Acacia

Acacias are heavily defended against herbivores. Different species have different combinations of defenses:

1. Thorns (except Australia)
2. Chemicals: bitter tannins and psychoactive alkaloids are common in Acacias. Some species have up to 40% tannins in their bark^[1]
3. Ants: in Africa and Central Americs, symbiosis with ants can deter all sizes of enemy, from elephants to caterpillars and stem-boring beetles.^[2] Some species of ants will also fight off competing plants around the acacia, cutting off the offending plant's leaves with their jaws and ultimately killing it. Other associated ant species appear to do nothing to benefit their hosts.

Products from the acacia plants have often been used for medicinal purposes.

Various species of acacia yield gum. True gum arabic is the product of Acacia senegal, abundant in dry tropical West Africa from Senegal to northern Nigeria.

Acacia arabica is the gum-Arabic tree of India, but yields a gum inferior to the true gum-Arabic.

This species of acacia grows two kinds of thorns. The main defense is provided by pairs 5cm long arranged nearly at a right angle. Smaller stipular spines grow between the pairs of big thorns. These emerge from hollow galls, bulbous swellings 2 to 3cm across. One of four different species of ant lives in each of these igloos, which they open up by cutting holes into them. A dying bush whistles as the wind blows over these entrances. Most acacias make toxins that rush to leaves that are under attack by browsers. The whistling thorn does not. It is infested with stinging ants that swarm out and prepare to bite anything they can when the branch is disturbed. Most browsers seem to avoid infested bushes, perhaps because the ants stink of formic acid.

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