from : http://spectregroup.wordpress.com/2010/01/08/acacia-self-defense/

Acacia Self-Defense
http://www.newscientist.com/article/mg12717361.200-antelope-activate-the-acacias-alarm-system.html
Antelope activate the acacia’s alarm system
“Acacia trees pass on an ‘alarm signal’ to other trees when antelope browse on their leaves, according to a zoologist from Pretoria University. Wouter Van Hoven says that acacias nibbled by antelope produce leaf tannin in quantities lethal to the browsers, and emit ethylene into the air which can travel up to 50 yards. The ethylene warns other trees of the impending danger, which then step up their own production of leaf tannin within just five to ten minutes. Van Hoven made his discovery when asked to investigate the sudden death of some 3000 South African antelope, called kudu, on game ranches in the Transvaal. He noticed that giraffe, roaming freely, browsed only on one acacia tree in ten, avoiding those trees which were downwind. Kudu, which are fenced in on the game ranches, have little other than acacia leaves to eat during the winter months. So the antelope continue to browse until the tannin from the leaves sets off a lethal metabolic chain reaction in their bodies.”

Interspecies Mutualism
http://news.bbc.co.uk/2/hi/science/nature/7179880.stm
“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. 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. 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.”

Bribes + Repellents
http://news.bbc.co.uk/2/hi/science/nature/8383577.stm
“That is the plus side for the plants. But being inhabited by aggressive insects could make one important aspect of a plant’s life difficult – flowering. One clever approach by the plant is a food “bribe”. Acacias produce structures called beltian bodies on the leaf tips. These, Dr Raine explains, are nutritious structures produced by the plant to feed its resident colony of ant-guards. But [when in bloom], it is a case of chemical warfare. Floral volatile compounds can act as signals – drawing in pollinators such as bees and hummingbirds in with their irresistible aromas. To the ants, however, they are far from irresistible. “The flowers seem to produce chemicals that are repellent to the ants,” said Dr Raine. “They release these particularly during the time when they’re producing lots of pollen, so the ants are kept off the flowers.” The researchers think that some of the repellents that acacias produce are chemical “mimics” of signalling pheromones that the ants use to communicate. “We put flowers into syringes and puffed the scent over the ant to see how they would respond, and they became quite agitated and aggressive” he explained. “The ants use a pheromone to signal danger; if they’re being attacked by a bird they will release that chemical that will quickly tell the other ants to retreat.” Dr Raine says this clever evolutionary system shows how the ants and their plants have evolved to protect, control and manipulate each other.”