Acacia trees, ants, giraffes in African ecosystems


From the BBC:

Big mammals key to tree-ant team

By Anna-Marie Lever

Science and nature reporter, BBC News

At first it may seem counter-intuitive: that preventing large African herbivores from browsing Acacia trees decreases their growth.

This, however, is precisely what researchers report in Science magazine.

It is all because of the Acacia’s mutually beneficial relationship with a biting ant.

Together they fend off Africa’s big grazing mammals; but it is these very antagonists that are needed to keep the plant-insect team working in concert.

“Simulating large mammal extinction, by experimentally excluding them from eating the trees, causes the ant-plant mutualism to break down,” said co-author Robert Pringle from Stanford University, US.

The whistling thorn tree (Acacia drepanolobium) and the biting ant (Crematogaster) that lives on it form a relationship, evolved over many millennia, in which both species co-operate and in turn benefit from each other.

Ant bodyguards

When this “mutualism” is working well, Acacia trees provide ants with swollen thorns, which serve as nesting sites; and nectar, which the ants collect from the bases of Acacia leaves.

In return for this investment, ants protect the tree from browsing mammals by aggressively swarming against anything that disturbs the tree. …

Mr Pringle explains: “It is as if the tree hires bodyguards, in the form of ants, to protect it from being eaten.”

The researchers disrupted this relationship by fencing off six plots of savanna land in Kenya by an 8,000-volt electric fence for 10 years.

Herbivores, such as giraffes and elephants, were no longer able to feed on the trees, causing a change in plant-ant dynamics.

“[The trees] diminish the rewards that they produce for the ant bodyguards, decreasing both the amount of housing and the amount of sugar-rich nectar they produce,” lead-author Dr Todd Palmer at the University of Florida, US, told the BBC News website.

He continued: “In essence, the trees begin to default on the co-operative bargain that they’ve made with the ants, because the trees no longer have need for protection from large browsing mammals like giraffes and elephants.”

It would seem that now the trees are better off, as they do not need to use their resources to support the ants – but the researchers have revealed that this is not the case.

Due to lack of housing and food, the mutualistic ant species becomes less aggressive, its colony size decreases and it loses its competitive edge.

Conservation implications

“The net result is a community-wide replacement of the ‘good’ mutualist ant by a decidedly ‘bad’ ant species that does not protect the trees from herbivores, and actually helps a wood-boring beetle to create tunnels throughout the main stem and branches of the acacia trees, which the bad ant then uses as nesting space,” Dr Palmer explains.

Trees occupied by this antagonist ant grow more slowly and experience double the death rate compared with trees occupied by the mutalistic ant.

Acacia trees harboring ants escaped elephant appetites, apparently because elephants don’t like ants up their trunks. Karen Hopkin reports: here.

Ant-acacia plants attract ants by offering specialized food and hollow thorns in which the ants live, while the ant colony in turn defends its acacia against herbivores. This mutualistic relationship only occurs in older plants. New findings by plant biologists identify the genetic pathway that appears to regulate the timing of the acacia’s ant-sustaining arsenal: here.

8 thoughts on “Acacia trees, ants, giraffes in African ecosystems

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