Mud humps?

[Andi Stephens]

The first field in Finley when you turn in off Finley Road is filled with mud humps.  They come up every year when the rains start.  Probably some sort of insect builds them -- does anyone who does this?

Thanks!

--Andi

[Lisa Millbank]

Those humps at Finley are built by ants and persist for many years, but from spring through early fall they are hidden by vegetation.  When the vegetation dies down and gets flattened by rain and wind, it can look like the mounds suddenly popped up.  Of course, there are also gopher and mole mounds that do appear overnight, but I don't think they would be confused with the thousands of ant mounds.  

I don't know much about these ants, and I'm not having much luck finding which species they are.  I haven't seen them on upland prairies, just wet prairies.  My guess is that there are different, non-mound-building ant species on upland prairies, and that the annual flooding of the wet prairies requires the ants there to build mounds so that the colony can be elevated above the saturated soil conditions.  One thing I've noticed is that the mounds seem to have some effect on plant diversity.  Some plants are less tolerant of growing in standing water, but can grow well on the mounds, while other plants like monkeyflowers and tufted hairgrass grow between the mounds in the water, so there's kind of a patchwork effect.

Lisa Millbank

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[Andi Stephens]

Very cool!  Thanks!

[Stephanie Hazen]

I have often wondered about those mounds.  Next time we tour Finley I will capture some ants, 

chill them, photograph them and submit the photos to BugGuide.net to see if I can get an identification. 

Stephanie Hazen 

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[Hendrik Herlyn]

Great idea, Stephanie! Please keep us posted if you find out anything.

Hendrik

__________________________
Hendrik G. Herlyn
Corvallis, OR

"Nature is not a place to visit. It is home."
     -- Gary Snyder

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[Linda Fink]

Oregon ants that build mounds are either Harvester Ants or Thatching
Ants, far as I know. I have mostly seen their mounds in open woods, like
our Ponderosa Pine forest and in openings in my parents Doug fir forest.
I have seen mounds close to four feet tall. Those ants pack a nasty
sting. Fortunately (from my point of view) Flickers eat them. In areas
where we are apt to walk, I knock down the mounds and let the Flickers
go to work. They especially like the underground larvae, I've read.

Linda Fink
SW Yamhill County

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[Mary Garrard]

Thanks, interesting. That reminds me of a story about mud lumps.

On a half-day birding outing (remember those???) one very frosty Saturday morning, we saw a lot of mud lumps in the field to the south of Finley Road just before the road turns south towards the overlook. We wondered about them. The sun must have been able to warm the lumps a bit and as we went by, the lumps stood up, shook themselves, and turned into Killdeer! 

Mary

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[Lisa Millbank]

Those thatch ants build the most impressive ant nests around here.  Here's an interesting article about them: https://www.trailkeepersoforegon.org/mound-builders-of-the-forest-western-thatch-ants/

I think the wet prairie ants are a different species.  The thatch ants' mounds are almost all vegetation rather than soil, and they seem to be forest dwellers. 

Lisa

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[Bill Gerth]

Hi folks,

According to a scientific article published in 1977, those mounds are built by the ant species Formica fusca. I don't have the paper, but here is the citation...

Landa, E. R. (1977). An unusual ant nest morphology for the ant Formica fusca Linne in western Oregon (Hymenoptera: Formicidae). Pan-Pacific Entomologist 53:250-252.

I'll try to get the paper through inter-library loan and will share it if I get a copy.

Cheers,

Bill

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[Owen Dell]

A few years ago I attended a field day at Finley. One of the staff led a discussion, and the subject of the mud humps came up. She mentioned that the prairie at that site is the only place in the entire Willamette Valley that has never been plowed. I was surprised to hear that, and while I can't verify it I will say that she seemed to be quite knowledgeable. 

Has anyone seen active ants on any of the humps? I often look but the humps seem rock-hard and barren, and I find myself wondering how old they might be and whether they are occupied or abandoned.

They're quite fascinating.

Owen

[Joel Geier]

As to how long these ant mounds take to develop:

The prairie restoration in our front yard/pasture (kept wet by our drain field) has some nice mounds that have developed over the course of 10 to 12 years. They're still active -- whenever I accidentally scalp one with our mower, the ants are visibly annoyed (OK, maybe a bit of anthropomorphizing there) and set about patching the mound.

One ecologist who was working for the Institute for Applied Ecology commented that, in managed prairie restoration, it seems like these mounds develop about as fast as their mowing cycle, on sites where annual burning isn't possible. So these might be an important element of prairie "microtopography" that might be hard to replicate in some prairie restorations.

Regarding places in the Willamette Valley that haven't ever been plowed: The main prairie at Finley is almost certainly the largest remaining example of native wet prairie. There are likely other places that were grazed but not plowed, for example, the upper part of Baskett Butte. The top of Coffin Butte is likely a smaller example -- there has been some disturbance but by all indications it was grazed rather than plowed.

Another restoration biologist noted a decade or so back, as we were starting to see California winemakers buying up similarly hilly lands in the Willamette Valley as a hedge against global warming, that there are likely more prairie remnants that were never characterized or brought into protection, especially up in Polk & Yamhill counties.

Joel Geier

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[Lisa Millbank]

Thanks Bill, for locating that paper!  Here's a digital version of it: https://www.biodiversitylibrary.org/page/56105570#page/12/mode/1up  It's only a few pages long.

BugGuide says that North American ants that were formerly called Formica fusca are now Formica subaenescens.  This species seems to be wide-ranging and can build different types of nests, such as under logs.  I wonder if our wet prairie version of these ants may someday be recognized as a separate species.

Lisa

[howard bruner]

Fascinating discussion about the architect ants.  Owen asks about actually getting a look at these unique insects.  

Many years ago, I worked on Finley as a field botanist. The permanent plots that we sampled are on the prairie south of the observation deck on Finley Road. Bob Frenkel, who was the lead investigator of the project, found me a student to record the data as I read the plots. She made a habit of sitting on the ant mounds while recording. Until she realized she was picking up hitchhikers when we moved to the next plot. Apparently, the ants were quite aware of disturbance on the exterior of their mounds. She found that if she knocked on the top of the mound with her pen, she could determine whether or not it was a good sitting mound, as there seemed to be an uneven response among the mounds to the disturbance. I think it was fairly successful (although still noticed a bit of antsy fidgeting) and her technique might be an easy way to find those little ones. 

H

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