The Occurrence of Seeds and Fruits Taken by Ants Versus Birds in Mediterranean Australia and Southern Africa, in Relation to the Availability of Soil Potassium

Abstract

This paper compares the distribution of certain floristic elements, and the animals associated with them, in like environments on different continents. The Barrens on the south coast of Western Australia and the Caledon coast in the southwestern Cape of South Africa have not only very similar mediterranean climates and landforms, but also soils comparable in their chemical fertilities. In both areas, plants growing on substrates very poor in nutrients bear dry, hard or dehiscent seed capsules displaying no obvious food body or a very small one attractive only to ants. Plants producing relatively large, soft food bodies attractive to vertebrates appear restricted to soils relatively rich in available potassium and with adequate calcium and phosphorus. It is suggested that the availability of potassium to plants is unusually low over most of the dry areas of Australia, owing mainly to geological peculiarities. The parent rocks contain very low concentrations of nutrients, and the soils have been further depleted by leaching followed by contamination with unusual accumulations of sodium of marine origin. As a result, plants with seeds taken by ants are widespread in Australia, while in dry areas of southern Africa and other continents we find instead plants of rainforest affinities producing fleshy fruits taken by birds. It is proposed that unusual interactions between plants and animals have arisen in Australia because of a unique physical environment rather than the isolation or genetic limitations of the flora and fauna.