Effect of Diaspore Characteristics on Removal of Seeds Adapted for Dispersal by Ants

Abstract

We investigated the role of ant behavior as a selective influence on the presentation and morphology of elaiosome—bearing seeds in Australian sclerophyll vegetation by manipulating characteristics of natural and artificial diaspores. We compared the response to these diaspores of three seed—removing ant species, two of which (Rhytidoponera "metallica" and Aphaenogaster longiceps), produce more favorable seed fates than the third species (Pheidole sp. 1). We measured the effect on removal rate of diaspore arrangement (degree of clumping), elaiosome removal, diaspore size (i.e., mass), elaiosome/load ratio, and composition of both the elaiosome and seed. Clumped seeds were removed significantly faster than single seeds by Pheidole sp. 1, a group forager. A. longiceps, a solitary forager, removed single seeds faster while R. "metallica" removed single and clumped seeds at the same rate. The presence of elaiosomes on seeds increased removal rates, especially by A. longiceps and R. "metallica." Both A. longiceps and R. "metallica" responded positively to elaiosome/load ratio while Pheidole sp. 1 responded mainly to diaspore size. These species—specific responses were such that diaspores with high elaiosome/seed ratios are more likely to be removed by A. longiceps or R. "metallica" than by Pheidole sp. 1. Diaspore characteristics other than size had inconsistent effects on removal rates. The differential response of the ant species to such characteristics as seed arrangement and elaiosome/seed ratios demonstrates the way in which ant behavior may have been an important selective force in the evolution and maintenance of myrmecochory.