January 11, 2008

Mutualism Good for Plants and Ants

The term "circle of life" may ring more true than some would think. For years, scientists have been studying the decline in large herbivores in Africa, and ten years ago, they decided to find out the effect of that decline on plant life.

The African acacia tree, a thorny African shrub has been providing food and shelter to aggressive biting ants for years. These ants, in turn protect the trees by attacking large herbivores and other animals that try and eat their leaves. This practice, called mutualism, is good for both the ants and the trees.

In order to study the decline's effect on the African acacia tree, scientists fenced off some of the acacias and left others unfenced. After just a few years, the fenced trees began to slow their growth and appear sick.

As a result of being fenced off, the large animals could not reach the trees to eat from them. Without animals eating the leaves, the trees lowered production of nectar and grew fewer swollen thorns for the ants to inhabit. Because of this, some ants reduced their defenses and began allowing other insects to live in the plants. Other ants were replaced by another species of ant which encourages the presence of wood-borer beetles which eat holes in the trees.

The researchers had thought that the plants might do better without the animals eating them, however, they were incorrect. Instead, those animals were the motivation for the trees to "pay" the ants for protecting them. Without those animals, the biting ants began to starve and give up.

Todd Palmer, an assistant zoology professor at the University of Florida said, "Although this mutualism between ants and plants has likely evolved over very long time-scales, it falls apart very, very rapidly. So, that's one lesson from the research, to me: The human-induced decline of big herbivores in Africa can have some pretty dramatic and non-intuitive consequences for the communities in which these large mammals live."

Palmer's research has now turned toward discovering if the system can be restored by bringing back the animals who eat it, and if so, how quickly it can be done.  


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