Acacia Ants

Another great symbiotic mutualism from Costa Rica!  The acacia tree (actually several species of Acacia) we saw at Santa Rosa National Park have a very close association with ants of the genus Pseudomyrmex.  The plant provides the ants with nest sites, carbohydrates and protein in return for defense by the ants.

The Acacia Tree

Acacia trees are part of a large genus - there are thousands of species worldwide, or, more accurately, in the southern hemisphere and in the tropical parts of the northern hemisphere.  A member of the bean family, they are also related to locust trees and the tamarind tree.  As  members of the bean family, Acacias may play host to Rhizobium bacteria which infect the rrots forming characteristic root nodules.  The bacteria obtain nitrogen from the air and convert it into an organic form which is shared with the tree.  The tree uses the nitrogen to make amino acids and thus proteins.  Presence of the bacteria allow the Acacia to grow better on soils with little nitrogen.  Worldwide, Acacia trees are a source of the material known as gum arabic, which is a thickener used in the production of many processed foods such as candies and ice creams.  Acacias also bear formidable thorns to deter mammalian predators.  Despite the thorns, herbivores such as giraffes feed routinely on acacias; in fact when giraffes were first brought to The Wilds in the 1990's I was struck by how quickly they began browsing on the locust trees (locusts being a thorn-bearing Acacia relative).

One of the more interesting aspects of the Acacia tree is its tendency to form symbiotic, mutualistic relationships with ants.  This happens in both the Americas and in Africa (and perhaps in other areas as well).  In Costa Rica, the association is usually with ants of the genus Pseudomyrmex.

 

The Ants and the Symbiosis

Left:  The Acacia tree provides the ants with sugars, protein and a nesting site.  You can see two of those benefits in this picture.  The enlarged thorns are hollow - the ants need only chew an entrance hole to gain access to the hollow inside of the thorn, which they can then raise their young in.  A colony of ants on a tree may occupy many such thorns.  The other lure for the ants are nectaries; these glands have a little depression that fills with tree sap, a good source of sugar (and water, something which should not be ignored in a tropical seasonal forest during the dry season.
Here is another view of a nectary.

 

Left:  To fulfill the protein needs of the ants, the tree also provides protein-rich Beltian Bodies, particularly on the tips of newly developed leaves.  These bodies serve no function for the plant, but they do help complete the nutritional needs of the ants (which also derive nutrition from insects that they kill on the acacia).  

 

Some recent studies from Africa (where the trees don't provide the Beltian bodies) show an interesting effect.  If the trees are fenced off (to prevent large mammals from grazing on them), then there is less damage to the plants.  Under these conditions, the plants cut back on the amount of sugar they provide to the ants, whose services are not as important without the herbivory occurring. Deprived of this sugar source the ants turn to another - they allow sap-sucking insects such as aphids or scale insects to attach to the plant and then get sugar exuding from the sap-suckers This ant-aphid symbiosis is common in the ant world - many species of ants do this, and they protect the aphids or scale insects from predators in return.  Thus, the plant goes from having the ants as an ally to having the ant as an enemy.  Ironically, it was this type of ant that was first co-opted by the acacias when they started producing nectaries and thus inducing the ants to abandon their sap-sucking proxies and go to work full-time for the tree.

 

 

The classic line is that the ants drive away all insects and other invertebrates from the Acacia.  They also deter vertebrate herbivores from chewing on the leaves, and even cut away epihytic plants.  In some cases they may come down from the Acacia tree and remove or kill (by girdling) competing vegetation in the immediate vicinity. 

 

Our observations in Costa Rica showed that the ants don't exclude all insects and other animals from the Acacia.  To the left, a spider is spinning its web adjacent to an inhabited thorn.

This beetle was crawling around on an Acacia leaf, of course it is possible that it would be run off or killed in short order.

This praying mantis was doing its preying on an Acacia, apparently with little interference from the ants.  Of course, having a preying mantis on the plant is good from the Acacia's point of view, but how do the ants know which insects to allow and which to drive off?  Perhaps there is a questionnaire...
Vertebrates sometimes make their homes in Acacia trees as well.  Bird nests in Acacias are not bothered by many of the traditional nest predators (snakes), but how the birds themselves avoid the ants isn't clear.

This wasp nest isn't in an Acacia (but the tree does have thorns); the thorns provide some protection against vertebrate predators who would love to get at the honey and protein-rich grubs inside - and who would be willing to take a few stings along the way.  At Santa Rosa we saw other wasp nests in acacia trees, where they are protected both by the thorns of the acacias and by the acacia ants.  Apparently the wasps smear a chemical deterrent on the branch where the nest is connected to the tree to keep the ants away from the nest. 
Speaking of chemical defense, stink bugs like this one were very common in the Acacia trees at Santa Rosa.  Stink bugs, as their name implies, have a good number of chemical defenses of their own, and presumable these were used to keep the Pseudomyrmex ants at bay.

 

 

 

This is a good reference on Acacias:  http://waynesword.palomar.edu/plaug99.htm

References:

In addition to the web references listed above, these two books are excellent!

Hölldobler B, Wilson EO.  1990.  The ants.  Harvard University Press.  732 pp.

Hölldobler B, Wilson EO.  1994.  Journey to the ants.  Harvard University Press.  224 pp.

Janzen, DH. (Editor). 1983. Costa Rican Natural History. The University of Chicago Press, Chicago.

As are  these articles:  

 

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