Ant-epiphytes of Australia

Abstract

Symbioses between ants and plants are widespread in the tropics. Trees are usually defended by their ants, while epiphytic plants probably obtain mineral nutrients via the ants. In northern Australia three Malesian genera of ant-epiphytes are found: Myrmecodia and Hydnophytum (Rubiaceae) and Dischidia (Asclepiadaceae). Myrmecodia and Hydnophytum form large tubers with cavities, some of which have specialized warts, probably for absorption. In Dischidia the leaves form sacs which are entered by adventitious roots. Ants, usually Iridomyrmex cordatus, occupy these cavities and sacs. They deposit organic remains in the cavities with warts. Radio-isotopes fed to the ants are later absorbed by the plants. The behaviour of the ants and other occupants of the ant-plant are discussed in relation to the symbiosis.

The three species of Myrmecodia and two of Hydnophytum in Australia require moist tropical conditions, but are absent from the rain forest. In coastal and savanna vegetation, where other epiphytes are rare, they are abundant, probably due to improved mineral nutrition from the symbiosis. In Dischidia transpirational water loss may be reduced by the symbiosis, enabling the plants to use drier habitats.

Illustration and a key to Australian species are given together with nomenclatural notes. Species of ants which have been found in ant-plants are tabulated.