Big mammals key to tree-ant team

By Anna-Marie Lever
Science and nature reporter, BBC News

A system in which there is mutal benefit

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."

Acacia trees hire ant bodyguards for protection from grazing animals

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.

Human activity is having unanticipated results on ecosystems

At present, the researchers do not fully understand the mechanisms that allow the tree to sense it is no longer being browsed and to turn off its investment in mutualistic ants, but they suggest it takes place over a 5-10-year period.

Dr Palmer said there were two important conservation implications of this research: "The first is that the decline of these charismatic [large animals] can have complex and cascading effects on entire ecosystems, with unanticipated results.

"The second is that classical conservation approaches talk about conserving species, but perhaps equally important is the conservation of 'interactions'."

The researchers suggest that the loss of large herbivores throughout Africa, due to ongoing human activity, may have strong and unanticipated consequences on the broader community.

Mr Pringle adds: "It is a cautionary tale."