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Have you ever heard a gardener comment, "Spurge plants always have ants," and wondered if it was true? I admit searching ground spurge (Chamaesyce prostrata) for ants when I find the weeds, and I am often rewarded. Ants are attracted to spurge seeds (previous post), but is there more to the story?

Let's take a look at a spurge with numerous ants milling around it. Most are Forelius.

The ants seem to be visiting the complex structures that pass for flowers in ground spurge.

In this photograph the ant on the right appears to be visiting the flower. But there is also an aphid on the same plant.

Yes, there are aphids on the plant.

The ants are also collecting honeydew from the aphids.

This leads to the question, how significant are the two sources of sweets? Ants are known to regularly visit the nectaries of the invasive weed, leafy spurge, but I wasn't able to find out much about ground spurge.

Another question arises about whether ants might be capable of pollinating ground spurge. Ants are thought to pollinate certain low-growing plants with small flowers that are tight to the stem, and ground spurge certainly fits the bill.

Selleck, et al. 1962 reported that leafy spurge was pollinated by ants. After reading their evidence, however, it might be a shaky conclusion.

What is up with ants and spurge? Do ants visit spurge where you live? Do the ants gather nectar? Any evidence of pollination?

Reference:

Selleck, G.W.; Coupland, R.T.; Frankton, C.(1962). Leafy spurge in Saskatchewan. Ecological Monographs. 32:1-29. free .pdf.

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Imagine you are a plant. You sit all day, not going anywhere, just growing and changing. You make a flower and eventually some seeds. Then you realize that if you simply drop those seeds in your shade, they won’t be able to grow. How are you going to get your seeds to someplace where they can thrive? You can throw them, but it is really hard without arms (although some plants can shoot their seeds away). You can make a sail or parachute and send them off in the wind, but only if you are in an open spot or if you are tall and the wind can catch them. You can make a fleshy fruit around your seeds so birds and mammals will take them away and eat them, but sometimes there isn’t enough water to make a juicy fruit.

What do you do if you are a low plant, a small unnoticed plant, or if you live in the desert and you don’t have a lot of water? Then you might want to encourage ants to take your seeds.

Wait, don’t ants eat seeds? Yes, some harvester ants specialize in collecting and eating seeds. But far more species of ants and plants have developed a civil system of reward for effort that benefits both the plants and the ants. The seeds of these plants have a special structure on them that is particularly yummy to ants. You could think of it as an ant potato chip, although the technical name is “elaiosome” which means oily, edible thing. Wait, doesn’t that describe a potato chip too?

These are acacia seeds in an open pod. The orange and red parts are elaiosomes.

Ants collect the seeds provided by the plant, carry them back to their nest, strip off the elaiosome/potato chip part, eat that and discard the still-intact seed into their garbage heap, a nutrient-rich pile of well-aerated soil. The seeds germinate, sending out new little plants that grow and thrive. Why don’t the ants just continue eating the seed after they’ve removed the elaiosome? Because most seeds with these structures are especially hard and/or slippery so they are just not that easy to eat.

Sound like science fiction? Scientists have found these structures on the seeds of about a third of the plant species, everything from acacia trees to violets. But simply having the structures might not mean much. How do researchers know ants are important for planting seeds? Wildflowers were rapidly disappearing in one area of New England. The researchers knew pesticides had been sprayed on the forest to kill gypsy moth caterpillars. Did the pesticides kill the wildflowers? No, the answer was that the pesticides killed all the ants, and ants are vitally important for planting wildflowers, like violets and trillium.

Besides providing a nice, well-aerated, moist and nutritious place for seeds to grow, ants may also be protecting seeds from rodents and other seed predators. Ant-planted seeds with elaiosomes are most common in areas with frequent wildfires. It makes sense the seeds would do better buried under the ground then exposed to the intense heat. Ant-planted seeds also do better in harsh climates, where the surface of the soil gets exceedingly hot.

What about the harvester ants that eat seeds and not just the elaiosomes? It turns out even harvester ants drop or discard perfectly good seeds in places where the seeds can grow. Often a fringe of emerging seedlings surround harvester ant trash heaps.

Humans have taken advantage of this seed-collecting behavior. In southern Africa the rooibos plant is used to make a tea, sometimes called “redbush” tea. Cultivation of the plant was difficult because the plant shoots out its seeds, making them nearly impossible for humans to gather. Farmers eventually discovered certain kinds of ants collect and store the seeds in their nests. They began gathering seeds from the ant nests and planting them successfully.

The bottom line is that ants are responsible for dispersing and planting thousands of species of plants, and we are learning more about these amazing relationships all the time.

Have you ever seen ants gathering seeds? Did you know ants planted seeds?

This post was prepared to accompany How Seeds get Around at Growing With Science blog.

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From the photo archives:

You have probably heard all about the relationship of ants and peony flower buds.

Peonies (Paeonia sp.) are small perennial shrubs that produce large, lovely flowers in the spring.  The flower buds produce nectar via extrafloral nectaries, which attract ants. The ants chase off potential herbivores until the buds open. A simple story, yet an entire garden mythology has grown up around it. You can do an Internet search for "peony ants" and find a wealth of funny, and at times sad, myths.

But there may be another piece of the story that is rarely mentioned.

Do you know what this plant structure is? (Quit looking at the ant :-))

Since I already mentioned peonies, you can probably guess it is the fruit of a peony. Inside each of those three "pods" are rows of seeds completing development.

As this is not a bud, what is an ant doing there? Take a look at the next few photographs and see what you think.

Mandibles agape...

Do you see the fly?

Has anyone seen this before? Do you know if the extrafloral nectaries are still active? Are ants just poor botanists?

If you want to find out more, try:

B L Bentley. (1977). Extrafloral Nectaries and Protection by Pugnacious Bodyguards. Annual Review of Ecology and Systematics, Vol. 8: 407 -427.

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Remember the post a few weeks back showing the snail shells in the harvester ant midden? Let's take a little closer look at what kinds of objects harvester ants collect and put on their mounds.

At first glance a harvester ant mound looks a bit like a pile of rubble.

If you start to study the mound, however, you begin to notice that the pebbles are roughly the same size. Harvester ants (Genus Pogonomyrmex) are known to gather various objects and deposit them around their nest entrances. Harvester ants in the western United States often gather pebbles, among other things.

Fossils

if you are interested in harvester ants, you should pick up the fascinating article by Daniel Adams in Smithsonian Magazine from 1984.  Adams described how paleontologist John Hatcher discovered a terrific place to find tiny fossils, such as the teeth of mouse-sized mammals. In fact he went from finding an average of 2 per day to over 87 per day.  What was his secret? Hatcher discovered that harvester ants pick up ant-sized fossils and drop them on their mounds. Hatcher simply had the locate ant mounds and sort through the piles. Much of what is known about the mammals that lived during the time of dinosaurs is due to the diligent collecting behavior of harvester ants.

Paleontologists and archaeologists both still use mounds as a source of tiny fossils today. The University of Colorado Museum of Natural History has an exhibit honoring the contribution of the western harvester ant (Pogonomyrmex occidentalis), called Tiny Collectors: Harvester Ants. This page has a photograph of a mound and more information about how the fossils are collected.

Most of the finds are in Montana, the Dakotas, Wyoming, and Colorado. I'm not sure, but there might even be something in the mound from Arizona.

Can you spot it in the upper right hand corner? To me it looks a bit like a tooth.

Beads

In 2009, Schoville et al. distributed beads of various sizes and colors around harvester ant mounds at measured distances to investigate how far harvester ants move artificial material. They found ants would bring back beads from as far as 48 m away, but most were collected within 20 m of the main entrance. They were interested in how harvester ants moving artifacts potentially effects archaeological findings.

Charcoal

Deborah Gordon first studied deposits of charcoal in Pogonomyrmex badius middens in 1984. She established that the pieces of charcoal were not incidental, because if she removed the charcoal bits, the ants quickly began replacing them. She concluded that the charcoal probably marked the ants' territories and deterred other ants.

Smith and Tschinkel re-visited what they called "non-food collection" by harvester ants. They evaluated mounds in Florida and found that pieces of charcoal were the most common objects, as Gordon had. They suggested that the objects reflect a significant amount of material, and that once again, the collecting behavior probably isn't incidental. They found a slight increase in temperature in mounds with charcoal versus those without, but found no evidence other ant species avoided the pieces.

Snail Shells

It turns out that other people have also noticed snail shells on harvester ant mounds.

PÁLL-GERGELY and SÓLYMOS are malacologists working in Turkey who found that harvester ant mounds can be a significant source of taxonomic material, especially for more cryptic species of snails.

They noted that the ants tended to collect the smaller species of snails, and the juveniles of larger species. They did not observe the ants feeding on snails, but noticed some shells were cleaned out and some were not. They emphasized that harvest ants are known to collect and feed on seeds.

Some ants do eat snails. Mark Moffett has a photo of a Basiceros singularis worker feeding a tiny snail to its larvae. (You might have to scroll through a few photos to find it.) Do harvester ants do the same?

Conclusions:

The bottom line is that harvester ants have some interesting and unusual things on their mounds, and as of yet, we don't have a very clear picture exactly why or how. In any case, we humans have found ways to use their activities to our benefit.

What unusual items have you found on a harvester ant mound?

References:

Adams, D.B. (1984). Fossil hunters best friend is an ant called pogo: paleontologists use insects to find
small bones. Smithsonian, 15: 99-104.

Gordon, D. M. (1984), The harvester ant (Pogonomyrmex badius) midden: refuse or boundary? Ecological Entomology, 9: 403–412.

PÁLL-GERGELY, B. and P. SÓLYMOS. (2009). Ants as shell collectors: notes on land snail shells found around ant nests. Malacologica Bohemoslovaca, 8:  14–18.

Schoville, B. J. Burris, L. E. and L. C. Todd. (2009).  Experimental Artifact Transport by Harvester Ants (Pogonomyrmex sp.): Implications for Patterns in the Archaeological Record Journal of Taphonomy, 7 (4):  285-303.

Smith C. R. and W. R. Tschinkel. (2005). Object Depots in the Genus Pogonomyrmex:  Exploring the “Who,” What, When, and Where. Journal of Insect Behavior, 18 (6):  859-879.

Smith C. R. and W. R. Tschinkel. (2007). The adaptive nature of non-food collection for the
Florida harvester ant, Pogonomyrmex badius. Ecological Entomology, 32:  105–112.