Ant-Plant Mutualism in the African Savanna

Posted on 19, Jan, 2008 at 18:00

Category: Science

There is an acacia tree in South Africa, the whistling-thorn tree, Acacia drepanolobium, that secretes a carbohydrate-rich nectar from glands at the base of its leaves. It also grows large hollow thorns. The carbohydrate nectar attracts ants that feed on this nectar.

The species of ants found in greatest number is Crematogaster mimosae. This ant feeds on the nectar and lays its eggs in base of the large hollow thorns. C. mimosae will aggressively defend a tree and the use of its reward. They will attack any large herbivores, such as elephants, that try to graze on the acacia tree leaves. They attack in such large numbers that they drive away the herbivore before too much damage in done to the tree. The elephants run-a-way with a bunch of angry biting ants up inside of their trunk.

In addition to driving away the herbivores, C. mimosae also keeps away the long-horned beetles (Cerambycidae). The larvae of these beetles excavate stem cavities that can harm the tree.

The next common species of ant that can be found on the acacia tree is C. sjostedti. These ants like to lay their eggs in the stem cavities created by the long-horned beetles, so they are not as aggressive in keeping the beetles off of the tress as C. mimosae.

Dr. Todd M. Palmer and his colleagues wondered what would happen to the acacia trees if the large herbivores were removed from the equation. So, they built a large fence around some trees in Kenya Science 11 January 2008, Vol. 319. no. 5860, pp. 192 195).

After ten years the acacia trees seemed to be doing worst without the herbivores instead of better. Without the outside stimulus of the herbivores, the acacia trees quit producing the nectar and the large hollow thorns.

This caused the proportion of the trees containing C. mimosae ants to drop by 30% and the proportion of tress with C. sjostedti to increase by 54%.

It was also found that without the nectar from the acacia tree, C. mimosae were twice as likely to tend to sap-sucking homopteran scale insects as a source of carbohydrates to replace the lost nectar from the trees. It was noted at C. mimosae were less aggressive in protecting an acacia tree from other ant species. This allowed C. sjostedti to move in new areas.

Because C. sjostedti used the stem cavities created by the long-horned beetles, C. Sjostedti were not as aggressive in keeping the beetles off of the trees. Thus those acacia trees suffered increased attacks from the stem-boring beetles, grew more slowly and had twice the mortality rate than the trees were herbivores were allowed to graze on trees.

This is a very interesting study that shows us how changing just one variable in a mutualistic relationship can greatly affect an ecosystem.


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