Trees That Hire Bodyguards

The latest issue of journal Science has fascinating facts about intricate web of life in Africa. Scientists report how elephants, giraffes and other large herbivorous spur Acacias to “hire” and support ants as bodyguards. 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. 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. Healthy trees have hundreds of the thorns, often containing more than 100,000 ants per tree. When the threat from these mammals stops or decreases, the trees slash their investment in ants, opening both to other attackers. Fewer colonies of weakened ants become less able to defend their territory from another species of ant that moves in which does not have a mutually beneficial relationship with Acacias. This new ant species feeds away from the tree and does not protect it from attackers. It actually encourages a destructive, wood-boring beetle whose cavities then serve as this ant’s home. The trees untouched by browsing mammals are infested with more of the beetles gradually weakening the trees. The trees wind up actually needing the mammals. Getting rid of the mammals causes individual trees to grow more slowly and die younger. The research has important implications for conservation. The cautionary note is that because many of the mammals are threatened, human activities like population growth, habitat fragmentation, over-hunting, can influence the ecosystem with unexpected consequences.