Pill Bug Mystery

Last November, we conducted a prescribed burn at a 3-year old restored prairie.  Two weeks later, I was surprised to be able to photograph green regrowth in that area.  Last week, I revisited the same burned area and got yet another surprise.  Thousands and thousands of dead pill bugs (aka sow bugs, roly polies, wood lice) lay scattered across the ground.

Pill bugs
Dead pill bugs in a prairie burned last November.
cluster
The distribution of dead critters wasn’t consistent across the site, but there were a lot of areas like this with big numbers.

After I found the first few, I noticed them everywhere.  White skeletons of pill bugs, lying on top of the ground – sometimes in large aggregations, other times just a few here and there.  If I’d randomly tossed a hoola hoop on the ground 100 times, I bet I’d have found at least a couple dead pill bugs inside the hoop after 97 out of 100 of those throws.

isopod
I think these pill bugs were probably Armadillidium vulgare, which is a pretty good descriptive name for them.

When I got home, I looked through my photos from last November to see if maybe the dead pill bugs had been on the ground back then too.  If they were still dark gray, instead of white, I might not have noticed them.  Out of about 30 photos, I did find one bug, and it was still dark.  It’s certainly possible there were many more bugs on the ground, hidden by a combination of their dark color and the remaining ash and debris that has since largely disappeared. I’d like to think I would have noticed that many, but I’m not very confident of that.

november
A single pill bug can be seen in this photo from mid-November last year.

I like mysteries, but I also like understanding ecological phenomena.  Pill bugs are detritivores; they feed on dead and decaying material on and below the soil surface.  One possibility is that the dead pill bugs had been feeding above ground, within the layer of thatchy dead vegetation, and were killed by the fire.  A second possibility is that they were belowground during the fire, but came up after the fire (maybe it got too cold and/or dry because the protective thatch was burned away?) and then died aboveground.  There are lots of other possibilities as well, not all of them related to fire.

I emailed photos and questions to several entomology friends, asking for help explaining what I’d seen.  None had ever seen something like this after a fire.  Based on their responses, though, my first proposed scenario (above) seems the most likely.  I just wish I’d looked more carefully after the fire last November, though there wasn’t any reason to do that at the time!  One of my friends also mentioned that I shouldn’t lose any sleep over what happened since the pill bugs are an introduced species (native to the Mediterranean region) and could be having negative impacts on the native detritivore community in prairies.

caught in grass
There were clusters of dead pill bugs in basal clumps of grasses like this one.  Maybe they got lodged here during a strong wind?

This is the kind of thing that keeps me interested in ecology.  Something killed a lot of pill bugs in that prairie.  It was probably related to the fire, but I can’t even say that for sure.  If it was the fire, were there other creatures similarly impacted, but in a less visible way? (We try not to burn entire prairies because of this kind of potential impact, especially on insects overwintering aboveground.) What impacts might the loss of that many pill bugs have on other detritivores, on the decomposition process in that prairie, or on other aspects of prairie ecology?  Lots to ponder; I love it!

Has anyone out there seen anything like this before?  Any other suggestions as to what might have happened?

PLANT GAME ANSWERS:

Thanks to everyone who played the plant game this week.  Over 360 people guessed on the first question, and over 60% guessed correctly that “Widespread Panic Grass” was made up.  Nice work, though I did purposefully try to make the first one easy.

The second question got many fewer guesses, only about 200, as of my writing this.  I got most of you on this one.  Almost half of the guesses were for Yerba Mansa, but that’s a real plant, folks.  It’s a rhizomatous semi-succulent plant in the lizard’s-tail family (Saururaceae) – I PROMISE I’M NOT MAKING THIS UP – and is most common in the southwestern United States, but has at least one record in western Nebraska.

The correct answer for the second question was “Jagged-edge milkwort,” which doesn’t exist.  It’s a little tricky because milkwort is a real thing, but there is no such thing as a “jagged-edge milkwort”.  Only 14% of guessers got it right.  Congratulations to you 30 people!

It looks like people enjoyed playing the game, so we’ll try it again in the near future.