • 4 arachnids; Wolf Spider, Harvestman, and 2 unknowns; 3 alive
  • 14 centipedes; 3 alive
  • 4 arachnids; All Harvestman; all alive.
  • 3 centipedes; 1 alive

Trial 3

  • 3 ants; One larger, flying ant, and 2 smaller ants; 2 alive
  • 1 beetle; alive
  • 5 centipedes; 2 alive
  • 1 arachnid; unknown, small; alive
  • 1 arachnid; Wolf Spider; alive
  • 1 cricket; black legs; alive

Trail 4

  • 1 Arachnid; very small; alive
  • 1 Pill Insect; alive
  • 1 ant; dead
  • 1 Ant; Small, being eaten by spider; Dead
  • 1 centipede; alive
  • 1 Arachnid; weaving web, eating ant; alive

Our group had a total of 3 sites and 4 trials.

During our surveys, we primarily found terrestrial arthropods (pill bugs, ants, etc.) and arachnids (wolf spiders, harvestman, etc.) in the traps. The most common arthropods we found in our traps were centipedes and spiders. We also found ants, beetles, crickets, and pill bugs. The most common spider we found was identified as a wolf spider, and we found about 4 of those.

Wolf Spider

Wolf Spider

Overall Site 2 had the most insects found (25 total) and also the most centipedes found, with 19 total and 5 alive. Site 3 lead the group with a total of 7 spiders found over 4 trials. Site 1 led the charge with a total of 3 ants over 4 trials. Site 1 had a total of 9 insects found over 4 trials. Finally, Site 3 had a total of 13 insects found over 4 trials.  Therefore, we found were more insects\arachnids near the old tree stump and the old log jam.

This means that environments that include a lot of vegetation, like Site 2 with the tree stump, provide more insects because of the availability of resources and shelter, due to the habitat. Another conclusion that may be considered is that distance from the water does not seem to affect the amount of insects found there. This is because at Site 1, there were less insects found than at Site 3, even though Site 1 was 4 meters from the water, and Site 3 was 33 meters from the water.

Because of where the traps were placed, we learned where several different types of insects\arachnids travel. Most of the centipedes seemed to prefer a large habitat, particularly where bark or wood is. Spiders seem to roam almost anywhere and were found at most of our sites, indicating that they don’t seem to favor any of our particular sites.

It was a shock to see that the water distance for the sites did not affect how the insect numbers varied. We thought that the insects would prefer places near water over places near the forest, because all animals including insects depend on water. However, this theory proved false, and we think that the site near the water may have attracted less than it would have, due to the sandy terrain near the site.

We really enjoyed our time surveying terrestrial insects near the Elwha River. Like Christmas in July, when the final gift was unwrapped, we totaled over 46 arthropods!

© 2015 North Olympic Peninsula Skills Center
905 West 9th Street
Port Angeles, WA 98363
(360) 565-1533