This week we’re going to be doing a little trash talking. About ant trash, that is.
Ant trash or “middens” are the discarded materials that ants pile around or near their nests or mounds. Today we’re picking through the middens of a colony of Pogonomyrmex rugosus harvester ants found at Veteran’s Oasis park in Chandler, AZ. This first photo was taken on November 29, 2012.
What do you see? Small bits of rocks, of course. Those are common around harvester ant nests.
Being familiar with the plants found in the area, it is also possible to pick out some discarded seed materials.
The fuzzy strips are from creosote bush seeds. (Links go to posts about the plant at Growing with Science blog.)
(Creosote bush seeds)
You can also find some desert mallow, Sphaeralcea sp.
The desert mallow seeds have a covering that is often seen in these middens.
This is what the desert mallow seeds look like with the covering removed.
Finally, the larger pale seed toward the upper right is a mesquite of some sort.
The next two photographs were taken on May 6, 2013.
Taking a closer look, it is apparent that for the most part these middens consist of discarded dark gray fringed seeds from brittlebush, Encelia farinosa.
Brittlebush seeds
In August 2014, although there were still brittlebush seeds, the mix was more varied.
The larger, ovoid brown seeds are apparently from a honey mesquite.
Honey mesquite seeds
Over the weekend I visited the same colony again (April 26, 2015).
See the brittlebush and desert mallow seeds near the top of the photograph? Some of those were being dropped by workers from outside the nest and picked up by other workers to be taken inside. Other were being taken out.
Around the nest, the ground is covered with plant material likely deposited by the ants.
Note: the brown round objects are jackrabbit scats, probably incidental.
Close-up, it looks like quite a few desert mallow, creosote bush and some brittlebush discards.
What does studying middens tell us?
First of all, from the photographs we can safely say that as the colony has matured it seems to be gathering a substantially larger amount of plant material. The amount of middens probably isn’t a clear indicator of colony size, however, because the quantity of middens likely also varies with season, habitat, and recent weather. Taber (1998) indicates that worker harvester ants may store trash in underground chambers. These trash chambers may be closed off, or periodically cleaned out and brought to the surface causing a flush of discarded materials.
We can also make some assumptions about what seeds the harvesters are gathering throughout the season. Thus, these Pogonomyrmex rugosus workers are gathering seeds from mostly local desert species of plants.
What ends up in the trash, however, may not accurately entirely reflect what is being consumed. It is likely some seeds are used completely and have no husks to discard. Think about it, how accurately does your trash reflect what you eat?
Have you studied ant middens? What did you find out?
I’ve spent a ridiculous amount of my life watching harvester ants in Arizona. One time I was enjoying a beer while doing so and ended up spitting out a mouthful onto the ground. The nearby ants started to drink the discarded beer and I’m assuming got drunk, because a presumably sober ant went and (non-aggressively) picked up one of the ants that had been drinking. I was curious about what the plans were for the ant that had partaken, so I followed them. I followed them all the way to the midden, where upon arrival, the sober ant set the drunken one down and started making its way back to the nest. I guess harvester ants have a zero tolerance policy for their colonies? I found it kind of funny and super interesting.